2014
FORAGE QUALITY FOR GRAZING HERBIVORES IN A SEMIARID SAVANNA SYSTEM AS AFFECTED BY FIRE AND
RAINFALL
(KRUGER NATIONAL PARK)
Armin H.W. Seydack , Wessel J. Vermeulen, Nicky Knox,
Navashni Govender, Izak P.J. Smit, Rina C. Grant and
Sandra MacFadyen
Scientific Report 03 / 2014
SOUTH AFRICAN
NATIONAL PARKS
South African National Parks
Forage quality for grazing herbivores in a semi-arid
savanna system as affected by fire and rainfall
(Kruger National Park)
Armin H.W. Seydack1,2 , Wessel J. Vermeulen1, Nicky Knox3, Navashni Govender4,
Izak P.J. Smit4, Rina C. Grant4 and Sandra MacFadyen4
1
Conservation Services Division
South African National Parks
P.O. Box 3542, 6570 Knysna, South Africa
Telephone: +27 (0)44 302 5612, Fax +27 (0)44 384 0136
E-mail: [email protected]
2
Honorary Research Associate, Department of Biological Sciences, University of Cape
Town, South Africa
3
Research and Development Group
South African National Space Agency (SANSA) – Earth Observation Division
P.O. Box 484, Silverton, 0127 Pretoria, South Africa
4
Scientific Services, Kruger National Park
South African National Parks
Private Bag x 402, 1350 Skukuza, South Africa
Citation
Seydack , A.H.W., Vermeulen, W.J., Knox, N., Govender, N.,
Smit, I.P.J., Grant, R.C. & MacFadyen, S. 2014. Forage quality
for grazing herbivores in a semi-arid savanna system as affected
by fire and rainfall (Kruger National Park). Scientific Report
number 03 / 2014, South African National Parks, Skukuza.
Forage quality for grazing herbivores in a semi-arid
savanna system as affected by fire and rainfall
(Kruger National Park)
·
Seydack, A.H.W., Vermeulen, W.J., Knox, N., Govender, N., Smit, I.P.J., Grant, R.C.
& MacFadyen, S. (2014a) Intra-specific modes of grass metabolic functionality in
response to fire and rainfall effects shape spatiotemporal patterns of forage quality in
an African semi-arid savanna system. Science Report No. 03 / 2014: 1-33, South
African National Parks, Skukuza.
·
Seydack, A.H.W., Vermeulen, W.J., Knox, N., Govender, N., Smit, I.P.J., Grant, R.C.
& MacFadyen, S. (2014b) Rainfall effects on dry season forage quality for grazing
herbivores in an African semi-arid savanna system. Science Report No. 03 / 2014: 3458, South African National Parks, Skukuza.
·
Seydack, A.H.W., Vermeulen, W.J., Knox, N., Govender, N., Smit, I.P.J., Grant, R.C.
& MacFadyen, S. (2014c) Fire and climate interactively shape grass forage quality for
grazing herbivores in an African semi-arid savanna system. Science Report No. 03 /
2014: 59-96, South African National Parks, Skukuza.
Summary
1. This study represents an exploration of grass metabolic response modes, induced by
the availability of resources for plant growth as shaped by fire and rainfall patterns,
in determining spatiotemporal variation in grass forage quality for larger grazing
herbivores in a semi-arid savanna system (South Africa).
2. Intra-specific metabolic response modes of C4 grass plants (inter alia indicated by
δ13C and δ15N values), responding to conditions of resource availability (light,
temperature, water: plant metabolic functionality) determined by fire frequency and
rainfall effects were identified; thereby establishing the eco-physiological basis
linking fire and rainfall effects with patterns of spatiotemporal variation in forage
quality.
3. Forage quality indicators and indices, based on nitrogen, non-structural
carbohydrates and digestibility of leaf and stem material were determined for three
perennial C4 grass species from grass swards subject to divergent wet season
rainfall over two years. Low wet season rainfall (260 mm) preceding the dry season
of 2007 and high wet season rainfall (463 mm) preceding the dry season of 2008
were recorded. Divergence of rain fall effects between these two years (2007: water
deficits; 2008: water surplus) had been corroborated by higher δ13C and δ15N
values of C4 grass material from the wetter year (2008) than of the drier year
(2007), underscoring the metabolic significance of the difference in rainfall
between the two years.
4. Wet season growth activity subject to water stress resulted in enhanced forage
quality in grass leaf and stem material in the dry season following a low-rainfall
wet season (regarding nitrogen concentrations and dry matter digestibility).
Conversely, grass metabolic functionality responding to conditions of accentuated
temperature and water availability encountered in years of high wet season rainfall
preceding the dry season was linked to reduced (dry season) forage quality.
5. The presence of grass swards of relatively enhanced/elevated forage quality during
the dry season is in agreement with population performance of certain grazer
antelope species being relatively unaffected by drought periods, notable in respect
of selective grazer antelope species.
6. Adjustments of grass metabolic functionality to resource constraints of relatively
lower levels of light, temperature and water availability experienced under grass
sward crowding conditions in grass swards subject to long fire intervals had
apparently induced modes of metabolic performance characterized by curbed
growth activity. Grass metabolic functionality responding to conditions of
accentuated temperature and water availability encountered in high-rainfall years or
when subject to acclimation to progressive nocturnal warming, was linked to
reduced (dry season) forage quality.
7. The results of this study underpin the significance of long fire intervals for the
enhanced availability of high quality grass forage in semi-arid savanna systems by
inducing modes of metabolic functionality in grasses which benefit dry season
forage quality by mitigating or neutralizing forage quality-reducing effects
associated with high-rainfall wet season growth or temperature acclimation to
nocturnal warming; thereby supporting grazer population performance, especially
of selective grazer antelope species (increased dry season forage quality).
Intra-specific modes of grass metabolic functionality in
response to fire and rainfall effects shape spatiotemporal
patterns of forage quality in an African semi-arid savanna
system
Armin H.W. Seydack1,2* , Wessel J. Vermeulen1, Nicky Knox3, Navashni Govender4,
Izak P.J. Smit4, Rina C. Grant4 and Sandra MacFadyen4
1
Conservation Services Division
South African National Parks
P.O. Box 3542, 6570 Knysna, South Africa
Telephone: +27 (0)44 302 5612, Fax +27 (0)44 384 0136
E-mail: [email protected]
2
Honorary Research Associate, Department of Biological Sciences, University of Cape
Town, South Africa
3
Research and Development Group
South African National Space Agency (SANSA) – Earth Observation Division
P.O. Box 484, Silverton, 0127 Pretoria, South Africa
4
Scientific Services, Kruger National Park
South African National Parks
Private Bag x 402, 1350 Skukuza, South Africa
1
Abstract
1.
This study represents an exploration of grass metabolic response modes, induced by
the availability of resources for plant growth as shaped by fire and rainfall patterns,
in determining spatiotemporal variation in grass forage quality for larger grazing
herbivores in a semi-arid savanna system (South Africa).
2. Intra-specific metabolic response modes of C4 grass plants (inter alia indicated by
δ13C and δ15N values), responding to conditions of resource availability (light,
temperature, water: plant metabolic functionality) determined by fire frequency and
rainfall effects were identified; thereby establishing the eco-physiological basis
linking fire and rainfall effects with patterns of spatiotemporal variation in forage
quality.
3.
Wet season growth activity subject to water stress (reduced/restricted growth
functionality) resulted in enhanced forage quality in dry (senescent) grass material in
the dry season following a low-rainfall wet season (increased nitrogen concentrations
and dry matter digestibility).
4. Adjustments of grass metabolic functionality to resource constraints of relatively
lower levels of light, temperature and water availability experienced under grass
sward crowding conditions in swards subject to long fire intervals had putatively
induced modes of metabolic performance characterized by curbed growth activity,
supporting enhanced forage quality. Grass metabolic functionality responding to
conditions of accentuated temperature and water availability as encountered in highrainfall years in grass swards subject to high fire frequencies was linked to reduced
(dry season) forage quality.
5. These effects of forage quality reductions in years with high wet season growth
activity were ameliorated and global warming impacts potentially mitigated by
growth-curbed metabolic functionality in grass swards at advanced post-fire ages;
thereby sustaining forage quality into the late dry season. Intra-specific grass
metabolic response modes reflect trade-off relationships similar to those manifesting
in interspecific fast-slow growth strategies in response to environmental resource
constraints.
6. Results of this study underpin the significance of long fire intervals for the
availability of relatively enhanced grass forage quality in semi-arid savanna systems
by inducing modes of metabolic functionality in grasses which benefit dry season
2
forage quality by mitigating or neutralizing forage quality-reducing effects associated
with high-rainfall wet season growth.
Keywords: dry season forage quality, fire return intervals, intra-specific grass metabolic
response modes, Kruger National Park, semi-arid savanna systems
Introduction
Large herbivore abundance in African savannas is largely determined by the availability of
food resources of adequate quality (East, 1984; Fritz and Duncan, 1994). Herbivore
population performance is often limited by plant nutritional quality (Van Soest, 1994), which
is highly variable at various spatiotemporal scales (Ellery et al., 1995; Mutanga et al., 2004;
Owen-Smith, 2007). Availability of forage resources of adequate quality for grazers is
generally linked to the presence of green grass in the landscape; most often having higher
nutrient concentrations than senescent dry grass material (Fynn, 2012). However, as
investigated and revealed in this study, substantial variation in dry leaf and stem forage
quality is evident during the dry season depending on modes of metabolic functionality
induced in grass plants as these respond to the availability levels of resources for growth
(light, temperature, water), as determined by fire and rainfall effects. The impact of post-fire
regrowth in savanna grazer nutrition has received much attention (Van de Vijver et al.,
1999a). In contrast, comparatively little is known about the effects of fire on dry season
forage quality of dry/senescent grass material despite the fact that nutritional shortfalls in
terms of essential nutrients are generally experienced by savanna ungulates during the dry
season (Illius and O’Connor, 2000; Fynn, 2012). This study aims to contribute in this context
through its focus on exploring the effects of fire and rainfall on specifically dry season forage
quality. Further studies of factors shaping dry season forage quality may be facilitated by
remote sensing methods of mapping dry season savanna forage quality (Knox et al., 2011).
According to inter-specific grass trait-environment correlations and trade-off
relationships relatively slower growth rates are associated with tolerance to soil moisture
stress and conditions of interplant crowding due to low disturbance rates by fire; requiring
tolerance to reduced irradiance and increased litter (Fynn et al., 2011). Grass plants also
respond to growth resource conditions intra-specifically (Furbank, 2011) as identified in this
study and represented by particular metabolic response modes. Divergent modes of grass
metabolic functionality, responding to growth resource conditions shaped by fire effects and
3
rainfall patterns, result in divergent levels of forage quality as here investigated in a semi-arid
savanna system (Kruger National Park, South Africa). Temperature and water availability
levels prominently affect plant metabolism and the nutritional quality of plant tissues
(Wilson, 1984). Forage quality is inversely related to the rate and extent of accumulation
(growth productivity) of structural carbon relative to non-structural tissue components (nonstructural carbohydrates and nutrient content). Plant growth productivity is prominently
driven by the combined availability of relatively high levels of temperature and water (TW
growth functionality). Fire effects and rainfall patterns influence the resource conditions
under which grass plants grow (Zimmermann et al., 2010) and both fire and rainfall are thus
expected to influence forage quality (as indicated by nitrogen, phosphorus, total nonstructural carbohydrate contents and dry matter digestibility). Through the elucidation of
underlying physiological mechanisms, as characterized with reference to metabolic
performance modes (Seydack et al., 2014), new perspectives are revealed on the factors
determining dry grass forage quality; thereby expanding the scope for interpretation of dry
season forage quality patterns affecting grazing herbivore population performance in semiarid savanna systems.
In this study plant physiological mechanisms linking forage quality patterns with fire
and rainfall effects were identified and characterized with reference to intra-specific
metabolic response modes which represent particular expressions of TW growth functionality
as plants adaptively respond to the nature of resources available to them. The nature of such
resource availabilities were shaped by inter alia rainfall patterns and fire frequency scenarios,
as explored in this study. The objectives of this study were twofold: (1) Elucidation of
physiological mechanisms (intra-specific grass plant metabolic response modes) linking fire
and rainfall effects to spatiotemporal variation in forage quality indicators and (2)
Interpretation of patterns of association of fire frequency and rainfall effects with variations
in forage quality over space and time. Establishing the underlying physiological mechanisms
was considered important in order to facilitate extrapolation of results beyond the
spatiotemporal context of the study.
Methods
STUDY AREA
The Kruger National Park (KNP) is situated in north-eastern South Africa (between latitudes
22º 20′ and 25º 32′ and longitudes 30º 53′ and 32º 02′) and represents a large (approximately
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2 000 000 ha) semi-arid savanna system. Covering an altitudinal range of between 200 and
700 m a.s.l., the KNP falls within two disparate climate zones as defined by the South
African Weather Service (Venter et al., 2003). Rainfall is strongly seasonal, falling mainly
during the austral summer (October-March: Fig. 1). The area north of the Olifants River is in
the northern arid bushveld zone, receiving 450-650 mm rain per year, whilst the southern part
of the park falls into the lowveld bushveld zone, with an average annual rainfall of 500-800
mm per year. The park is longitudinally divided into granitic substrates forming relatively
nutrient-poor sandy soils in the west and more nutrient-rich, basalt-derived clay soils in the
east (Venter et al., 2003). The vegetation of the northern KNP is characteristically dominated
by mopane (Colophospermum mopane) and this tree species is well represented in broadleaved bushveld vegetation types on granites and broad-leaved shrubveld associated with
basalts (Gertenbach, 1983; Venter et al., 2003). The herbaceous layer of the KNP is
dominated by C4 grass species (Kennedy et al., 2003) and the more nutrient-rich savanna
vegetation types on clay soils carry dense stands of nutritious, high-bulk grasses (Venter et
al., 2003).
The study area involved sampling areas in and around the N’waswhitshumbe
enclosure (304 ha) on the northern basalt plains (Fig. 2; 382 m a.s.l.; mean annual rainfall,
1984-2012: 486 mm). This selective grazer breeding camp, mainly for roan antelope
(Hippotragus equinus), excluded other large herbivores and predators. It was established in
1967 and extended in 1984 by 48 ha to include a vlei grassland. The enclosure occurs within
the Colophospermum mopane shrubveld on basalts (Gertenbach, 1983); characterized by
freely spaced mopane shrubs and a dense herbaceous layer, resulting in an open tree and
shrub savanna (Mopane Basalt Shrubveld: Mucina and Rutherford, 2006). Three fire
frequency constellations were represented in and around the enclosure (Table 1; Fig. 2): O,
located outside of the N’washitshumbe enclosure (high fire frequency, 1 and 3 years since
last fire for the two dry season sampling events respectively); inside of the N’washitshumbe
enclosure: A (intermediate fire frequency, 5 and 6 years since last fire), BE and BF (low fire
frequencies over the past circa 20 years; time spans since last fires for B and E in 2007: 22/8
and for B and F in 2008: 23/20 years, respectively).
STUDY APPROACH
5
We investigated whether fire effects (fire frequency, post-fire sward age, post-fire grass
growth conditions) would induce particular intra-specific modes of metabolic functionality
(metabolic response modes) in grasses of the post-fire period. Post-fire growth conditions
relate to rainfall, especially during the first year of regrowth, and sward age/crowding effects
with progressively increasing post-fire sward ages. δ13C and δ15N values were considered
valuable for the identification of metabolic performance modes and associated responses to
resource conditions relating to light, temperature and water availability levels (Buchmann et
al., 1996; Saliendra et al., 1996; Schulze et al., 1996; Robinson et al., 2000; Codron et al.,
2008; Craine et al., 2009a). Such responses are known to affect tissue concentrations of
selected forage quality indicators (N: nitrogen, as proxy for protein content, TNC: total nonstructural carbohydrates; important source of energy and DMD: dry matter digestibility;
important for rate of intake). Relatively high forage quality indices, that is, combinations of
selected forage quality indicators are furthermore of importance for optimal herbivore
nutrition. A nitrogen quality index (NQ = N x DMD) and a forage quality index (FQI = N x
DMD x TNC) were defined to capture the co-occurrence in forage of forage quality
indicators. Fire and conditions of climate (temperature and water) were thus considered to
interactively induce particular metabolic performance modes in grass plants which, as they
differentially respond to resource conditions, in turn would determine grass quality indicator
and index values of grass leaf and stem material. Spatiotemporal forage quality effects over
areas subject to divergent fire frequency scenarios and over years with high and low rainfall
respectively (sampling layout), were then interpreted in order to reveal and explain various
rainfall- and fire-forage quality patterns.
SAMPJNG LAYOUT
Leaf and stem material of three perennial C4 grass species Panicum maximum, Panicum
coloratum and Themeda triandra was collected in and beyond the roan enclosure
(N’washitshumbe, northern KNP: Fig. 2) during the dry seasons in September 2007and
August 2008. These three species commonly occur and are generally favoured for grazing by
especially selective grazer antelope species (Knoop and Owen-Smith, 2006). The apical 20
cm of grass stems (including any attached flower heads) were collected as samples of stem
material. Multiple plants of each species were collected at each replication locality and
bulked for analyses. Sampling was undertaken in four treatment blocks (three inside and one
outside of the enclosure) characterized by divergence in the incidence of fire. Two late dry
6
season sampling events of dry grass material were involved. For each treatment block (Fig. 2;
late dry season 2007: O, A, B and E; late dry season 2008: O, A, B and F) three sampling
localities (replications) were involved; representing a total of 12 samples per species and
sampling event. Additionally, remotely-sensed forage quality levels of relevant grass swards
for the early dry season of 2008 (May 2008) were represented by Carnegie Airborne
Observatory (CAO) imagery (Fig. 3) and associated data (Asner et al., 2007), involving the
forage quality indicators nitrogen, phosphorus and fibre for the grass swards (Knox et al.,
2011).
Forage quality indicators (percentage concentrations of nitrogen, total non-structural
carbohydrates and in vitro dry matter digestibility) were determined for the sampled grass
material of treatment blocks (Fig. 2) representing two fire frequency scenarios (Table 1; Fig.
3) according to the incidence of fire over the last decade (since 1987): high fire frequency
scenario involving treatment blocks A and O and low fire frequency scenarios involving
treatment blocks B and E (2007) and B and F (2008). Sampling involved two late dry season
sampling events representing dry and wet year conditions (Fig. 1: September 2007: dry
season preceded by a relatively dry wet season and August 2008: dry season preceded by a
high-rainfall wet season).
LABORATORY ANALYSES
The collected grass material was dried at 55 °C overnight and mill ground through a 1-mm
sieve to form homogeneous powdered samples. The concentrations of ash minerals and total
non-structural carbohydrates (TNC) were determined; as well as in vitro dry matter
digestibility (DMD) of the powdered sample material (Agricultural Research Council, ARC:
Irene). In vitro DMD was determined following an adaptation of the two-phase technique
described by Tilley and Terry (1963), with some modification introduced by Engels and Van
der Merwe (1967). Ashing was performed in a furnace at 600 °C. TNC concentrations were
analyzed as reducing sugars after complete enzymatic hydrolysis to monosaccharides. The
method entailed gelatinization of all starch in the sample by autoclaving, followed by
enzymatic hydrolysis of the starch to glucose and determination of glucose content by
spectrometric measurement. Duplicate results were within 3% of the mean for each pair. For
the determination of δ13C and δ15N, samples were combusted in an automated Thermo1112
Elemental Analyser (Carlo Erba, Milan). The resultant CO2 and N2 gases were then
introduced into a Thermo Delta stable light isotope XP mass spectrometer via a continuous
7
flow-through inlet system (Thermo Conflo III). 13C/12C and 15N/14N ratios were expressed
in the delta (δ) notation in parts per thousand (‰) relative to international standards (Pee Dee
Belemnite standard for carbon; N2 air for nitrogen). Standard deviations of repeated
measurements were less than 0.1 ‰ for δ13C (Stable Light Isotope Unit, Department of
Archaeology, University of Cape Town). From these analyses percent nitrogen and percent
carbon (by weight) were also provided for each sample.
STATISTICAL ANALYSES
The area outside of the enclosure (treatment block O: high fire frequency) was subject to
mainly bulk grazing by buffalo, zebra and blue wildebeest (relatively high grazing pressure),
whereas selective grazing by roan antelope (relatively low grazing pressure) occurred in fire
treatment blocks inside of the enclosure (A: relatively high fire frequency; B, E and F: low
fire frequencies). Conditions of high fire frequencies may furthermore attract increased
grazing intensities thereby re-enforcing fire effects (Trollope et al., 2014). Since higher fire
frequencies and increased grazing intensities are thus spatially associated, grazing effects
may potentially confound fire effects. However, the spatial association of increased grazing
intensities with higher fire frequencies and lower grazing intensities with lower fire
frequencies (Zimmermann et al., 2010) also applies in the absence of enclosure (Smith et al.,
2012; Trollope et al., 2014). Therefore, in the context of this study, any grazing effects
associated with high fire frequencies are considered an integral part of high fire frequency
effects.
The data set involved two sequential year sampling events with four treatments blocks
per sampling event (year). The sampling layout did not represent conditions of repeated
measurements (treatment block E sampled in 2007 and F in 2008) and treatment block effects
were different in each year (year x treatment effects interaction: responding to rainfall effects
in the dry year 2007 and fire frequency effects in the wet year 2008. Repeated measures
ANOVA involving both years was thus not applicable. Treatment block effects in respect of
the response variables (N, DMD, TNC, NQ and FQI) were accordingly analysed with multifactor ANOVA for each year separately regarding treatment blocks O, A, B and E for 2007
and O, A, B and F for 2008 (4 treatment blocks x 3 species with 3 replications/species within
treatment blocks: n = 4 x 3 x 3 = 36; involving 9 data points/treatment block). Two fire
frequency treatments/scenarios were involved (Sampling layout): a high fire frequency
scenario in A and O and a low fire frequency scenario in B, E and F (Table 1; Figs 2 and 3).
8
Each of these two fire frequency scenarios was accordingly represented by two treatment
replicates within each sampling year (2007: AO-BE; 2008: AO-BF).
Results
GRASS METABOLIC RESPONSE MODES AND FORAGE QUALITY
Results relate to two parts: Identification of grass metabolic response modes as associated
with respective treatment blocks (based on and with reference to known plant physiological
responses and mechanisms) and explanation of spatiotemporal patterns in forage quality
(over treatment blocks and sampling years) with reference to these metabolic response
modes. Intra-specific grass metabolic response modes were identified as induced by fire
effects (metabolic performance modes), coping with water resource availability levels and
fluctuations (NADP-NAD metabolic performance types) and responding to water availability
during wet season growth (wet season rainfall). Metabolic response modes, as derived for
particular treatment blocks (Table 2), consist of three parts: the metabolic performance mode
determined in relation to fire frequency effects, the metabolic performance type, based on
metabolic functionality in adaptation to water resource variability and divergence of
metabolic activity in relation to wet season rainfall status as applicable in dry and wet years
(Table 3). A metabolic performance mode may either be LT (metabolic functionality a high
levels of light and temperature linked to high fire frequency scenarios) or Clt (capacity for
metabolic functionality at relatively low levels of light and temperature under conditions of
grass sward crowding, C, linked to low fire frequency scenarios). Metabolic performance
types represent intra-specific expressions of NADP-ME/NAD-ME photosynthetic subpathway functionality. Furthermore, metabolic growth activity may either occur under
relatively low (wet season rainfall status: w) or high (wet season rainfall status: W) levels of
water availability in dry and wet years respectively (Table 3). The derivation of metabolic
response modes as applicable to particular treatment blocks is explained in designated
sections below.
Metabolic response modes determine the extent of TW growth activity (growth
functionality at relatively high levels of temperature and water) which prominently shapes
forage quality (Wilson, 1984). Associations between metabolic response modes (Tables 2 and
3) and patterns of dry season forage quality (Table 4) for treatment blocks subject to either
high or low fire frequency scenarios and under conditions of divergent wet season rainfall
9
(dry year 2007 and wet year 2008) thus allowed for the explanation of spatiotemporal
divergence in forage quality on the basis of underlying physiological mechanisms.
FIRE FREQUENCY SCENARIOS AND GRASS METABOLIC PERFORMANCE MODES
Divergent scenarios of fire incidence associated with fire treatment blocks O, A and BEF
(Fig. 2) were noted (Table 1). Fire treatment block O had been subject to high fire
frequencies (Table 1). Of the fires, 60% had occurred in years of comparatively high rainfall,
notably the last two fires. In fire treatment block A fires at intermediate frequency had
occurred mostly in years of low rainfall (Table 1). Water-stressed post-fire regrowth
conditions, exacerbated by hot, clean burns (2002 and 1998) had prevailed in respect of the
last three fires in fire treatment block A (Table 1). Fire treatment blocks B, E and F had been
subject to relatively low fire frequencies and these fire treatment blocks were at advanced
post-fire ages when sampled (Table 1). Based on the incidence of fires particularly since
1987 two fire frequency scenarios were identified (Table 1; a high fire frequency scenario
applicable to treatment blocks O and A and a low fire frequency scenario involving treatment
blocks B, E and F).
Both grass height and cover are generally positively correlated with time since last
fire and negatively so with fire return frequency (Ferwerda et al., 2006). Fire results in
increased grass plant mortality rates and high fire frequencies over extended time periods are
accordingly expected to result in relatively reduced grass plant densities; with more resources
for growth (water, nutrients, light) available for surviving grass plants (Zimmermann et al.,
2010; Fynn et al., 2011). With increasing post-fire age and associated increasing grass
plant/sward bulk densities, crowding effects increasingly shape the conditions under which
seasonal grass regrowth has to grow and survive. Necromass, which shades new growth, is
usually considerably higher in swards of advanced post-fire ages (Scholes and Walker, 1993);
as is competition for soil resources (Zimmerman et al., 2010). Temperature optima for
photosynthesis decrease with decreasing radiance flux density (Cresswell et al., 1982). Under
conditions of reduced light (shading) and (soil) temperature levels encountered in denser
grass swards, C4 grass growth is usually suppressed, resulting in metabolic adjustments
towards optimal functionality at lower light and temperature levels (Cresswell et al., 1982;
Lin et al., 2001). Resource level surplus conditions are then seldom experienced by individual
grass plants due to effects of sustained interplant crowding and competition. In congruence,
Ludwig et al. (2001) found that, under shade, grass production was relatively reduced in the
10
wet season, but continued for longer into the dry season. Intra-specific metabolic adjustments
of seasonal regrowth in grass plants to prevailing sward crowding/age conditions (interplant
crowding, necromass shading and associated microclimatic conditions) accordingly were
expected to have resulted in the induction of metabolic response modes geared to cope with
resource availability conditions of comparatively limited light, temperature and water
availability levels then prevailing (sustained functionality at moderate to relatively low
resource availability levels).
Applicable metabolic performance modes for fire treatment blocks B, E and F were
accordingly designated Clt (Tables 2 and 3: C for crowding; lt reflecting reduced maximum
functionality rates in response to light and temperature levels in comparison with those in fire
treatment blocks O: LT and A: LT). Comparatively accentuated realization of high TW
growth functionality (associated with relatively low forage quality: Table 3; wet year 2008:
Table 4) is expected to prevail during early leaf and stem growth of less crowded grass plants
(under conditions of high fire frequency scenarios as applicable for treatment blocks O and
A: Table 1) when subject to high rainfall during the onset of wet season growth preceding the
dry season of 2008: Fig. 1). Grouping of fire treatment blocks BEF (low fire frequency
scenarios) and OA (high fire frequency scenario) according to their designated metabolic
performance modes is strikingly congruent with patterns of forage quality in respect of these
fire treatment blocks (Fig. 3).
METABOLIC PERFORMANCE TYPES COPING WITH DIVERGENT WATER RESOURCE
CONDITIONS
NAD-ME (nicotinamide adenine dinucleotide-malic enzyme) type metabolic activity of C4
grasses (relatively low Rubisco: PEP carboxylation ratio) indicated to be prevalent in A and
B by comparatively lower δ13C and higher δ15N values than in O and E/F with higher δ13C
values indicative of higher Rubisco: PEP carboxylation ratios of NADP-ME (nicotinamide
adenine dinucleotide phosphate-malic enzyme) type activity (Table 2; Schulze et al., 1996;
Codron et al., 2008; Codron et al., 2009). Following the results of Codron et al., (2005)
seasonal shifts in plant δ15N corresponded positively with seasonal summer rainfall (TW
conditions: concurrently high temperature and water levels) whereas winter rainfall (tW
functionality) is associated with lower δ15N values (Craine et al., 2009a); indicating more
sustained, stress tolerant growth performance or activity (Robinson et al., 2000). These
findings support the interpretation that δ15N values index the degree of resource-responsive
11
TW functionality (Seydack et al., 2012a) and is consistent with NAD-ME decarboxylation
being associated with both drought tolerance (survival efficiency) and the capacity for
opportunistic utilization of short events of water surplus (Schulze et al., 1996). Conversely,
relatively low δ15N values are indicative of resource-level buffered, i.e. sustained metabolic
performance of NADP-ME functionality (sustained growth efficiency at functionality settings
at both high or relatively low levels of light and temperature).
Heavier clay soils (mopane-type soils) accentuate water availability deficits and
fluctuations for grass plants in treatment blocks A and B in comparison with predominantly
marula-type soils in treatment blocks E and F. Mopane trees (Colophospermun mopane)
predominate in treatment blocks A and B, whereas most marula trees (Sclerocarya birrea) in
the enclosure occur in treatment blocks E and F. This predominance of mopane trees, being
able to cope with heavier clay soils (Frazer et al., 1987) than marula trees (showing a
preference for well-drained soils: Shackleton et al., 2002), supports the interpretation that
heavier clay soils underpinning pronounced water deficits/surplus fluctuations (Murphy et al.,
2000), are prevalent in treatment blocks A and B. After a rain-free period grass growth can
resume with substantially less rainfall on sandy soils, but is estimated to require three to four
times as much rainfall on clayey soils (Kumar et al.,, 2002). NAD-ME type metabolic
performance in grasses of treatment blocks A and B was associated and taken to have been
induced by pronounced fluctuations of water availability as a result of mopane-type heavier
clay soils. NAD-ME type metabolism is geared to cope with such fluctuations (resourceresponsive growth) under generally low-rainfall conditions (Schulze et al., 1996) and
relatively low δ13C and high δ15N values (Table 2) congruently indicate NAD-ME metabolic
performance (Codron et al., 2008; Codron et al., 2009) to have been prevalent in fire
treatment blocks A and B (metabolic performance types A: NAD and B: NAD; Tables 2 and
3).
With a more efficient carbon economy, NADP species sustain high productivity
(growth efficiency: resource-level buffered growth capacity) under conditions of variable but
relatively high water availability (Schulze et al., 1996). δ13C values are positively correlated
with Rubisco: PEP carboxylation ratios and associated CO2 assimilation and shoot growth
rates and decline with increasing water stress (Buchmann et al., 1996; Saliendra et al., 1996).
NADP-ME type metabolism (high Rubisco: PEP carboxylation ratio activity), as indicated to
be prevalent in grasses of treatment blocks O, E and F (relatively high δ13C and low δ15N
values: Table 2), involves sustained growth efficiency over variable levels of water
availability (Schulze et al., 1996), involving high growth rates at high water availability
12
levels and sustained lower growth rates at relatively low water availability levels.
Accordingly, conditions of relatively high and/or seasonally extended water availability
support NADP-type productivity. The associated growth capacity is at a premium under
conditions of disturbance due to high fire frequencies and grazing (Fynn et al., 2011), as
applicable in treatment block O (Table 1). Sustained growth performance of NADP-type
metabolism is also taken to be favoured under conditions of seasonally extended soil
moisture availability as expected for marula-type soils, coupled with improved soil water
retention under grass swards at advanced post-fire ages (Van de Vijver et al., 1999b; Smit &
Asner, 2012) prevalent in treatment blocks E and F (metabolic performance types E: NADP
and F: NADP; Tables 2 and 3).
INTERSPECIFIC CONGRUENCE OF INTRA-SPECIFIC METABOLIC RESPONSES
The δ13C and δ15N indicators of NADP/NAD type metabolic performance were consistent for
all three species regarding different treatment blocks, irrespective of species-specific
carboxylation subtypes of particular species (Table 2: > and < indicate intra-specific
consistency with expected patterns congruent with metabolic performance types). Whereas
Panicum maximum as a species shows NAD-ME type functionality (relatively low δ13C and
high δ15N values: Table 2), andropogonoid species such as Themeda triandra are generally of
the NADP-ME subtype (relatively high δ13C and low δ15N values; Table 2: statistical
significance: 1 > 2 > 3: SP column). Indicative of intraspecific plasticity all three species
show consistent trends of responding towards either more NADP (δ13C↑; δ15N↓) or NAD
(δ13C↓; δ15N↑) functionality in line with prevailing resource conditions optimally requiring
either more NADP metabolic performance (treatment blocks O, E and F) or NAD metabolic
performance (treatment blocks A and B) (δ13C: P. maximum: AB -13.62 < OEF -13.47, P <
0.05; P. coloratum: AB -13.27 ≤ OEF -13.18, P = 0.210; T. triandra: AB -13.23 < OEF 13.07, P< 0.05; δ15N: P. maximum: AB + 0.947 > OEF -0.089, P < 0.002; P. coloratum: AB 0.315 ≥ OEF -0.829, P = 0.129; T. triandra: AB -0.673 ≈ OEF -0.765, ns). Functionality
according to these carboxylation types is apparently not genetically species-specifically fixed,
but adaptive shifts can partially be induced intra-specifically by environmental conditions
(Furbank, 2011).
METABOLIC PERFORMANCE TYPES AND THEIR RESPONSES SUBJECT TO DIVERGENT WATER
RESOURCE CONDTITIONS IN DRY AND WET YEARS
13
Wet season rainfall preceding the September 2007 late dry season sampling event was
notably lower than that preceding the August 2008 late dry season sampling event (Fig. 1).
Low-wet season rainfall conditions (Fig. 1: October 2006-March 2007) were reflected in
lower (more negative) δ13C values of C4 grass sample material collected in the following dry
season (2007: Table 2), denoting water-stressed reduced rates of metabolic functionality
(Buchmann et al., 1996) during the preceding low-rainfall wet season (Table 2). Curbed
growth functionality was also indicated by relatively higher carbon concentrations (Table 2);
associated with higher leaf nitrogen levels (Table 4) in dry season grass leaf material (2007)
originating from low-rainfall wet season growth (2006/2007) in comparison with values for
material originating from high-rainfall wet season growth (2007/2008; consistent with Jung et
al., 2014). Lower δ13C and higher C metabolic indicator values for sample event means
(Table 2: M column) thus indicated water-constrained low-rainfall wet season growth
conditions (w) during the dry year 2007 and relative water-surplus high-rainfall wet season
growth conditions (W) during the wet year 2008 (Table 3: wet season rainfall status). For
treatment blocks O and E in dry season samples (2007), originating from low-rainfall wet
season growth (resource-based functionality: w), we accordingly derive the O: LT-NADP-w
and E: Clt-NADP-w metabolic response modes (Table 3). Corresponding metabolic response
modes applicable for dry season samples (2008) originating from high-rainfall wet season
growth (resource-based functionality: W) for fire treatment blocks with metabolic
performance modes of O: LT and F: Clt (based on fire frequency effects) were designated as
O: LT-NADP-W and F: Clt-NADP-W respectively (Table 2: as indicated by higher δ13C and
lower C values for 2008 than 2007 grass material; column M).
NAD-ME metabolism is associated with drought tolerance and the capacity for
opportunistic utilization of short rain events (Schulze et al., 1996). Thus, NAD-ME type
metabolism (relatively low Rubisco: PEP carboxylation activity: δ13C ↓: Saliendra et al.,
1996) combines greater water stress tolerance (efficient photosynthesis capacity under water
stress, but at substantially reduced growth rates; photosynthesis/growth ratio ↑) with the
capacity for opportunistic use of water surplus conditions (accentuated TW growth
functionality at relatively high transpiration rates: δ15N ↑; growth/photosynthesis ratio ↑;
resource-level responsive growth: steep functionality-level to resource availability-level
response curve). This implies that growth activity under NAD-type performance is more
restricted over time to conditions of water surplus (high growth rates over short time periods),
whereas NADP-type performance involves growth efficiency also at functionality settings
14
with metabolic performance at relatively low light, temperature and water availability levels.
For treatment blocks A and B (NAD metabolic performance types) under conditions of wet
season water stress (resource-based functionality: w), the associated metabolic response
modes A: LT-NAD-w and B: Clt-NAD-w would exhibit high photosynthesis/growth
functionality during the dry year 2007 (Fig. 1) and when opportunistically making use of
relative water surplus conditions as expected during the wet year 2008 (resource-based
functionality: W), accentuated TW growth/high growth/photosynthesis functionality would
prevail (associated metabolic response modes: A: LT-NAD-W and B: Clt- NAD-W; Tables 2
and 3).
DRY SEASON FORAGE QUALITY PATTERNS AS SHAPED BY RAINFALL AND FIRE EFFECTS
Combining metabolic performance modes (LT, Clt) and types (NADP, NAD) with resourcelevel functional responses (w: low- or W: high-rainfall, wet season rainfall status) resulted in
the identification of metabolic response modes linked to specific treatment blocks (Tables 2,
3 and 4; dry year 2007 dry season samples: O: LT-NADP-w, A: LT-NAD-w↓, B: Clt-NADw↓, E: Clt-NADP-w; wet year 2008 dry season samples: O: LT-NADP-W, A: LT-NAD-W,
B: Clt-NAD-W, F: Clt-NADP-W). Patterns of association between metabolic response modes
(Tables 2 and 3), as shaped by divergent conditions of fire frequency effects and water
availability levels (rainfall), and forage quality indicators (Table 4) could now be used to
interpret effects of various fire frequency and rainfall scenarios on forage quality with
reference to underlying physiological mechanisms (as represented by metabolic response
modes).
Metabolic functionality under constraints of water availability (w growth conditions),
as experienced with low-rainfall wet season growth (2006/2007: Fig. 1) was associated with
higher nitrogen, dry matter digestibility, nitrogen quality (NQ) and forage quality index
values of both grass leaf and stem material in the following dry season (2007) than in dry
season material of 2008, originating from high-rainfall wet season growth (W growth
conditions; Table 4: M column: 1 > 2). An inverse pattern was revealed for total nonstructural carbohydrate contents (TNC), where higher dry season levels prevailed in grass
leaf material following high-rainfall wet season growth (Table 4). Generally however, the
results indicate that drier wet seasons result in higher forage quality, notably so also in
subsequent late dry seasons.
15
Divergent metabolic responses to resource conditions and their effects on forage
quality in treatment blocks with different metabolic response modes were revealed for the
late dry seasons of 2007 and 2008 (following low- and high-rainfall wet seasons
respectively). In respect of the late dry season in 2007, accentuated NAD-w type
functionality (restricted TW growth functionality: Table 3; relatively high
photosynthesis/growth functionality) in treatment blocks A (metabolic response mode LTNAD-w) and B (metabolic response mode Clt-NAD-w) was associated with relatively
elevated nitrogen and TNC, as well as nitrogen quality and forage quality index values (leaf
and stem material) in comparison with those encountered in treatment blocks O and E; with
NADP metabolic performance types (Table 4). In contrast, for the wet year 2008 late dry
season (Table 4), relatively high nitrogen, DMD, nitrogen quality and forage quality indices
were positively associated with CltW functionality (curbed TW growth functionality: Table
3) as in treatment blocks B (Clt-NAD-W) and F (Clt-NADP-W) subject to low fire frequency
scenarios. Consistently higher forage quality indicators (nitrogen, phosphorus, low fibre
levels) in treatment blocks B, E and F (Clt metabolic performance response modes) in
comparison with A and O (LT metabolic performance response modes) were also evident for
the early dry season of the wet year 2008 (Fig. 3).
As a general pattern, relatively enhanced dry season forage quality is associated with
conditions of restricted or curbed TW growth functionality (Table 3). Fire frequency effects
were absent in the dry year (2007), but had shaped dry season forage quality in the year with
high rainfall wet season growth (2008; Table 3). Thus, within the dry year 2007 dry season
forage quality patterns were determined by metabolic performance types (AB: NAD versus
OE: NADP), whereas within the wet year dry season forage quality depended on metabolic
performance modes linked to fire frequency effects (BF: Clt versus AO: LT). Across these
two years dry season forage quality was generally higher in the dry year (Table 4).
Discussion
GRASS METABOLIC RESPONSE MODES AND FORAGE QUALITY
Conditions of resource availability to grass plants, as shaped by fire frequency effects and
rainfall patterns, were associated with particular intra-specific metabolic response modes
which in turn determined the expression of TW growth activity and associated patterns of dry
season forage quality variation in space and time (Table 3). Considerable evidence exists that
16
growth at high availability levels of light (Wilson, 1982), temperature (Wilson, 1984; An et
al., 2005) and water (Wilson, 1984; Wilson, 1983; Van Soest et al., 1978) is associated with
reduced forage quality. This is attributed to high TW functionality which is associated with
increased content of structural to non-structural material, resulting in dilution of metabolites
(light: Deinum et al., 1996; temperature: An et al., 2005; water: Wilson, 1984) and generally
compromising dry matter digestibility of plant material (Van Soest et al., 1978). Growth
under conditions of water surplus and high temperatures (TW growth) is associated with
rapid tissue development and maturation, whereas curbed growth under Tw/tW conditions
delays tissue ageing and the rate of progressive decline in nitrogen content and dry matter
digestibility with ageing is reduced (Wilson, 1984); resulting in the maintenance of forage
quality into the dry season.
The results of this study revealed how fire effects (features of fire frequency),
interacting with conditions of water availability (rainfall/soil type), had induced divergent
grass metabolic response modes. Depending on their metabolic performance types (NADP,
NAD) and modes (LT, Clt) and associated metabolic response modes (dry year 2007: O: LTNADP-w, A: LT-NAD-w, B:Clt-NAD-w, E: Clt-NADP-w; wet year 2008: O: LT-NADP-W,
A: LT-NAD-W, B:Clt-NAD-W, F: Clt-NADP-w), plants responded differently to resource
conditions prevailing during seasonal or post-fire regrowth; accentuated or curbed TW wet
season growth activity generally being associated with low or high forage quality of grass
tissues respectively (Table 3). Wet season growth activity subject to water stress generally
resulted in enhanced forage quality in grass leaf and stem material in the dry season
following a low-rainfall wet season (regarding nitrogen concentrations, dry matter
digestibility and forage quality indices: Table 4). Low-rainfall wet season enhancement of
dry season forage quality was particularly pronounced when intra-specific NAD-ME type of
metabolic activity prevailed. The forage quality-reducing effect of accentuated TW metabolic
functionality in respect of dry season forage originating from high-rainfall wet season growth
was counteracted/neutralized, i.e. forage quality was partly sustained into the dry season in
grass swards at advanced post-fire ages (low fire frequency scenario: Table 4; Fig. 3).
Intra-specific metabolic performance modes and types reflect trade-off relationships
between the capacities for tolerance to resource deficits and high growth performance,
thereby determining the expression of TW growth metabolic functionality in response to
prevailing resource conditions; with higher forage quality being associated with curbed
growth functionality. These intra-specific metabolic response modes embody similar tradeoff relationships representing slow-growth strategies of grass species dominating in
17
undisturbed (low fire frequency conditions) and water-stressed grasslands contrasting those
species with fast-growth strategies dominant in highly productive habitat with some
disturbance (Fynn et al., 2011).
PREDICTING SPATIOTEMPORAL FORAGE QUALITY PATTERNS WITH
METABOLIC RESPONSE MODES
Metabolic response modes represent the physiological mechanisms explaining how fire
effects and rainfall patterns result in particular spatiotemporal patterns of variation in forage
quality as primarily shaped by prevalent expressions of wet-season growth activity (curbed
versus accentuated growth activity). They thereby thus facilitate the prediction and
explanation of forage quality patterns beyond the spatiotemporal context of this study. High
TW growth functionality, as associated with high-rainfall wet season growth, expectedly
results in reduced forage quality mainly through nitrogen/nutrient dilution. As shown in this
study, dry season forage quality, notably nitrogen quality (N x DMD) of grass leaf and stem
material from high-rainfall wet season growth was compromised (Table 4). Reduced forage
quality associated with high TW growth activity under conditions of particularly high rainfall
expectedly results in reduced forage intake and depressed population performance of
herbivores; consistent with findings of and possibly explaining the relatively lowered weight
gains reported for cattle (Fynn and O’Connor, 2000) and bison (Craine et al., 2009b) under
conditions of high TW grass growth functionality.
Disproportionately accentuated enhancement of dry season forage quality in dry years
is associated with NAD-type metabolic performance (metabolic response modes LT-NAD-w
and Cl-NADt-w: Tables 2 and 4) and the forage quality-reducing effect in wet years is
counteracted by growth-curbed metabolic activity associated with grass swards at advanced
post-fire ages (Clt performance: metabolic response modes Clt-NAD-W and Clt-NADP-W:
Tables 2 and 4). For grass swards during the early dry season higher forage quality (higher N
and P and lower fiber levels: Fig. 3) is clearly evident in treatment blocks BEF (Fig. 2)
subject to low fire frequencies (Table 1; Fig. 3). Thus patterns of rainfall and fire frequency
both play a prominent role in shaping dry season forage quality under conditions prevalent in
semi-arid savanna systems. Nutrient and energy intake of herbivores subsisting on senescent
dry season grass swards generally drops below maintenance levels (Owen-Smith, 2007),
requiring them to rely on body stores (Fynn, 2012). Maintenance of body stores for survival
and reproduction is facilitated by functional heterogeneity of forage resources which
18
generally involves the presence of areas providing for nutritious wet-season grazing and areas
with adequate quality forage during the dry season (Fynn, 2012). Enhanced forage quality in
particular areas during the dry season is usually linked to the presence of moisture permitting
some green grass growth activity, i.e. conditions promoted by relatively high rainfall (Fynn,
2012). In the absence of moisture availability promoting green grass growth, dry season
forage quality depends on the quality of senescent grass material. In this study it was found
that enhanced dry season forage quality (senescent grass material) may also arise due to
curbed wet season growth activity linked to low rainfall; thereby representing another
dimension of functional resource heterogeneity.
Investigated over a period of 60 years (1944-2003) in the Kruger National Park the
population performance of grazer species with limiting forage quantity requirements (buffalo,
Syncerus caffer; waterbuck, Kobus ellipsiprymnus) was positively correlated with rainfall
(Seydack et al., 2012b); i. e. population numbers declined in drought years (Owen-Smith and
Ogutu, 2003). Such drought effects were not detected in grazer species where forage quality
levels largely determine the availability of suitable forage; such as for the selective grazer
species sable (Hippotragus niger) and roan antelope (Hippotragus equinus), tsessebe
(Damaliscus lunatus lunatus) and eland (Taurotragus oryx), and in bulk grazer species with a
preference for short grass; blue wildebeest, Connochaetes taurinus and zebra, Equus
burchelli (Seydack et al., 2012a,b). Enhanced forage quality in dry years is here interpreted to
contribute substantially towards population performance of these species to be relatively
unaffected by drought years (Walker et al., 1987).
CLIMATE CHANGE EFFECTS ON FORAGE QUALITY POTENTIALLY MITIGATED
At the long-term multi-annual scale increasing ambient temperatures were identified as
promoting grass productivity, with negative implications for grass forage quality (An et al.,
2005). Analyses by Craine et al. (2010) showed a general pattern of increased temperature
being associated with decreased forage quality (lowered dietary crude protein and organic
matter digestibility). To the extent that fire and rainfall effects shape grass metabolic modes
of functionality, climate change impacts on forage quality are potentially modified
(mitigation or aggravation) by prevailing fire frequency or rainfall scenarios. Diverse fire and
rainfall patterns may accordingly be of significance in relation to climate-induced long-term
trends in population performance of grazers as implicated by Seydack et al. (2012b).
19
Temperature acclimation to progressively increasing atmospheric temperatures is
expected to accentuate TW growth functionality (An et al., 2005; Wan et al., 2005).
Experimental warming enhanced biomass production due to stimulatory effects of
temperature on plant growth of C4 species and consequently decreased nitrogen
concentrations in both green and senescent grass leaf tissues (An et al., 2005). As revealed in
this study, forage quality-reducing metabolic functionality at high/surplus availability levels
of temperature and water (high/accentuated TW functionality: Table 3), as in A and O (LT
metabolic response modes; high fire frequency scenario: wet year 2008) was constrained in
grass swards experiencing long fire intervals as in B and F (Clt-W metabolic functionality;
Tables 2 and 3). Thus, Clt-W metabolic functionality, as applicable to treatment blocks B, E
and F inside of the enclosure, would expectedly have neutralized or counteracted any
climatically driven accentuation of TW functionality of grasses; thereby sustaining dry
season forage quality (Table 4). Accordingly, long fire intervals are expected to create
conditions mitigating the negative effects of climate warming on forage quality implicated to
have resulted in the long-term decline of selective grazer population performance in the
Kruger National Park (Seydack et al., 2012b).
Conclusions
Plant physiological mechanisms linking forage quality patterns with fire and rainfall effects
were identified and characterized with reference to intra-specific metabolic response modes
representing expressions of TW growth functionality of grass plants adaptively responding to
resources for growth and survival. This was possible due to intra-specific plasticity of grass
plants responding to resource conditions subject to similar trade-off constraints as
manifesting in interspecific slow-fast growth strategies. Enhanced forage quality associated
with drier years was interpreted to play an important role in buffering drought effects on the
population performance of grazer species with relatively high forage quality requirements.
Interpretations of grazer population performance patterns based on the results of this study
furthermore suggest that the impact of reduced forage quality linked to progressive climate
warming (Seydack et al., 2012 a,b) and in wetter years in general (high/accentuated TW
functionality) is mitigated by grass metabolic performance modes (curbed TW functionality)
induced by grass sward crowding/age effects (Clt-W metabolic performance modes: Table 3).
These results therefore underpin the significance of long fire intervals for the enhanced
availability of high quality grass forage in semi-arid savanna systems by inducing modes of
20
metabolic functionality in grasses which benefit dry season forage quality of senescent grass
material. Such fire x climate forage quality effects thus have profound ecological
consequences and also implications for fire management in semi-arid savanna systems.
Acknowledgements
We acknowledge efforts of rangers, field staff and scientists involved in the recording of fire
data inside and outside of the N’washitshumbe enclosure and the effective management of
the enclosure over many decades, without which this study would not have been possible. For
field work collecting grass samples we acknowledge the contributions made by Diba
Rikhotso, Hylton Herd and Lizette Moolman, as well as of the two research assistants who
accompanied us: Jacob Mlangeni and Vilssone Binda. The South African Weather Service
provided temperature and rainfall data. The CAO data were collected and processed by
Gregory Asner, David Knapp, Ty Kennedy-Bowdoin, Roberta Martin and colleagues of the
Department of Global Ecology, Carnegie Institution for Science. The Carnegie Airborne
Observatory is made possible by the Gordon and Betty Moore Foundation, the John D. and
Catherine T. MacArthur Foundation, W. M. Keck Foundation, the Margaret A. Cargill
Foundation, Grantham Foundation for the Protection of the Environment, Mary Anne Nyburg
Baker and G. Leonard Baker Jr., and William R. Hearst III. This study represents an output of
the specialist scientist research program in systems ecology conducted by members of the
Conservation Services Division (Knysna and Skukuza) of South African National Parks. The
main author, in his capacity as honorary research associate at the Department of Biological
Sciences (University of Cape Town), thankfully acknowledges general support received from
this institution.
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25
Table 1 Fire frequency scenarios based on fire history (years of fires: 1978-2007) of
treatment blocks within (A, B, E, F) and outside (O) of the N’washitshumbe enclosure (Fig.
2; Kruger National Park, South Africa)
Fire
treatment
blocks
Year of fires and
associated annual
rainfall (Vlakteplaas)
Three fire frequency constellations (O, A, BEF) grouped into
two fire frequency scenarios (high and low)
(number of fires 1978-2007/1987-2007)
High:
O (10/7): High; A (6/4): Intermediate
Low:
B (2/0), E (4/1), F (4/1): Low
O
O
2007
693
2004
507
O
O
O
O
O
O
O
O
2002
1999
1997
1995
1988
1982
1979
1978
219
899
278
469
469
194
427
557
A
A
A
A
A
A
2002
1998
1990
1987
1985
1982
219
449
371
462
768
194
1985
768
1980
660
1999
899
1986
1984
1978
310
428
557
1988
469
1985
1981
1979
768
618
427
B
B
E
E
E
E
F
F
F
F
Post-fire regrowth
under mesic
conditions
High intensity fire
Hot clean burn
Post-fire sward age
(2007): 22 years
Post-fire sward age
(2007): 8 years
Post-fire sward age
(2008): 20 years
Annual rainfall indications > 450 mm are shaded (Weather recording locality at Vlakteplaas: 11 km southerly of
study site).
26
Table 2 Grass metabolic functionality indicators (carbon isotope, d13C and nitrogen isotope,
d15N values; carbon content: % DM) of grass leaf material for treatment blocks with
divergent post-fire ages and long-term fire frequencies for dry and wet season sampling
events (northern plains: Kruger National Park, South Africa)
SP
Dry season (Late dry season 2007)
(following a low-rainfall wet season )
Metabolic performance modes*
Metabolic performance types
Carbon isotope ratios (d13C)*
Panicum maximum
Panicum coloratum
Themeda triandra
Nitrogen isotope ratios (d15N)
Panicum maximum
Panicum coloratum
Themeda triandra
O
2
1
1
1
2
2
Carbon content (% DM)
Dry season (Late dry season 2008)
(following a high-rainfall wet season )
Metabolic response modes*
Metabolic performance types
Carbon isotope ratios (d13C)*
Panicum maximum
Panicum coloratum
Themeda triandra
Nitrogen isotope ratios (d15N)
Panicum maximum
Panicum coloratum
Themeda triandra
Carbon content (% DM)
1
2
3
LT
NADP
-13.13
-13.43
-13.02
-12.92
-0.53(b)
-0.05
-0.75
-0.81
X
*
Comparison of values over sampling
units (treatment blocks)
X
*
A
B
E
LT
NAD
-13.38
-13.57
-13.17
-13.38
-0.35a
0.82
-0.69
-1.18
Clt
NAD
-13.30
-13.59
-13.21
-13.09
-0.18a
0.33
-0.15
-0.74
39.4
39.5
40.0
39.7
O
A
B
F
LT
NADP
-12.81(a)
-12.70
-12.95
-12.78
-0.48(b)
-0.50
-0.55
-0.40
LT
NAD
-13.09(b)
-12.93
-13.06
-13.26
0.24(a)
1.79
-0.77
-0.30
Clt
NAD
-13.10(b)
-13.27
-12.94
-13.09
0.24(a)
0.84
0.35
-0.47
38.7ab
38.3(b)
39.0(a)
>
>
>
<
<
>
>
>
<
<
<
<
>
>
>
Clt
NADP
-13.12
-13.60
-12.86
-12.90
-1.16c
-0.40
-1.47
-1.60
Clt
NADP
-12.99(ab)
< -12.96
-12.92
-13.09
0.07(ab)
>
0.59
>
-0.56
-0.25
39.1a
M*
-13.232
-0.552
39.61
-13.001
0.021
38.82
* (d13C): highest d13C value of three replicate sample patches. * X: Indication of intra-specific conformity to
expected trend: d13C: NADP > NAD; d15N: NADP < NAD. Values in adjacent columns expectedly higher (>) or
lower (<). Multi-factor ANOVA: Statistically significance between treatment blocks (containing grass swards
with divergent post-fire ages: O vs A vs B vs E/F) within sampling events (dry season 2007, dry season 2008):
indicated by superscripts a, b and c for P < 0.05 or (a), (b) and (c) for P ˃ 0.05 and P < 0.10 (Duncan range test
comparisons). For means across sampling events (M* column): indicated by 1 > 2 at P < 0.05 and for species
means across species within sampling events: indicated by 1 > 2 > 3 at P < 0.05.* Metabolic response modes:
LTw/LTW (high productivity at relatively high levels of light, temperature and water availability; NADP-ME
type functionality), LT-NAD-w (efficient functionality under water stress; NAD-ME type functionality), LTNAD-W (high peak functionality under transient water surplus conditions; NAD-ME type functionality),
Cltw/CltW (Clt metabolic performance modes induced by longer-term grass sward crowding C/age effects, due
to long fire intervals; optimal functionality at relatively low resource level maxima; curbed growth).
27
Table 3 Associations between metabolic performance modes and types, metabolic response
modes and links with growth functionality and dry season forage quality (dry grass leaf
material) for treatment blocks subject to divergent fire frequency scenarios in respect of dry
year 2007 and wet year 2008.
Treatment
blocks
Fire
frequency
scenarios
Fig. 2
Table 1
Metabolic
performance
mode
Metabolic
performance
type
Table 2
Wet
season
rainfall
status
Fig. 1
Metabolic
response
modes a
TW growth
functionality
Table 2
Dry year 2007: October 2006 – September 2007: low-rainfall wet season preceding 2007 dry season
O
High
LT
NADP
Low
LT-NADP-w
Reduced
(w)
A
High
LT
NAD
Low
LT-NAD-w
Restricted
(w)
B
Low
Clt
NAD
Low
Clt-NADw
Restricted
(w)
E
Low
Clt
NADP
Low
Clt-NADP-w
Reduced
(w)
Wet year 2008: October 2007 – September 2008: high-rainfall wet season preceding 2008 dry season
O
High
LT
NADP
High
LT-NADP-W
High
(W)
A
High
LT
NAD
High
LT-NAD-W
Accentuated
(W)
B
Low
Clt
NAD
High
Clt-NAD-W
Curbed
(W)
F
Low
Clt
NADP
High
Clt-NADP-W
Curbed
(W)
a
Forage
quality
level b
Table 4
↑
↑↑
↑↑
↑
↓
↓
↑
↑↑
Metabolic response modes: LT-NADP-w sustaind productivity at relatively high levels of light and
temperatures; NADP-ME type functionality: high growth activity); LT-NAD-w efficient functionality under
water stress; LT-NAD-W: high peak functionality under transient water surplus conditions; Clt-W (Clt
metabolic performance mode induced by longer-term grass sward crowding C/age effects, due to long fire
intervals; optimal functionality at relatively low resource level maxima; curbed growth). Clt-NAD-w (efficient
functionality under water stress; NAD-ME type functionality), Clt-W (Clt metabolic performance due to grass
sward crowding effects curbing TW growth functionality). b Relative forage quality indication in respect of fire
treatment blocks (Table 4): ↑- relatively high and relatively low ↓; ↑↑- notably higher.
28
Table 4 Forage quality indicators and forage quality indices for grass leaf and stem
material, as differentiated for treatment blocks (areas with divergent post-fire ages of the
grass swards and long-term fire frequencies) over sampling events (2007 and 2008 dry
seasons); species pooled (northern plains: Kruger National Park, South Africa)
Values of grass leaf material for
Comparison of values over fire
composition (% air-dried mass), in
treatment blocks
vitro dry matter digestibility (%) and
(post-fire sward age in
forage quality indices
parentheses)
Forage quality indicator or index values for grass leaf material (unless indicated as for stem)
Late dry season (2007)
(following a low-rainfall wet season )
Plant metabolic response modes*
O
A
B
E
(3)
(5)
(22)
(8)
LTNADP-w
LTNAD-w
CltNAD-w
CltNADP-w
Nitrogen
Dry matter digestibility
Total non-structural carbohydrates
Nitrogen quality (NQ)
Forage Quality Index (FQI)
0.62b
44.4
6.52(b)
27.7b
176b
0.72a
44.0
7.35(a)
32.0(a)
227a
0.72a
45.4
7.23(a)
33.3a
233a
0.66ab
46.3
6.51(b)
30.9ab
196(b)
0.681
45.01
6.902
31.01
2081
Nitrogen quality (NQ): Stem
Forage Quality Index (FQI): Stem
17.4b
95b
19.4ab
133(ab)
23.0a
137(a)
17.5b
98c
19.31
1161
Late dry season (2008)
(following a high-rainfall wet season )
Plant metabolic response modes*
O
A
B
M*
F
(1)
(6)
(23)
(20)
LTNADP-W
LTNAD-W
CltNAD-w
CltNADP-W
Nitrogen
Dry matter digestibility
Total non-structural carbohydrates
Nitrogen quality (NQ)
Forage Quality Index (FQI)
0.44c
29.0b
8.39a
12.8c
105b
0.52b
24.4b(c)
7.95ab
13.2(c)
101b
0.58(b)
28.7b
8.00ab
16.8b
128b
0.65a
36.7a
7.43b
24.4a
173a
0.552
29.72
7.941
16.82
1272
Nitrogen quality (NQ): Stem
Forage Quality Index (FQI): Stem
10.3ab
68
7.4(b)
48
10.0ab
61
12.3(a)
76
10.02
632
Multi-factor ANOVA: fire treatment blocks (O vs A vs B vs E/F) x species (3) with 3 replications (n = 4 x 3 x 3
= 36). Statistically significance between fire treatment blocks within sampling events (late dry season 2007, late
dry season 2008): indicated by superscripts a, b and c for P < 0.05 or (a), (b) and (c) for P ˃ 0.05 and P < 0.10
(Duncan range test comparisons). M* column: sampling event means across sampling events indicated by 1 > 2
at P < 0.05. NQ: nitrogen quality = nitrogen x dry matter digestibility; FQI: forage quality index = nitrogen x
dry matter digestibility x total non-structural carbohydrate concentrations. a Metabolic response modes: LT:
high productivity at relatively high levels of light and temperatures; NADP-ME type functionality: high growth
activity); LT-NAD-w/Clt-NAD-w efficient functionality under water stress; LT-NAD-W: high peak
functionality under transient water surplus conditions; Clt-W (Clt metabolic performance modes induced by
longer-term grass sward crowding C/age effects, due to long fire intervals; optimal functionality at relatively
low resource level maxima; curbed growth).
29
Monthly rainfall (mm)
300
Dry year 2007
Wet year 2008
250
200
150
100
50
0
J A S OND
2006
J F M AM J J A S O N D
J F M AM J J A S O N D
2007
2008
J F MAM J
2009
Fig. 1. Monthly rainfall at Vlakteplaas, Kruger National Park, South Africa (sampling
events: September 2007, August 2008). Dry year 2007: October 2006 – September 2007:
low-rainfall wet season preceding 2007 dry season; Wet year 2008: October 2007 –
September 2008: high-rainfall wet season preceding 2008 dry season.
30
C
6ha
O (outside)
N
A
63ha
D
B
22ha
65ha
H
E
13ha
F
63ha
62ha
G
6ha
Fig. 2. Layout of the N’washitshumbe enclosure (northern plains, Kruger National Park)
indicating the positions of treatment blocks in which sampling took place: A, B, E, F and O
(outside of the enclosure). Northern basalt plains, Kruger National Park (South Africa).
31
a)
b)
32
c)
Fig. 3. Early dry season (1 May 2008) nutrient maps generated for the area covering
treatment blocks (Fig. 2) O and A (high fire frequency scenarios) and B, E and F (low fire
frequency scenarios) depicting the spatial variation in (a) nitrogen, (b) phosphorus and (c)
fibre percentage contents of the grass sward. The colour scale bar represents the % DM of the
respective nutrients (Knox et al. 2011). Nitrogen (%DM means for O: 0.38, A: 0.41, B: 0.56,
E: 0.56, F: 0.55): BEF > OA, P < 0.01; Phosphorus (%DM means for O: 0.13, A: 0.16, B:
0.25, E: 0.25, F: 0.23): BEF > OA, P < 0.01; Fibre (%DM means for O: 46.7, A: 46.0, B:
44.0, E: 44.0, F: 43.5): BEF < AO, P < 0.01.
33
Rainfall effects on dry season grass forage quality for
grazing herbivores in an African semi-arid savanna
system
Running head: Rainfall effects shaping dry season forage quality
Armin H.W. Seydack1,2* , Wessel J. Vermeulen1, Nicky Knox3, Navashni Govender4,
Izak P.J. Smit4, Rina C. Grant4 and Sandra MacFadyen4
1
Conservation Services Division
South African National Parks
P.O. Box 3542, 6570 Knysna, South Africa
Telephone: +27 (0)44 302 5612, Fax +27 (0)44 384 0136
E-mail: [email protected]
2
Honorary Research Associate, Department of Biological Sciences, University of Cape
Town, South Africa
3
Research and Development Group
South African National Space Agency (SANSA) – Earth Observation Division
P.O. Box 484, Silverton, 0127 Pretoria, South Africa
4
Scientific Services, Kruger National Park
South African National Parks
Private Bag x 402, 1350 Skukuza, South Africa
34
Summary
1. This study represents an exploration of how rainfall shapes dry season grass forage
quality for larger grazing herbivores in a semi-arid savanna system (Kruger National
Park, South Africa).
2. Forage quality indicators and indices, based on nitrogen, non-structural carbohydrates
and digestibility of leaf and stem material were determined for three perennial C4 grass
species from grass swards subject to divergent wet season rainfall over two years. Low
wet season rainfall (260 mm) preceding the dry season of 2007 and high wet season
rainfall (463 mm) preceding the dry season of 2008 were recorded. Divergence of rain
fall effects between these two years (2007: water deficits; 2008: water surplus) had been
corroborated by higher δ13C and δ15N values of C4 grass material from the wetter year
(2008) than of the drier year (2007), underscoring the metabolic significance of the
difference in rainfall between the two years.
3. Wet season growth activity subject to water stress resulted in enhanced forage quality
in grass leaf and stem material in the dry season following a low-rainfall wet season
(regarding nitrogen concentrations and dry matter digestibility). Conversely, grass
metabolic functionality responding to conditions of accentuated temperature and water
availability encountered in years of high wet season rainfall preceding the dry season
was linked to reduced (dry season) forage quality.
4. Sites in arid areas subject to pronounced fluctuations in water availability to plants
(clayey soils) were conducive to intra-specific NAD-ME metabolic activity involving
restricted and accentuated growth activity in dry and wet years respectively. Particularly
high forage quality of grass material from such sites in the dry year was revealed
(relatively high nitrogen and TNC values being associated with restricted NAD growth
activity).
5. The presence of grass swards of relatively enhanced/elevated forage quality during the
dry season is in agreement with population performance of certain grazer antelope
species being relatively unaffected by drought periods, notable in respect of selective
grazer antelope species.
Key-words: grass forage quality, Kruger National Park, large herbivores, semi-arid
savannah, rainfall effects
35
Introduction
Rainfall supports grass production (McNaughton et al., 1989; Fritz and Duncan, 1994);
thereby affecting grazer population performance; notably in semi-arid savannas. Forage
quality is generally inversely related to grass plant productivity, as linked to rainfall
(McNaughton et al., 1989) and manifesting in grass percentage cover or biomass (Mutanga et
al., 2004). Spatial variation in forage quality affects the spatial distribution of grazing
herbivores (McNaughton, 1988; Heitkönig and Owen-Smith, 1998). In this study variation in
grass forage quality, as prominently shaped rainfall in a semi-arid savanna system (Kruger
National Park, South Africa), was investigated. Temperature and water availability levels
effectively determine plant metabolism and the nutritional quality of plant tissues (Wilson,
1984). These levels vary spatiotemporally at multiple scales.
Comparatively little is known about the underlying factors determining the
spatiotemporal variability of dry season forage quality (senescent grass material). The amount
and timing of rainfall determines the availability of green grass within the dry season; thereby
providing relatively nutritious forage in the dry season. For the most part however only
senescent grass material is available in the dry season in semi-arid savanna systems. This
study is focusing on the effects of wet season rainfall on the forage quality in subsequent dry
seasons. A greater understanding in this regard is of particular importance since nutritional
short falls of essential nutrients are generally experienced by savanna grazers during the dry
season (Illius and O’Connor, 2000; Fynn, 2012).
The objective of this study was to reveal how rainfall (inter-year variations in
rainfall), interacting with fire effects, would shape grass forage quality for large grazing
herbivores in a semi-arid savanna system. Some implications for fire management are
anticipated to follow from the findings of this study.
Methods
STUDY AREA
Representing a large semi-arid savanna system (approximately 2 000 000 ha) the Kruger
National Park (KNP) is situated in north-eastern South Africa (between latitudes 22º 20′ and
25º 32′ and longitudes 30º 53′ and 32º 02′). The KNP (covering an altitudinal range of
between 250 and 850 m a.s.l.) falls within two disparate climate zones as defined by the
36
South African Weather Service (Venter et al., 2003). Rainfall is strongly seasonal, falling
mainly during the austral summer (October-March). The area north of the Olifants River is in
the northern arid bushveld zone, receiving 450-650 mm rain per year, whilst the southern part
of the park falls into the lowveld bushveld zone, with average annual rainfall generally
between 500-800 mm per year.
The vegetation of the northern KNP is characteristically dominated by mopane
(Colophospermum mopane) and, following the vegetation classification by Mucina and
Rutherford (2006), this tree species is well represented in broad-leaved bushveld vegetation
types on granites (Tsende Mopaneveld; NW-KNP: 450-650 mm rainfall per year) and broadleaved shrubveld associated with basalts (Mopane Basalt Shrubland; NE-KNP: 400-500 mm
rain per year). Characteristic vegetation units on granites of the southwestern KNP include
Granite Lowveld and Pretoriuskop Sour Bushveld (a large-leafed deciduous woodland with a
tall dense herbaceous layer dominated by sour grasses; 550-800 mm rain per year).
Sclerocarya birrea/Acacia nigrescens Savanna, an open tree savanna with a dense
herbaceous layer, predominates on basalts of southeastern KNP (Gertenbach, 1983; 500-600
rain per year). The herbaceous layer of the KNP is dominated by C4 grass species and the
more nutrient-rich savanna vegetation types on clay soils carry dense stands of nutritious,
high-bulk grasses (Venter et al., 2003).
The study area involved sampling areas in and around the N’waswhitshumbe
enclosure (304 ha) on the northern basalt plains (Fig. 1; 382 m a.s.l.; mean annual rainfall,
1984-2012: 486 mm). This selective grazer breeding camp, mainly for roan antelope
(Hippotragus equinus), excluded other large herbivores and predators. It was established in
1967 and extended in 1984 by 48 ha to include a vlei grassland. The enclosure occurs within
the Mopane Basalt Shrubland (Mucina and Rutherford, 2006; Gertenbach, 1983);
characterized by freely spaced mopane shrubs and a dense herbaceous layer, resulting in an
open tree and shrub savanna.
STUDY APPROACH
We investigated associations between rainfall effects and grass forage quality. Intra-specific
metabolic performance modes of grasses responding to conditions light, temperature and
water availability levels, as shaped by fire and rainfall effects, had been established (Seydack
et al., 2014). Such responses are known to affect tissue concentrations of selected forage
quality indicators (N: nitrogen, as proxy for protein content, TNC: total non-structural
37
carbohydrates and DMD: dry matter digestibility). Relatively high forage quality indices, that
is, combinations of selected forage quality indicators are furthermore of importance for
optimal herbivore nutrition. A nitrogen quality index (NQ = N x DMD) and a forage quality
index (FQI = N x DMD x TNC) were defined to capture the co-occurrence in forage of
forage quality indicators. Fire and conditions of climate (temperature and water) were thus
considered to have interactively induced particular metabolic performance modes in grass
plants which, as they differentially respond to resource conditions, in turn would determine
grass quality indicator and index values of grass leaf and stem material. Metabolic response
modes of grasses induced by resource conditions of water and temperature availability as
shaped by fire frequency and wet season rainfall effects had been identified; providing the
causative mechanism of these effects in determining forage quality patterns (Seydack et al.,
2014) and the basis for the interpretation of spatiotemporal forage quality patterns
manifesting in treatment blocks over years with high and low rainfall respectively (refer to
Grass metabolic response modes and forage quality: Seydack et al. 2014a below).
SAMPLING LAYOUT
During the dry seasons of 2007 (September) and 2008 (August) senescent leaf and stem
material and during 2009 (February: late wet season; green leaf material only) of three
perennial C4 grass species Panicum maximum, Panicum coloratum and Themeda triandra
were collected in and beyond the roan enclosure (N’washitshumbe, northern KNP: Fig. 1)
The apical 20 cm of grass stems (including any attached flower heads) were collected as
samples of stem material. Multiple plants of each species were collected at each replication
locality and bulked for analyses. Three sampling localities (replications) were involved
regarding each treatment block (2007: O, A, B and E; 2008/2009: O, A, B and F);
representing a total of 12 samples per species and sampling event. Forage quality indicators
(percentage concentrations of nitrogen, total non-structural carbohydrates and in vitro dry
matter digestibility) were determined for the sampled grass material.
Two fire frequency scenarios had been identified (Seydack et al., 2014): treatment
blocks O (1987-2007: 7 fires; sward post-fire age in 2007: 3 years) and A (1987-2007: 4
fires; sward post-fire age in 2007: 5 years) represented high fire frequency scenarios.
Treatment blocks B, E and F, representing low fire frequency scenarios, were subject to
relatively low fire frequencies and these treatment blocks were at advanced post-fire ages
when sampled (B 1987-2007: 0 fires; sward post-fire age 2007: 22 years; E 1987- 2007: 1
38
fire; sward post-fire age 2007: 8 years and F 1987-2007: 1 fire; post-fire age 2008: 20 years).
In this study data from the late dry season sampling event in September 2007 (dry year 2007:
October 2006-September 2007: low rainfall wet season preceding 2007 dry season) and in
August 2008 (wet year 2008: October 2007-September 2008: high rainfall wet season
preceding 2008 dry season) were involved. Rainfall for the wet season months November,
December and January at Vlakteplaas (north-eastern KNP) of the dry 2007 and wet 2008
years had been 260 and 463 mm over the three months respectively. Significantly higher δ13C
and δ15N values, and lower carbon contents (% DM) of the grass material from the wetter
year (2008) than of the drier year (2007) underscored the metabolic significance of the
difference in rainfall between the two years (Seydack et al., 2014; Buchmann et al., 1996).
LABORATORY ANALYSES
The collected grass material was dried at 55 °C overnight and mill ground through a 1-mm
sieve to form homogeneous powdered samples. Concentrations of ash minerals and total nonstructural carbohydrates (TNC) of the powdered sample material were determined according
to standard procedures (Agricultural Research Council, ARC: Irene). In vitro dry matter
digestibility (DMD) was determined following Tilley and Terry (1963) and Engels and Van
der Merwe (1967). Carbon (δ13C) and nitrogen (δ15N) isotope values were determined
according to international standards (Stable Light Isotope Unit, Department of Archaeology,
University of Cape Town). From these analyses percent nitrogen and percent carbon (by
weight) were also provided for each sample. More details on methods performed are provided
in Seydack et al., (2014).
STATISTICAL ANALYSIS
The spatial association of increased grazing intensities with higher fire frequencies and lower
grazing intensities with lower fire frequencies (Zimmermann et al., 2010) also applies in the
absence of enclosure (Smith et al., 2013; Trollope et al., 2014). In the context of this study,
any grazing effects associated with high fire frequencies outside of the enclosure were
therefore considered an integral part of high fire frequency effects. Since the sampling layout
did not represent conditions of repeated measurements and treatment block effects were
different in each year (responding to rainfall effects in the dry year 2007 and fire frequency
39
effects in the wet year 2008), legacy effects were deemed insubstantial and repeated measures
ANOVA over the two years was thus not considered to be applicable. Treatment block
effects in respect of the response variables (forage quality indicators: N, DMD, TNC and
indices: NQ, FQI) were accordingly analysed with multi-factor ANOVA regarding treatment
blocks O, A, B and E for the dry year 2007 (4 treatment blocks x 3 species with 3 replications
within treatment blocks: n = 4 x 3 x 3 = 36). Two fire frequency scenarios were involved: a
high fire frequency scenario in A and O and a low fire frequency scenario in B, E and F
(Table 1; Fig. 1) and two groupings of water resource responsive, site-based metabolic
performance types (O and E: NADP; A and B: NAD; Table 1). Each of the two fire
frequency scenarios was accordingly represented by two treatment replicates within each
sampling year (2007: AO-BE; 2008: AO-BF) and correspondingly also two treatment
replicates in respect of NADP/NAD performance types (2007: OE-AB; 2008: OF-AB).
Grass metabolic response modes and forage quality (Seydack et al., 2014)
Intra-specific grass metabolic response modes had been identified (Seydack et al., 2014) as
induced by fire effects (metabolic performance modes), coping with water resource
availability levels and fluctuations (NADP-NAD metabolic performance types) and
responding to water availability during wet season growth (differentially for low- and highrainfall years).
Intra-specific NAD-ME type metabolism in treatment blocks A and B was identified
by low δ13C and high δ15N values relative to treatment blocks O, E and F with higher δ13C
and lower δ15N values denoting NADP-ME type metabolism (Seydack et al., 2014 and
supporting references therein). Pronounced fluctuations of water availability were indicated
to prevail in treatment blocks A and B as a result of mopane-type heavier clay soils. After a
rain-free period grass growth can resume with substantially less rainfall on sandy soils, but is
estimated to require three to four times as much rainfall on clayey soils (Kumar et al., 2002).
NAD-ME type metabolism is geared to cope with such fluctuations under generally lowrainfall conditions as it is characterized by drought tolerance and the capacity for
opportunistic utilization of short rain events (Schulze et al. 1996); thus combining greater
water stress tolerance (enhanced w functionality: photosynthesis/growth ratio ↑) with the
capacity for opportunistic use of water surplus conditions (accentuated W functionality at
relatively high transpiration rates: growth/photosynthesis ratio ↑). Therefore, under
conditions of wet season water stress, the associated metabolic response modes for treatment
40
blocks A and B (NAD metabolic performance types) would exhibit A: LT-NAD-w and B:
Clt-NAD-w functionality (accentuated w functionality: as expected during the low-rainfall
wet season preceding the 2007 dry season) and when opportunistically making use of relative
water surplus conditions (accentuated W functionality: as expected during the high-rainfall
wet season preceding the 2008 dry season; A: LT-NAD-W and B: Clt-NAD-W functionality
would prevail). NADP-ME type metabolism, with a more efficient carbon economy (Schulze
et al., 1996: high Rubisco: PEP carboxylation ratio activity), as indicated to be prevalent in
grasses of treatment blocks O, E and F (relatively high δ13C and low δ15N values: (Seydack et
al., 2014), involves sustained growth efficiency over time with the metabolic capacity for
high growth productivity at high levels of water availability, but also some capacity for
sustained growth with functionality settings at relatively low resource availability levels of
light, temperature and water (resource-level buffered growth capacity: flat functionality-level
to resource availability-level response curve). Conversely, NAD-ME type metabolism
combines the capacities of drought tolerance (conditions of water deficits) and opportunistic
utilization of short events of water surplus (resource-level responsive growth: steep
functionality-level to resource availability-level response curve).
With increasing post-fire age and associated increasing grass plant and sward bulk
densities, crowding effects increasingly shape the conditions under which seasonal grass
regrowth has to grow and survive. Due to interplant crowding with increasing post-fire sward
age (Zimmermann et al., 2010), necromass shading (Scholes and Walker, 1993) and
associated microclimatic conditions, resource-level surplus conditions are seldom
encountered by individual grass plants, resulting in intra-specific adjustments of metabolic
capacity to sustain metabolic activity at moderate to relatively low resource availability levels
in terms of light and temperature (Seydack et al., 2014). In congruence, grass production
under shade was relatively reduced in the wet season, but continued for longer into the dry
season (Ludwig et al., 2001a). This sustained productivity was attributed to higher water
availability due to decreased evaporation and increased water use efficiency as a result of
lower plant transpiration (Ludwig et al., 2001b).
Derived applicable metabolic performance modes for B, E and F, treatment blocks
subject to low fire frequency scenarios, were accordingly designated as BEF: Clt, as induced
by prolonged crowding effects in grass swards at advanced post-fire ages (C for crowding:
constrained resource availability per plant; lt reflecting reduced maximum functionality rates
in response to light and temperature levels in comparison with those in treatment blocks
subject to high fire frequency scenarios O: LT and A: LT). Combining metabolic
41
performance modes (LT, Clt) and types (NADP, NAD) with responses to water availability
status (low- or high-rainfall wet season growth conditions) resulted in the identification of
metabolic response modes linked to specific treatment blocks; differentiated for years of low
and high wet season rainfall preceding the respective dry seasons. Metabolic response modes
for treatment blocks with NADP-type metabolic performance were accordingly designated as
O: LT-NADP-w and E: Clt-NADP-w for the dry year 2007 and O: LT-NADP-W and F: CltNADP-W for the wet year 2008. Corresponding metabolic response modes for treatment
blocks with NAD-type performance were designated as A: LT-NAD-w/B: Clt-NAD-w and
A: LT-NAD-W/B: Clt-NAD-W for the dry and wet years respectively (NAD: denoting
resource-level responsive metabolism: w: restricted growth and W: accentuated growth).
High TW metabolic functionality (growth under conditions of water surplus and high
temperatures) is associated with rapid tissue development and maturation and increased
content of structural to non-structural material, resulting in dilution of metabolites (Wilson,
1983; Wilson, 1984; An et al., 2005; Dwyer et al., 2007) and generally compromising dry
matter digestibility of plant material (Van Soest et al., 1978). Curbed growth under Tw/tW
conditions delays tissue ageing and the rate of progressive decline in nitrogen content and
dry matter digestibility with ageing is reduced (Wilson, 1984); sustaining forage quality into
the dry season. Thus, as a general pattern, elevated TW growth functionality is associated
with compromised forage quality. Metabolic response modes, as linked to treatment blocks,
incorporate the effects of fire frequency scenarios and water status responses in determining
TW growth functionality, thereby provide the mechanistic link between these effects and
forage quality (Seydack et al., 2014). Based on these considerations three settings of
enhanced forage quality were identified: dry-year enhanced forage quality setting NADP-w
(relative to wet year); dry-year elevated forage quality setting NAD-w (within dry year and
relative to wet year) and low fire frequency forage quality setting Clt-W (relatively increased
forage quality within wet year; Table 1).
Results
DRY SEASON FORAGE QUALITY PATTERNS IN RESPONSE TO PRECEDING LOW- OR HIGHRAINFALL WET SEASONS
Metabolic functionality under constraints of water availability, as experienced during the dry
year 2007 was reflected in lower δ13C values (reduced/restricted TW growth functionality;
42
Table 1) and associated with higher nitrogen, dry matter digestibility, nitrogen quality (NQ =
N x DMD) and forage quality index (FQI) values (Table 2) than those of the wet year for
both grass leaf and stem material (consistent over all three grass species: Fig. 2). Notably
higher dry matter digestibility values were evident for late dry season leaf and stem material
originating from low-rainfall wet season growth (Table 2); largely unaffected by divergence
in metabolic performance types (Table 3). An inverse pattern was revealed for TNC, where
higher dry season levels prevailed in grass leaf and stem material following high-rainfall wet
season growth (Table 2). Generally however, the results indicate that higher dry season
forage quality is encountered during dry years; notably also in respect of late dry seasons.
DIFFERENTIATED DRY SEASON FORAGE QUALITY ENHANCEMENT ACCORDING TO
DIVERGENT INTRA-SPECIFIC NAD/NADP METABOLIC PERFORMANCE TYPES
For the dry season of the dry year two differentiated forage quality settings of enhanced
forage quality had been identified (Seydack et al., 2014). Accentuated NAD-type w functionality
(Table 1; dry-year elevated forage quality setting NAD-w) in treatment blocks A and B
(Seydack et al., 2014; A: LT-NAD-w and B: Clt-NAD-w metabolic response modes) was
associated with relatively elevated nitrogen and TNC levels (Table 3: leaf and stem material).
This combination of relatively high N and TNC levels is reflected in relatively elevated
forage quality index values of the forage quality setting NAD-w (Table 4: for both leaf and
stem material) in comparison with those encountered in treatment blocks O and E,
representing forage quality settings NADP-w (dry-year enhanced forage quality setting
NADP-w). These elevated late dry season forage quality levels of the forage quality setting
NAD-w (associated with sward patches/areas where NAD-w type of intraspecific metabolic
performance prevails; Table 1) can be expected under site conditions with pronounced
fluctuations of water availability (heavy clay soils) in generally arid areas (Seydack et al.,
2014).
Thus, as a general pattern, enhanced forage quality is expected and associated with
conditions of depressed/curbed TW growth activity, as encountered during dry years
(reflected in forage quality indicators of the 2007 dry season following a low-rainfall wet
season) and in wetter years prevalent in grass swards subject to low fire frequencies (Table 1:
BF: Clt metabolic mode functionality in 2008 late dry season samples originating from highrainfall wet season growth of grass swards subject to low fire frequencies: Seydack et al.,
2014).
43
SPECIES-SPECIFIC PATTERNS IN FORAGE QUALITY
Contrasting patterns in forage quality between Panicum maximum, a generally highly
favoured forage species for many bulk and selective grazer species and Themeda triandra,
representative of andropogonoid grass species which are of notable importance for selective
grazers (Seydack et al., 2012a,b and references therein), were evident. Leaf material of P.
maximum had consistently higher nitrogen, dry matter digestibility and nitrogen quality
values (Tables 3 and 4: SP column: 1 > 2/3; Fig. 2a,b) than T. triandra, which had higher
TNC (total non-structural carbohydrate) contents (Fig. 2c). Leaf late dry season nitrogen
sample patch maxima of P. maximum reached 0.8-1.0 % DM in the dry year 2007 (Fig. 2a);
whereas T. triandra showed relatively high TNC contents of around 10 % during the dry
season of the wet year (Fig. 2c). P. maximum had higher leaf dry season nitrogen quality
(NQ = N x DMD) and forage quality index values (FQI = N x DMD x TNC) than T. triandra
in the dry (Fig. 2d,e; Table 4: SP column: 1 > 2) but not significantly so in the wet year (data
not shown). An opposite trend was revealed for stem material, with T. triandra tending to
have higher NQ and FQI values than P. maximum, notably so for dry season stem material of
the drier year (Table 4: SP column: 1 > 2). These results indicate the occurrence of enhanced
forage quality of dry grass leaf and stem material during dry years; particularly so in respect
of P. maximum leaf material and of T. triandra stem material (T. triandra representing
andropogonoid grass species).
Discussion
FORAGE QUALITY SETTINGS OF ENHANCED DRY SEASON FORAGE QUALITY IN RELATION TO
RAINFALL AND FIRE EFFECTS
Five settings of relatively enhanced forage quality can be identified. Two of these involve
green grass: wet season post-fire regrowth (Van de Vijver et al., 1999) and post-fire/rainfalllinked dry season regrowth (Owen-Smith and Ogutu, 2003; Parrini and Owen-Smith, 2010;
Seydack et al., 2012b). For dry (senescent) grass, three forage quality settings of enhanced
dry season forage quality were identified (Table 1): dry-year enhanced forage quality
(NADP-w); dry-year elevated forage quality (NAD-w) and sustained/enhanced forage quality
in wetter years of grass swards under low fire frequency scenarios (Clt-W). Overall, dry
44
season forage quality (nitrogen, dry matter digestibility, nitrogen quality and forage quality
indices) of brown/senescent grass material was higher in grass swards which had been subject
to a preceding low-rainfall wet season (Table 2). Low-wet season rainfall enhancement of
forage quality was particularly pronounced when intra-specific NAD-ME type of metabolic
activity prevailed (Table 1: forage quality setting NAD-w). These results are consistent with
forage quality remaining higher during the entire growth period under conditions of low
water availability (Kumar et al. 2002).
Dry-year enhanced (forage quality setting NADP-w) and dry-year elevated (forage
quality setting NAD-w) dry season forage quality both involved relatively increased NQ (N x
DMD) and FQI (N x DMD X TNC) values in relation to wet year levels for both leaf and
stem material (Tables 3 and 4). Comparatively elevated and concurrently higher leaf N and
TNC levels and hence higher FQI values for the NAD-w than the NADP-w forage quality
setting were evident (Table 3: leaf N and TNC; Table 4: leaf and stem FQI). Late dry season
leaf nitrogen concentrations of Panicum maximum in the dry year 2007 (forage quality setting
NAD-w) and for grass swards subject to low fire frequencies during the wet year (forage
quality setting Clt-W) approached/reached maintenance levels for ruminants (Table 3; Fig.
2a); calculated as 1% on a dry matter basis according to Prins and Beekman (1989) and
exceeded the recommended minimum nitrogen concentrations in wild herbivore diets of 0.8
% (according to Schmidt and Snyman 2002).
Greater prevalence of the dry-year elevated forage quality setting (NAD-w) is
expected in relatively arid areas with well-defined short wet seasons (northern Kruger
National Park) and locally where edaphic conditions promote pronounced fluctuations in
water availability (such as due to clay soils; Seydack et al., 2014); whereas the dry-year
enhanced forage quality setting (NADP-w) would be expected to occur more so in relatively
mesic areas with seasonally more extended wet seasons (southern/ southwestern Kruger
National Park). According to this study, conditions prevalent in grass swards at advanced
post-fire age and low-rainfall wet season growth in general resulted in growth-curbed
metabolic functionality during the growing season, resulting in enhanced forage quality,
notably of brown/senescent grass material, during subsequent dry seasons. The associated
settings of enhanced /elevated/sustained dry season forage quality (Table 1) represent further
dimensions of functional resource heterogeneity (Fynn 2012), with profound implications for
the population performance of grazing herbivores in semi-arid savanna systems.
45
DRY SEASON FORAGE QUALITY SETTINGS AND PATTERNS OF GRAZING BY SELECTIVE
GRAZERS
Selectively grazing antelope species (sable antelope, roan antelope, tsessebe, eland) require
dry season diets with reasonably high forage quality indices (FQI: N x DMD x TNC) in order
to maintain body condition (Seydack et al., 2012 a,b). Whereas energy (inter alia derived
from dietary TNC) is considered a nutritional currency for survival, protein and minerals are
important for reproduction (Parker et al., 2009).
Panicum maximum as a good source of nitrogen and andropogonoid grass species with their
relatively high TNC contents thus play an important role in achieving relatively high N-TNC
diets (high FQI: forage quality index values), notably in the dry season (Seydack et al.,
2012a).
Total non-structural carbohydrates (TNC) contents of plant tissues are a product of
metabolic functionality at relatively high photosynthesis to growth ratios (storage allocation;
Estiarte and Peñuelas, 1999) and increased levels are expectedly encountered under
conditions of curbed growth functionality, notably resulting in concurrently high TNC and N
concentrations in dry season grass material in dry years (especially so in dry-year elevated
NAD-w forage quality settings). Faecal nitrogen levels for sable (Macandza et al., 2013) and
roan antelope (Knoop and Owen-Smith, 2006) in the more arid northern KNP (greater
availability of NAD-w settings) were higher in the dry season of the drier year; consistent
with higher N (Table 2; Table 3: NAD > NADP), NQ and FQI values (Table 2: Table 4:
NAD > NADP) of both leaf and stem material during the drier year (2007) as recorded in this
study. Dry season faecal nitrogen levels for sable during the drier year were also higher than
those recorded in the more mesic SW-KNP (Macandza et al., 2013); which may be explained
by the lower availability of NAD-w forage quality settings in more mesic areas (2007: Tables
3: leaf N: NAD > NADP ; Table 4; leaf and stem FQI: NAD > NADP). In contrast, as a result
of higher retention of greenness in SW-KNP with its higher rainfall than in NW-KNP, higher
faecal nitrogen levels were encountered there during the wetter year (Macandza et al., 2013).
Sable antelope in SW-KNP increased the contribution of Hyperthelia dissoluta
(Andropogoneae) to their diet and seemed to favour grasses with many stems during the late
dry season of the drier year (Le Roux, 2010). These foraging patterns can be explained with
reference to the results of this study: higher NQ and FQI values for both leaf and stem
material were encountered during the dry year (2007, Table 2; Table 4: SP column: 1 > 2);
disproportionately so for leaf material of P. maximum (Table 4: SP column: 1 > 2) and stem
46
material of Themeda triandra (Andropogoneae; Table 4: SP column: 1 > 2). Contrasting the
situation for sable antelope in the more arid nort-western KNP, faecal nitrogen levels of sable
herds of the more mesic upper southwestern KNP decreased to below minimum maintenance
levels during the dry season of the drier year; associated with relatively higher faecal crude
fibre levels, especially during the late dry season (Macandza et al., 2013). Under such
circumstances higher forage quality (Table 2), notably of stem material of andropogonoid
grass species, as reflected in Themeda triandra in this study (higher leaf TNC and higher
stem N, TNC and FQI values of T. triandra > P. maximum; 1 > 2, SP column: Tables 3 and
4) facilitates a disproportionately increased intake of fibrous, high TNC/N dietary items of
andropogonoid grass species; as reported by Le Roux (2010). As a consequence relatively
low faecal nitrogen levels due to faecal dilution of nutrients are implicated (but refer to
Macandza et al., 2013 for a divergent interpretation). Relatively low dry season faecal
nitrogen levels also on record for roan antelope (Knoop and Owen-Smith, 2006; Codron et
al., 2009), another selective grazer species can be similarly explained. Digestion trials with
roan antelope have revealed relatively fast rates of digestive passage, suggesting increased
capacity to cope with high dietary fibre levels (Heitkönig, 1994). Such capacity to handle
fibrous and TNC-rich diets typically procurable from stemmy andropogonoid grass species,
notably in the dry season of drier years, is here implicated to form the basis of sustained body
condition during dry seasons and demographic resilience in coping with drought years.
DIVERGENT FIRE-FORAGE QUALITY EFFECTS INTERACTING WITH RAINFALL
Blue wildebeest and zebra represent a species group of short grass bulk grazers preferring
growth-curbed green grass material (high nitrogen quality = N x DMD: Seydack et al.,
2012a; Traill, 2004). High nitrogen quality is usually associated with wet season growth and
is increased when the green grass growth is curbed by relatively low wet season rainfall.
Good nitrogen quality forage is accordingly expected to be associated with low-rainfall wet
season growth (as reflected in dry season forage quality following a low-rainfall wet season;
Table 2; Fig. 2 a,b,d: 2007); but during prolonged high-rainfall periods nitrogen quality is
compromised as a result of higher stem/leaf ratios, nitrogen dilution and low digestibility
(Table 2; Fig. 2d, as reflected in lower nitrogen and DMD values of 2008 dry season
material; following a high-rainfall wet season). Poor population performance of blue
wildebeest and zebra has been observed during such years (Whyte and Joubert, 1988) and
47
moderate drought conditions do not adversely affect these two species (Walker et al., 1987;
Seydack et al., 2012b).
An increased proportional contribution of certain preferred grass species (mainly
Panicum and andropogonoid species) in the diets of selective grazers during the late dry
seasons of drier years (sable antelope: Le Roux, 2010; roan antelope: Knoop and OwenSmith, 2006) is here interpreted to be the result of an increased availability of relevant forage
items of acceptable forage quality, notably of senescent grass material, due to enhancement
of forage quality of the grasses having grown under conditions of water stress (compare dry
season forage quality indicators and indices of 2007 following a low-rainfall wet season with
those of 2008 following a high-rainfall wet season (Table 2: increased leaf and stem NQ and
FQI values, especially of corresponding values for stem material of andropogonoid species as
represented by Themeda triandra; Fig. 2). Improved dry season forage quality during lowrainfall periods may accordingly partly compensate for any reduced incidence of dry season
greenness in such years. This is in agreement with selective grazer antelope population
performance being relatively unaffected during periods of low total annual rainfall (Walker et
al., 1987; Seydack et al., 2012b), although sensitive to reduced dry season rainfall (Seydack
et al., 2012b).
Acknowledgements
We acknowledge efforts of rangers, field staff and scientists involved in the recording of fire
data inside and outside of the N’washitshumbe enclosure and the effective management of
the enclosure over many decades, without which this study would not have been possible. For
field work collecting grass samples we acknowledge the contributions made by Diba
Rikhotso, Hylton Herd and Lizette Moolman, as well as of the two research assistants who
accompanied us: Jacob Mlangeni and Vilssone Binda. The South African Weather Service
provided temperature and rainfall data. This study represents an output of the specialist
scientist research program in systems ecology conducted by members of the Conservation
Services Division (Knysna and Skukuza) of South African National Parks. The main author,
in his capacity as honorary research associate at the Department of Biological Sciences
(University of Cape Town), thankfully acknowledges general support received from this
institution.
48
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51
Table 1. Metabolic response modes associated with treatment blocks O, A, B, E, and F and
associated dry season forage quality patterns as linked to growth functionality, fire frequency
scenarios and water status response types in respect of the dry year 2007 and the wet year
2008 (Seydack et al., 2014).
Metabolic
performance
modes *
Fire
frequency
scenarios
Metabolic
performance
types *
TW growth
functionality
Forage
quality
status
Forage quality
settings
(Enhanced)
Dry year 2007: October 2006 – September 2007
(low-rainfall wet season preceding 2007 dry season)
O: LTw
A: LTw
B: Cltw
E: Cltw
High
High
Low
Low
NADP-w
NAD-w
NAD-w
NADP-w
Reduced
Restricted
Restricted
Reduced
Enhanced
Elevated
Elevated
Enhanced
NADP-w
NAD-w
NAD-w
NADP-w
Wet year 2008: October 2007 – September 2008
(high-rainfall wet season preceding 2008 dry season)
O: LTW
A: LTW
B: CltW
F: CltW
High
High
Low
Low
NADP-W
NAD-W
NAD-W
NADP-W
High
Accentuated
Curbed
Curbed
Lowered
Lowered
Sustained
Sustained
Clt-W
Clt-W
* Metabolic response modes/types (Seydack et al., 2014): LTw/LTW (high productivity at relatively high
levels of light, temperature and water availability; NADP-ME type functionality), LT-NAD-w (efficient
functionality under water stress; NAD-ME type functionality), LT-NAD-W (high peak functionality under
transient water surplus conditions; NAD-ME type functionality), Clt-W (Clt metabolic performance modes
induced by longer-term grass sward crowding C/age effects due to long fire intervals; optimal functionality at
relatively low resource level maxima; curbed growth).
52
Table 2. Forage quality indicators for senescent grass leaf and stem material for the dry
season of the dry year 2007(D) in comparison with the wet year 2008 (W); differentiated for
treatment blocks (Fig. 1: areas with divergent post-fire ages of grass swards and long-term
fire frequencies; northern basalt plains: Kruger National Park, South Africa)
Values of grass leaf material for
composition (% air-dried mass), in
vitro dry matter digestibility (%) and
forage quality indices
Late dry season (2007)
(following a low-rainfall wet season )
Plant metabolic response modes
and types *
Dry leaf material
Nitrogen
Dry matter digestibility
Total non-structural carbohydrates
Nitrogen quality (NQ = N x DMD)
Forage Quality Index (FQI)
(FQI = N x DMD x TNC)
Dry stem material
Nitrogen
Dry matter digestibility
Total non-structural carbohydrates
Nitrogen quality (NQ = N x DMD)
Forage Quality Index (FQI)
(FQI = N x DMD x TNC)
Comparison of values over fire treatment
blocks
(post-fire sward age in parentheses)
D*
W
O
A
(3)
B
E
(5)
(22)
(8)
LTw
NADP
LTw
NAD
Cltw
NAD
Cltw
NADP
D
W
D
W
D
W
D
W
D
W
0.62a
0.44b
44.4a
29.0b
6.52
8.39
27.7a
12.8b
176a
105b
0.72a
0.52b
44.0a
24.4b
7.35
7.95
32.0a
13.2b
227a
101b
0.72a
0.58b
45.4a
28.7b
7.23
8.00
33.3a
16.8b
233a
128b
0.66
0.65
46.3a
36.7b
6.51
7.43
30.9
24.4
196
173
Average
0.68a
0.55b
45.0a
29.7b
6.90b
7.94a
31.0a
16.8b
208a
127b
D
W
D
W
D
W
D
W
D
W
0.41
0.37
42.7a
27.0b
5.38b
6.64a
17.4a
10.3b
95
68
0.47a
0.35(b)
41.4a
21.9b
6.52
6.45
19.4a
7.4b
133a
48b
0.53a
0.40(b)
43.7a
24.8b
5.80
6.30
23.0a
10.0b
137a
61b
0.42
0.43
41.5a
28.4b
5.53
6.12
17.5a
12.3(b)
98
76
0.46a
0.39b
42.3a
25.5b
5.81b
6.37b
19.3a
10.0b
116a
64b
*Metabolic response modes/types: Table 1 (Seydack et al., 2014). Multi-factor ANOVA: Dry (*D: 2007)
versus wet (*W: 2008) year x treatment blocks (O vs A vs B vs E) x species (3) with 3 replications (n = 2 x 4 x
3 x 3 = 72). Statistically significance between D and W sampling events for fire treatment blocks (late dry
season 2007, late dry season 2008): indicated by superscripts a and b (a > b) for P < 0.05 or (a), (b) and (c) for P
˃ 0.05 and P < 0.10 (Duncan range test comparisons).
53
Table 3. Forage quality indicators for senescent grass leaf and stem material for the dry
season of the dry year 2007 (differentiated for treatment blocks: Fig. 1: areas with divergent
post-fire ages of grass swards and long-term fire frequencies; northern basalt plains: Kruger
National Park, South Africa)
Values of grass leaf material for
composition (% air-dried mass), in
vitro dry matter digestibility (%) and
forage quality indices
Late dry season (2007)
(following a low-rainfall wet season )
Plant metabolic response modes
and types *
Dry leaf material
Nitrogen
Panicum maximum
Panicum coloratum
Themeda triandra
Dry matter digestibility
Panicum maximum
Panicum coloratum
Themeda triandra
Total non-structural carbohydrates
Panicum maximum
Panicum coloratum
Themeda triandra
Dry stem material
Nitrogen
Panicum maximum
Panicum coloratum
Themeda triandra
Dry matter digestibility
Panicum maximum
Panicum coloratum
Themeda triandra
Total non-structural carbohydrates
Panicum maximum
Panicum coloratum
Themeda triandra
Comparison of values over treatment blocks
(post-fire sward age in parentheses)
SP
1
2
2
1
2
3
2
1
1
(2)
3
1
2
1
2
2
2
1
O
A
B
E
(3)
(5)
(22)
(8)
LTw
NADP
LTw
NAD
Cltw
NAD
Cltw
NADP
0.62b
0.75b
0.54
0.56(b)
44.4
50.7
44.0
38.5
6.52(b)
5.50
6.96
7.10(b)
0.72a
0.91a
0.62
0.62
44.0
48.7
43.1
40.2
7.35(a)
6.13(a)
7.79
8.12
0.72a
0.87(a)
0.63
0.67(a)
45.4
49.7
46.0
40.6
7.23(a)
5.71
7.70
8.29(a)
0.66ab
0.76b
0.59
0.63
46.3
51.7
45.4
41.7
6.51(b)
5.08(b)
7.18
7.28
0.41
0.37b
0.35
0.52a
42.7
37.9
47.6
42.6
5.38b
4.87(b)
5.20(b)
6.07
0.47
0.66a
0.33
0.41(b)
41.4
40.3
43.3
40.7
6.52a
6.34a
6.25a
7.00
0.53
0.48a
0.43
0.68a
43.7
43.1
45.6
42.3
5.80(b)
5.60a
5.39a
6.40
0.42
0.40(b)
0.35
0.52a
41.5
38.9
43.6
42.1
5.53b
4.85(b)
5.41(b)
6.32
Multi-factor ANOVA: treatment blocks (O vs A vs B vs E) x species (3) with 3 replications (n = 4 x 3 x 3 =
36). Statistically significance between treatment blocks within the late dry season 2007 sampling event:
indicated by superscripts a, b and c for P < 0.05 or (a), (b) and (c) for P ˃ 0.05 and P < 0.10 (Duncan range test
comparisons between treatment blocks) and in SP column for sample event means across species (1 > 2 > 3: P <
0.05 or P ˃ 0.05 and P < 0.10 when in parenthesis). *Metabolic performance modes or types Table 1 (Seydack
et al., 2014): Clt: low fire frequency scenario; LT: high fire frequency scenario; intra-specific NAD type
metabolism: high photosynthesis /growth ratio), NAD-w: low TW stress tolerant growth and NAD-W
accentuated TW growth. NADP: intra-specific NADP functionality (high growth productivity).
54
Table 4. Forage quality indices for senescent grass leaf and stem material for the dry season
of the dry year 2007 (as differentiated for treatment blocks; Fig. 1: areas with divergent postfire ages of the grass swards and long-term fire frequencies; northern basalt plains: Kruger
National Park, South Africa)
Values of grass leaf material for
composition (% air-dried mass), in
vitro dry matter digestibility (%) and
forage quality indices
Late dry season (2007)
(following a low-rainfall wet season )
Plant metabolic response modes
and types *
Nitrogen quality (NQ): Foliage
Panicum maximum
Panicum coloratum
Themeda triandra
Forage Quality Index (FQI): Foliage
Panicum maximum
Panicum coloratum
Themeda triandra
Nitrogen quality (NQ): Stem
Panicum maximum
Panicum coloratum
Themeda triandra
Forage Quality Index (FQI): Stem
Panicum maximum
Panicum coloratum
Themeda triandra
Comparison of values over treatment blocks
(post-fire sward age in parentheses)
SP
1
2
2
1
2
2
(2)
(2)
(1)
2
2
1
O
A
B
E
(3)
(5)
(22)
(8)
LTw
NADP
27.7b
37.8(b)
23.9
21.5
176b
208b
166(b)
152c
LTw
NAD
32.0(a)
44.1(a)
26.7
25.1
227a
269a
208(ab)
204(b)
Cltw
NAD
33.3a
43.4(ab)
29.0
27.3
233a
247ab
224(a)
226a
Cltw
NADP
30.9ab
39.7(ab)
26.6
26.3
196(b)
204b
192(ab)
192(abc)
17.4b
13.9
16.3
22.0
95b
68b
84
135
19.4ab
27.2
14.5
16.6
133(ab)
203a
89
115
23.0a
20.6
19.7
28.9
137(a)
120ab
106
187
17.5b
15.4
15.2
21.9
98c
75b
82
138
Multi-factor ANOVA: treatment blocks (O vs A vs B vs E) x species (3) with 3 replications (n = 4 x 3 x 3 = 36).
Statistically significance between treatment blocks within the late dry season 2007 sampling event: indicated by
superscripts a, b and c for P < 0.05 or (a), (b) and (c) for P ˃ 0.05 and P < 0.10 (Duncan range test comparisons
between fire treatment blocks) and in SP column for sample event means across species (1 > 2 > 3: P < 0.05 or P
˃ 0.05 and P < 0.10 when in parenthesis). *Metabolic performance modes or types Table 1 (Seydack et al.,
2014): Clt: low fire frequency scenario; LT: high fire frequency scenario; intra-specific NAD type metabolism:
high photosynthesis /growth ratio), NAD-w: low TW stress tolerant growth and NAD-W accentuated TW
growth. NADP: intra-specific NADP functionality (high growth productivity).. NQ: nitrogen quality = nitrogen
x dry matter digestibility; FQI: forage quality index = nitrogen x dry matter digestibility x total non-structural
carbohydrate concentrations.
55
C
6ha
O (outside)
N
A
63ha
D
B
22ha
65ha
H
E
13ha
F
63ha
62ha
G
6ha
Fig. 1. Layout of the N’washitshumbe enclosure (northern plains, Kruger National Park)
indicating the positions of treatment blocks A, B, E, F and O (outside of the enclosure).
Northern basalt plains, Kruger National Park (South Africa).
56
57
d) Dry season Nitrogen Quality (N x DMD)
60
Sample patch maximum
50
Sample patch average
40
30
20
*
*
*
*
*
*
10
*
*
*
O
A
BEF
Panicum maximum
O
A
BEF
O
Panicum coloratum
A
2008
2007
2008
2007
2008
2007
2008
2007
2008
2007
2008
2007
2008
2007
2008
2007
2008
2007
0
BEF
Themeda triandra
Fig. 2. Forage quality indicators (grass leaf material) for late dry season sampling events
during 2007 (relatively dry preceding wet season) and 2008 (relatively moist preceding wet
season) in respect of treatment block groupings O, A and BEF: O/A/BE during 2007 and
O/A/BF during 2008 (Figure 1). a) Nitrogen, b) Dry matter digestibility, c) Total nonstructural carbohydrates, d) Nitrogen quality = nitrogen x dry matter digestibility, e) Forage
quality index = nitrogen x dry matter digestibility x total non-structural carbohydrates.
* Indicating mean values for sampling events (in 2007 and 2008) being statistically
significantly higher (P < 0.05) than for the other year of the 2007/2008 pair.
58
Fire and climate interactively shape grass forage quality
for grazing herbivores in an African semi-arid savanna
system
Running head: Fire-climate interactions shaping forage quality
Armin H.W. Seydack1,2* , Wessel J. Vermeulen1, Nicky Knox3, Navashni Govender4,
Izak P.J. Smit4, Rina C. Grant4 and Sandra MacFadyen4
1
Conservation Services Division
South African National Parks
P.O. Box 3542, 6570 Knysna, South Africa
Telephone: +27 (0)44 302 5612, Fax +27 (0)44 384 0136
E-mail: [email protected]
2
Honorary Research Associate, Department of Biological Sciences, University of Cape
Town, South Africa
3
Research and Development Group
South African National Space Agency (SANSA) – Earth Observation Division
P.O. Box 484, Silverton, 0127 Pretoria, South Africa
4
Scientific Services, Kruger National Park
South African National Parks
Private Bag x 402, 1350 Skukuza, South Africa
59
Summary
1. This study represents an exploration of how fire (fire frequency), interacting with
rainfall, shapes grass forage quality for larger grazing herbivores in a semi-arid savanna
system (Kruger National Park, South Africa). Forage quality indicators (nitrogen, nonstructural carbohydrates and digestibility of leaf and stem material) were determined for
three perennial C4 grass species from grass swards subject to divergent fire frequency
scenarios.
2. Effects of long fire intervals on the incidence and size of woody species and relative
densities of grass species with divergent forage quality levels were also investigated.
3. Adjustments of grass metabolic functionality to resource constraints of relatively
lower levels of light, temperature and water availability experienced under grass sward
crowding conditions in grass swards subject to long fire intervals, had apparently
induced a mode of metabolic performance characterized by curbed growth activity.
Grass metabolic functionality responding to conditions of accentuated temperature and
water availability encountered in high-rainfall years or when subject to acclimation to
progressive nocturnal warming, was linked to reduced (dry season) forage quality. These
effects of forage quality reductions during above-average rainfall years were ameliorated
and nocturnal warming impacts seemingly mitigated by metabolic functionality in grass
swards at advanced post-fire ages; thereby supporting grazer population performance,
especially of selective grazer antelope species (increased dry season forage quality).
4. Long fire intervals were associated with an increased incidence of relatively largersized woody individuals and of grass sward species composition of increased forage
value; especially for selective grazer antelope species.
5. The results of this study underpin the significance of long fire intervals for the
enhanced availability of high quality grass forage in semi-arid savanna systems by
inducing modes of metabolic functionality in grasses which benefit dry season forage
quality by mitigating or neutralizing forage quality-reducing effects associated with
high-rainfall wet season growth or temperature acclimation to nocturnal warming. These
fire x climate-forage quality effects have profound ecological consequences for large
grazing herbivores and thus implications for fire management of semi-arid savanna
systems.
60
Key-words: climate effects mitigation, fire return intervals, grass forage quality, Kruger
National Park, large herbivores
Introduction
Abiotic factors such as rainfall and soil fertility support grass production (McNaughton et al.,
1989; Fritz and Duncan, 1994), affecting the abundance of grazing herbivores Forage quality
is often inversely related to grass plant productivity, as linked to rainfall (McNaughton et al.,
1989) and manifesting in grass percentage cover or biomass (Mutanga et al., 2004).
Divergent spatial distributions of grazing herbivores are often influenced by spatial variation
in forage quality (McNaughton, 1988; Heitkönig and Owen-Smith, 1998). In this study
variation in grass forage quality, as shaped interactively by fire and climate (rainfall,
nocturnal warming) in a semi-arid savanna system (Kruger National Park, South Africa), was
investigated. Particular emphasis was placed on the effects of fire return periods on dry
season grass forage quality. Fire-forage quality effects along pathways of fire-climate-forage
quality interactions and some associated ecological consequences are considered.
Temperature and water availability levels prominently affect plant metabolism and the
nutritional quality of plant tissues (Wilson, 1984). These levels vary spatiotemporally at
multiple scales. Fires influence the resource conditions under which grass plants grow and
both fire and climate are expected to interactively determine forage quality (as indicated by
nitrogen, phosphorus, total non-structural carbohydrate contents and dry matter digestibility).
At the long-term multi-annual scale increasing ambient temperatures were identified as
promoting grass productivity, with negative implications for grass forage quality (Dwyer et
al., 2007; An et al., 2005; Wan et al., 2005). Analyses by Craine et al. (2010) showed a
general pattern of increased temperature being associated with decreased forage quality.
Seydack et al. (2012a) developed a climate-vegetation response model linking plant
metabolic performance to climate and its effects on forage quality. Using this explanatory
model large herbivore population fluctuations in the Kruger National Park over the past seven
decades were interpreted, also implicating nocturnal warming (Tmin) to be causally involved
in the population declines of selective grazers (sable antelope Hippotragus niger, roan
antelope Hippotragus equinus, tsessebe Damaliscus lunatus lunatus and eland Taurotragus
oryx) to low levels of abundance (Seydack et al., 2012b). Since features of a fire regime
(notably fire frequency) will also influence forage quality it was important to identify any
61
ameliorative or aggravating influences on forage quality interacting with climate change
effects.
Fire regimes, as characterized by fire return period, seasonality, intensity and fire size
configurations, influence forage quality in various ways. When promoting the occurrence of
large/older trees in the landscape through long fire intervals and intense spring fires (Smit et
al., 2010), the incidence of nutrient spots associated with such trees (Belsky et al., 1989;
Treydte et al., 2007; Ludwig et al., 2008) is increased or maintained. The role of post-fire
regrowth in savanna grazer nutrition has received much attention. Herbivores are attracted to
this regrowth which constitutes a feeding resource of increased nutrient concentrations and
leaf: stem ratios during the post-fire regrowth season (Van de Vijver et al., 1999). The value
of post-fire regrowth during the dry season for herbivore population performance has been
recorded in the context of numerous studies (Magome et al., 2008; Parrini and Owen-Smith,
2010). Spatial-scale variation in forage quality is furthermore influenced by the interaction
between grazing pressure and localized fire frequency (Trollope et al., 2014). However much
less is known about the effects of fire on dry season forage quality of brown/senescent grass
material despite the fact that nutritional shortfalls in terms of essential nutrients are generally
experienced by savanna ungulates during the dry season (Illius and O’Connor, 2000; Fynn,
2012). This study aims to inter alia fill this gap by exploring the effects of fire on dry season
forage quality. Further studies of factors shaping dry season forage quality may be facilitated
by remote sensing methods of mapping dry season savanna forage quality (Knox et al.,
2011).
The objective of this study was to reveal how fire, notably fire frequency, interacting
with climate (inter-year variations in rainfall), would shape grass forage quality for large
herbivores in a semi-arid savanna system. The aim was to reveal and explain multiple
interacting fire-forage quality effects and associated ecological consequences. Of particular
interest was whether enhanced dry season forage quality, as associated with long fire
intervals, was likely to mitigate negative climate effects on forage quality (Seydack et al.,
2012 a,b). Some implications for fire management are anticipated to follow from the findings
of this study.
Methods
STUDY AREA
62
Representing a large semi-arid savanna system (approximately 2 000 000 ha) the Kruger
National Park (KNP) is situated in north-eastern South Africa (between latitudes 22º 20′ and
25º 32′ and longitudes 30º 53′ and 32º 02′). The KNP (covering an altitudinal range of
between 250 and 850 m a.s.l.) falls within two disparate climate zones as defined by the
South African Weather Service (Venter et al., 2003). Rainfall is strongly seasonal, falling
mainly during the austral summer (October-March). The area north of the Olifants River is in
the northern arid bushveld zone, receiving 450-650 mm rain per year, whilst the southern part
of the park falls into the lowveld bushveld zone, with average annual rainfall generally
between 500-800 mm per year.
The vegetation of the northern KNP is characteristically dominated by mopane
(Colophospermum mopane) and, following the vegetation classification by Mucina and
Rutherford (2006), this tree species is well represented in broad-leaved bushveld vegetation
types on granites (Tsende Mopaneveld; NW-KNP: 450-650 mm rainfall per year) and broadleaved shrubveld associated with basalts (Mopane Basalt Shrubland; NE-KNP: 400-500 mm
rain per year). Characteristic vegetation units on granites of the southwestern KNP include
Granite Lowveld and Pretoriuskop Sour Bushveld (a large-leafed deciduous woodland with a
tall dense herbaceous layer dominated by sour grasses; 550-800 mm rain per year).
Sclerocarya birrea/Acacia nigrescens Savanna, an open tree savanna with a dense
herbaceous layer, predominates on basalts of southeastern KNP (Gertenbach, 1983; 500-600
rain per year). The herbaceous layer of the KNP is dominated by C4 grass species and the
more nutrient-rich savanna vegetation types on clay soils carry dense stands of nutritious,
high-bulk grasses (Venter et al., 2003).
The study area involved sampling areas in and around the N’waswhitshumbe
enclosure (304 ha) on the northern basalt plains (Fig. 1; 382 m a.s.l.; mean annual rainfall,
1984-2012: 486 mm). This selective grazer breeding camp, mainly for roan antelope
(Hippotragus equinus), excluded other large herbivores and predators. It was established in
1967 and extended in 1984 by 48 ha to include a vlei grassland. The enclosure occurs within
the Mopane Basalt Shrubland (Mucina and Rutherford, 2006; Gertenbach, 1983);
characterized by freely spaced mopane shrubs and a dense herbaceous layer, resulting in an
open tree and shrub savanna.
STUDY APPROACH
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We investigated associations between fire (fire frequency, post-fire sward age, post-fire grass
growth conditions) and grass forage quality. Intra-specific metabolic performance modes of
grasses responding to conditions light, temperature and water availability levels, as shaped by
fire and rainfall effects, had been established (Seydack et al., 2014a). Such responses are
known to affect tissue concentrations of selected forage quality indicators (N: nitrogen, as
proxy for protein content, TNC: total non-structural carbohydrates and DMD: dry matter
digestibility). Relatively high forage quality indices, that is, combinations of selected forage
quality indicators are furthermore of importance for optimal herbivore nutrition. A nitrogen
quality index (NQ = N x DMD) and a forage quality index (FQI = N x DMD x TNC) were
defined to capture the co-occurrence in forage of forage quality indicators. Fire and
conditions of climate (temperature and water) were thus considered to have interactively
induced particular metabolic performance modes in grass plants which, as they differentially
respond to resource conditions, in turn would determine grass quality indicator and index
values of grass leaf and stem material. Metabolic response modes of grasses induced by
resource conditions of water and temperature availability as shaped by fire frequency and wet
season rainfall effects had been identified; providing the causative mechanism of these effects
in determining forage quality patterns (Seydack et al. 2014a) and therewith also the basis for
the interpretation of spatiotemporal forage quality patterns over areas subject to divergent fire
frequency scenarios (refer to Grass metabolic performance modes and forage quality:
Seydack et al., 2014a below). Since grass species differ in forage value/quality (Trollope et
al., 2014), fire frequency effects on the relative abundance of such species were also of
relevance and investigated in the context of this study.
SAMPLING LAYOUT
During the dry seasons of 2007 (September) and 2008 (August) senescent leaf and stem
material and during 2009 (February: late wet season; green leaf material only) of three
perennial C4 grass species Panicum maximum, Panicum coloratum and Themeda triandra
were collected in and beyond the roan enclosure (N’washitshumbe, northern KNP: Fig. 1)
The apical 20 cm of grass stems (including any attached flower heads) were collected as
samples of stem material. Multiple plants of each species were collected at each replication
locality and bulked for analyses. Three sampling localities (replications) were involved
regarding each treatment block (2007: O, A, B and E; 2008/2009: O, A, B and F);
representing a total of 12 samples per species and sampling event. This part of the study
64
involved the analysis of treatment effects in respect of treatment blocks O and A (high fire
frequency scenario) and B and F (low fire frequency scenario) for the years 2008 and 2009.
Forage quality indicators (percentage concentrations of nitrogen, total non-structural
carbohydrates and in vitro dry matter digestibility) were determined for the sampled grass
material.
Three divergent fire frequency constellations associated with fire treatment blocks O,
A and BEF (Fig. 1) were identified (Seydack et al., 2014). Treatment block O had been
subject to a high fire frequency (1978-2007: 10 fires; 1987-2007: 7 fires). Of the fires, 60%
had occurred in years of comparatively high rainfall, notably the last two fires. In treatment
block A, fires of intermediate frequency (1978-2007: 6 fires; 1987-2007: 4 fires) had
occurred mostly in years of low rainfall. The two fire frequency constellations prevalent in
fire treatment blocks O and A were grouped to represent replicates of a high fire frequency
scenario. Low fire frequencies had prevailed in treatment blocks B (1978-2007: 2 fires; 19872007: 0 fires; sward post-fire age 2007: 22 years), E (1978-2007: 4 fires; 1987- 2007: 1 fire;
sward post-fire age 2007: 8 years) and F (1978-2007: 4 fires; 1987-2007: 1 fire; post-fire age
2008: 20 years). Treatment blocks B, E and F were accordingly subject to relatively low fire
frequencies and these fire treatment blocks were at advanced post-fire ages when sampled.
Fire treatment blocks B and E (2007 dry season sampling event) and B and F (2008 dry
season sampling event) represented replicates of a low fire frequency scenario. This part of
the study involved analyses and interpretations of sampling events in the late dry season of
the wet year (August 2008: preceded by a high-rainfall wet season) and a wet season 6months post-fire sampling event (2/2009). Early dry season forage quality (May 2008) was
represented by Carnegie Airborne Observatory (CAO) imagery and associated data (Asner et
al., 2007), involving grass sward forage quality indicators nitrogen, phosphorus and fibre
(Knox et al., 2011).
LABORATORY ANALYSES
The collected grass material was dried at 55 °C overnight and mill ground through a 1-mm
sieve to form homogeneous powdered samples. Concentrations of ash minerals and total nonstructural carbohydrates (TNC) of the powdered sample material were determined according
to standard procedures (Agricultural Research Council, ARC: Irene). In vitro dry matter
digestibility (DMD) was determined following Tilley and Terry (1963) and Engels and Van
der Merwe (1967). Carbon (δ13C) and nitrogen (δ15N) isotope values were determined
65
according to international standards (Stable Light Isotope Unit, Department of Archaeology,
University of Cape Town). From these analyses percent nitrogen and percent carbon (by
weight) were also provided for each sample. More details on methods performed are provided
in Seydack et al. (2014).
GRASS SPECIES COMPOSITION SHIFTS
Within the N’waswhitshumbe enclosure relatively low fire frequencies had prevailed during
1988-2007 (mean fire interval: 16 years) in comparison with an earlier preceding period
(1968-1987: 5.7 years) and outside of the enclosure (1988-2007: 3.3 years). A high local
density of roan antelope (a selective grazer) and the near absence of other grazers occurred
inside of the enclosure; with free access to relatively frequently burned areas for bulk grazers
outside of the enclosure (Grant et al., 2002). Fire frequency and/or grazing effects on grass
species composition and associated conditions of forage availability were explored by
comparing relative densities of 13 relevant grass species in respect of the two time periods
inside of the enclosure in relation to those outside of the enclosure. Relative densities were
determined as the total number of tussocks of a grass species expressed as a percentage of the
total number of tussocks of all grass species. Data required for these determinations were
extracted from Joubert (1976) for representing circumstances in 1970 inside of the enclosure
and from VCA survey sites inside and corresponding transects outside of the enclosure
(Grant and Freitag-Ronaldson, 2004). Forage factors (on a scale of 0 to 10) for grass species
indicating their relative potential to produce nutritious forage for grazing ungulates were
assigned to each species (Trollope, 1990; Trollope et al., 2014). The relative forage values of
grass species assemblages were assessed with Spearman rank correlations between relative
density and forage factor rankings. Good positive correlations were indicative of relatively
high forage value of the species assemblage (forage value = sum of products of relative
densities of grass species in the assemblage and their forage factors).
STATISTICAL ANALYSIS
Treatment block effects in respect of the response variables (N, DMD, TNC, NQ, FQI) were
analysed with multi-factor ANOVA for each year separately regarding treatment blocks O, A,
B and F for the years 2008 and 2009 (4 treatment blocks x 3 species with 3 replications
within treatment blocks: n = 4 x 3 x 3 = 36). Two fire frequency scenarios were involved: a
66
high fire frequency scenario in A and O and a low fire frequency scenario in B, E and F (Fig.
1). Each of these two fire frequency scenarios was accordingly represented by two treatment
replicates within each sampling year (2007: AO-BE; 2008 and 2009: AO-BF).
Grass metabolic performance modes and forage quality (Seydack et al., 2014a)
Intra-specific grass metabolic response modes had been identified (Seydack et al., 2014a) as
induced by fire effects (metabolic performance modes), coping with water resource
availability levels and fluctuations (NADP-NAD metabolic performance types) and
responding to water availability during wet season growth (differentially for low- and highrainfall years).
Intra-specific NAD-ME type metabolism in treatment blocks A and B was identified
by low δ13C and high δ15N values relative to treatment blocks O, E and F with higher δ13C
and lower δ15N values denoting NADP-ME type metabolism (Seydack et al., 2014a and
supporting references therein). Pronounced fluctuations of water availability were indicated
to prevail in fire treatment blocks A and B as a result of mopane-type heavier clay soils. After
a rain-free period grass growth can resume with substantially less rainfall on sandy soils, but
is estimated to require three to four times as much rainfall on clayey soils (Kumar et al.,
2002). NAD-ME type metabolism is geared to cope with such fluctuations under generally
low-rainfall conditions as it is characterized by drought tolerance and the capacity for
opportunistic utilization of short rain events (Schulze et al., 1996); thus combining greater
water stress tolerance (enhanced w functionality: photosynthesis/growth ratio ↑) with the
capacity for opportunistic use of water surplus conditions (accentuated W functionality at
relatively high transpiration rates: growth/photosynthesis ratio ↑). Therefore, under
conditions of wet season water stress, the associated metabolic response modes for treatment
blocks A and B (NAD metabolic performance types) would exhibit A: LT-NAD-w and B:
Clt-NAD-w functionality (accentuated w functionality: as expected during the low-rainfall
wet season preceding the 2007 dry season) and when opportunistically making use of relative
water surplus conditions (accentuated W functionality: as expected during the high-rainfall
wet season preceding the 2008 dry season), A: LT-NAD-W and B: Clt-NAD-W functionality
would prevail). NADP-ME type metabolism, with a more efficient carbon economy (Schulze
et al., 1996: high Rubisco: PEP carboxylation ratio activity), as indicated to be prevalent in
grasses of treatment blocks O, E and F (relatively high δ13C and low δ15N values: Seydack et
al., 2014a), involves sustained growth efficiency over time with the metabolic capacity for
67
high growth productivity at high levels of water availability, but also some capacity for
sustained growth with functionality settings at relatively low resource availability levels of
light, temperature and water (resource-level buffered growth capacity: flat functionality-level
to resource availability-level response curve). Conversely, NAD-ME type metabolism
combines the capacities of drought tolerance (conditions of water deficits) and opportunistic
utilization of short events of water surplus (resource-level responsive growth: steep
functionality-level to resource availability-level response curve).
With increasing post-fire age and associated increasing grass plant and sward bulk
densities, crowding effects increasingly shape the conditions under which seasonal grass
regrowth has to grow and survive. Due to interplant crowding with increasing post-fire sward
age (Zimmermann et al., 2010), necromass shading (Scholes and Walker, 1993) and
associated microclimatic conditions resource-level surplus conditions are seldom encountered
by individual grass plants, resulting in intra-specific adjustments intra-specific metabolic
capacity adjustments to sustain metabolic activity at moderate to relatively low resource
availability levels in terms of light and temperature (Seydack et al., 2014a). In congruence,
grass production under shade was relatively reduced in the wet season, but continued for
longer into the dry season (Ludwig et al., 2001a). This sustained productivity was attributed
to higher water availability due to decreased evaporation and increased water use efficiency
as a result of lower plant transpiration (Ludwig et al., 2001b).
Derived applicable metabolic performance modes for B, E and F, treatment blocks
subject to low fire frequency scenarios, were accordingly designated as BEF: Clt, as induced
by prolonged crowding effects in grass swards at advanced post-fire ages (C for crowding:
constrained resource availability per plant; lt reflecting reduced maximum functionality rates
in response to light and temperature levels in comparison with those in treatment blocks
subject to high fire frequency scenarios O: LT and A: LT). Combining metabolic
performance modes (LT, Clt) and types (NADP, NAD) with responses to water availability
status (low- or high-rainfall wet season growth conditions) resulted in the identification of
metabolic response modes linked to specific treatment blocks differentiated for years of low
and high wet season rainfall preceding the respective dry seasons. Metabolic response modes
for treatment blocks with NADP-type metabolic performance were accordingly designated as
O: LT-NADP-w and E: Clt-NADP-w for the dry year 2007 and O: LT-NADP-W/F: CltNADP-W for the wet year 2008. Corresponding metabolic response modes for treatment
blocks with NAD-type performance were designated as A: LT-NAD-w/B: Clt-NAD-w and
A: LT-NAD-W/B: Clt-NAD-W for the dry and wet years respectively.
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High TW metabolic functionality (growth under conditions of water surplus and high
temperatures) is associated with rapid tissue development and maturation and increased
content of structural to non-structural material, resulting in dilution of metabolites (Wilson
1983; Wilson 1984; An et al. 2005; Dwyer et al. 2007) and generally compromising dry
matter digestibility of plant material (Van Soest et al. 1978). Curbed growth under Tw/tW
conditions delays tissue ageing and the rate of progressive decline in nitrogen content and
dry matter digestibility with ageing is reduced (Wilson 1984); sustaining forage quality into
the dry season. Thus, as a general pattern, elevated TW growth functionality is associated
with compromised forage quality. Metabolic response modes, as linked to fire treatment
blocks, incorporate the effects of fire frequency scenarios and water status responses in
determining TW growth functionality, thereby provide the mechanistic link between these
effects and forage quality (Seydack et al., 2014a).
Results
ENHANCED FORAGE QUALITY OF POST-FIRE REGROWTH
Forage quality indicator nitrogen, nitrogen quality (N x DMD) and the forage quality index
(FQI = N x DMD x TNC) of wet season green grass leaf material were higher in treatment
block O (post-fire regrowth six months after the fire) than those of wet season growth in
treatment blocks with post-fire ages exceeding 6.5 years (A, B and F: Table 1: 2009). These
results confirmed the occurrence of enhanced forage quality associated with post-fire
regrowth. In contrast, nitrogen concentrations and nitrogen quality (NQ = N x DMD) of
senescent dry season grass leaf material in treatment block O (metabolic response mode O:
LT-NADP-W) were lower than those determined for B and F (metabolic response modes B:
Clt-NAD-W and F: Clt-NADP-W) in dry season leaf material originating during the wet year
2008 (Tables 2 and 3).
ENHANCED FORAGE QUALITY ASSOCIATED WITH LOW FIRE FREQUENCY SCENARIOS
Higher values of forage quality indicators (nitrogen, phosphorus, low fibre levels) in
treatment blocks B, E and F (Clt metabolic performance modes: low fire frequency scenario)
in comparison with A and O (LT metabolic performance modes: high fire frequency
scenario) were evident for the early dry season of the 2008 wet year (Fig. 2; percentage DM
69
means: nitrogen: BEF > OA, P < 0.01; Phosphorus: BEF > OA, P < 0.01; Fibre: BEF < AO,
P < 0.01; Seydack et al., 2014a). In adjustment to grass sward crowding effects the Clt
metabolic performance mode is representative for grass plants cued to sustain growth at
relatively low light/temperature levels (which could be experienced, for example, earlier in
the morning or under cloudy conditions), whereas grass plants with the LT metabolic
performance mode routinely function at relatively high light/temperature levels (Seydack et
al., 2014a). During the following late dry season relatively higher nitrogen, DMD, nitrogen
quality and forage quality index values of grass leaf material (wet year 2008: Table 2: N and
DMD; Table 3: NQ and FQI) were similarly associated with treatment blocks F and B (low
fire frequency scenario); prominently so under NADP-type metabolic circumstances (F: CltNADP-W > O: LT-NADP-W) and less pronounced under NAD-type circumstances (B: CltNAD-W/ A: LT-NAD-W. Values for these forage quality indicators and indices showed a
similar pattern for stem material; although statistically non-significant in many cases.
Functionality in terms of photosynthesis and growth at relatively high availability levels of
light and water promote high TNC concentrations in grass leaf and stem tissue (e.g. Wilson
and Wong 1982, who found lower concentrations of TNC under shade). NADP-type
metabolic performance (resource-level buffered) involves the capacity for sustained growth
productivity over moderately fluctuating relatively high levels of light/temperature and water
(LT-NADP-W) and sustained growth under conditions of relatively low resource availability
levels when cued to performance at low light/temperature levels (Clt-NADP-W). Particularly
high and low TNC levels are accordingly found in treatment blocks O and F as associated
with the LT-NADP-W and Clt-NADP-W metabolic performance types/modes (Table 2). On
the other hand, for NAD-type metabolic performance (resource-level responsive: steep
functionality level to resource level response curve) high growth functionality is restricted
over time to conditions of combined surplus levels of water and relatively high
light/temperature levels, i.e. both high growth productivity and growth at low
temperature/light settings are restricted; resulting in intermediate TNC levels for grass
material of treatment blocks A and B (Table 2).
Forage quality normally declines progressively from the late wet towards the early dry
season, reaching lowest levels during the late dry season. As the results suggest, this
progressive decline in forage quality was slowed down or neutralized in grass swards subject
to crowding effects associated with low fire frequency scenarios (Clt metabolic performance),
but notably more so in grass swards with NADP-type metabolic performance (Tables 2 and
3). It is well established that high TW metabolic functionality (growth under conditions of
70
water surplus and high temperatures) is associated with rapid tissue development and
maturation and associated faster rates of decline of nitrogen content and dry matter
digestibility progressively over time (Wilson, 1983; Wilson, 1984). Under (transient)
conditions of water surplus, as expected to occur mainly in the wet seasons of wet years (as
2008), high TW growth activity is expected to occur in treatment blocks O (O: LT-NADP-W)
and A (A: LT-NAD-W), but at reduced rates in treatment blocks subject to low fire
frequencies, namely B (B: Clt-NAD-W) and F (F: Clt-NADP-W). The latter two associated
with enhanced forage quality, notably F: Clt-NADP-W (Tables 2 and 3). Regarding F:
NADP-CltW, as explained above, this is related to resource-buffered NADP-type metabolic
performance facilitating sustained growth activity at low light/temperature levels (sustained
curbed growth); whereas resource-level responsive NAD-type metabolism is subject to
restrictions of growth activity at reduced levels of light and water availability (steep
functionality-level to resource-level response curve). Particularly growth-curbed functionality
of the F-Clt-NADP metabolic scenario is indicated by higher stem carbon (stem C: NADP: F:
Clt: 41.8 > 40.5 O: LT, P < 0.02; NAD: B: Clt: 40.9 ≥ 40.7 A: LT, ns) and lower leaf ash
concentrations (leaf ash: NADP: F: Clt: 13.2 < 14.9 O: LT, P < 0.006; NAD: B: Clt: 14.2 ≤
14.6 A: LT, ns). Rates of growth (cell expansion) are more sensitive to moisture stress than
photosynthesis (Turner and Begg 1978). As conditions of water deficits persist, leaf growth
and shoot development (cell enlargement) are initially restricted more so than rates of cell
division (Brown 1995). This results in smaller cells with smaller vacuoles and curbed growth
is thus associated with higher carbon (on a volume basis) and reduced ash concentrations due
to smaller cell vacuoles. NAD-ME type metabolism is consistent with resource-responsive
TW growth functionality; whereas NADP-ME type metabolism is relatively resource-level
buffered, i.e. involves sustained growth activity at moderate resource availability levels
(Seydack et al., 2014a). This implies that growth activity under NAD-type performance is
more restricted to conditions of water surplus, whereas NADP-type performance involves
growth efficiency also at functionality settings with metabolic performance at relatively low
temperature and water availability levels (as cued by grass sward crowding effects associated
with low fire frequency scenarios). Grass sward crowding effects (low fire frequency
scenario) accordingly result in more curbed-growth productivity of grasses (as indicated by
lower leaf ash and higher stem carbon concentrations) with higher forage quality sustained
from the early into the late dry season when performing under the NADP metabolic
performance mode.
71
Particularly enhanced forage quality (N, DMD, NQ, FQI) of dry season leaf material
in grass swards subject to low fire frequencies was accordingly evident in grass swards with
NADP-type metabolic performance at low light/temperature functionality settings (Tables 2
and 3). An inverse pattern was revealed for TNC levels which were positively associated with
growth functionality at higher levels of light/temperature and water availability (Table 2). In
summary, low fire frequency scenarios are conducive to enhanced dry season forage quality
of grass material during wetter years (2008), notably sustained into the late dry season under
NADP-type metabolic performance. Forage quality enhancement due to low-rainfall wet
seasons (drier years, 2007) were found to override/pre-empt any fire frequency effects
(Seydack et al., 2014b).
SPATIAL-SCALE HETEROGENEITY OF DRY SEASON FORAGE QUALITY
As indicated by higher nitrogen and phosphorus and lower fibre levels (Fig. 2) forage quality
of grass sward material during the early dry season of the wet year 2008 was not only
substantially higher in treatment blocks B, E and F (BEF: Clt metabolic performance modes;
low fire frequency scenarios) than for A and O (high fire frequency scenarios), but was also
characterized by pronounced patch-wise heterogeneity (Figs 2 and 3). For early dry season
grass swards (2008) relatively high nitrogen and phosphorus concentrations were determined
to occur in patches in BEF (low fire frequency scenario), whilst patches with nitrogen levels
> 0.41 % DM and phosphorus levels > 0.20 % DM were virtually absent in O and A (Fig.
3a,b). Patches with grass material of low fibre contents and thus with higher dry matter
digestibility levels occurred over considerable areas in BEF (Fig. 3c; curbed TW growth
functionality: BEF: Clt-W; Seydack et al., 2014a). In contrast, areas subject to high TW
functionality (A: LT-W and O: LT-W) were covered by grass swards with higher fibre levels,
i. e. depressed dry matter digestibility (Figs 2c and 3c).
Patch-scale heterogeneity in late dry season forage quality was also revealed when the
three replicate sample patches in each treatment block were differentiated according to δ13C
rankings (high, intermediate and low). δ13C rankings index LTW growth activity (metabolic
functionality at relatively high levels of light, temperature and water availability: Seydack et
al., 2014a). Forage quality was depressed for late dry season samples of the wet year 2008
from treatment blocks O (LT-W) and A (LT-W) in sampled patches with high δ13C values
(Fig. 4; high δ13C indicating relatively high LT-W growth activity). This effect was absent in
BF samples (Fig. 4: 2008 with BF: Clt metabolic response modes in swards subject to low
72
fire frequency scenarios: Seydack et al., 2014a). Variably higher forage quality levels in B
and F (low fire frequency scenario) in comparison with A and O (Figs 2 and 3) were
accordingly sustained into the late dry season (2008: Fig. 4); whereas in patches with high
wet season TW growth activity (high δ13C patches) in A and O (high fire frequency scenario)
notably depressed forage quality levels were encountered (Fig. 4: 2008). Relatively high
sample patch maxima were notably evident for Panicum maximum in respect of late dry
season leaf nitrogen (BF: 0.88 > O: 0.65 % DM), dry matter digestibility (approaching 60 %
and 40 % for BF and O: LT respectively) and forage quality indices under conditions of
curbed growth functionality as applicable in treatment blocks B and F (low fire frequency
scenarios: Clt-W forage quality setting).
FIRE FREQUENCY AND GRASS SPECIES COMPOSITION
Within the enclosure, Panicum maximum had greatly increased from a relative density of
0.02 of thirteen relevant forage grass species (1970: Joubert, 1976) to 23.7 % in 2004 (Table
4). This increase in the frequency of Panicum maximum, a species typically occurring with
and close to woody species, was associated with an increase in the incidence and height of
woody species, mainly mopane (Colophospermum mopane) shrubs, inside the enclosure since
1987 (Levick, 2001). Between 1967, when the enclosure was established, and 1987, any
increase in woody cover was relatively slow and comparable to that outside of the enclosure
(cf. Levick, 2001). During this early time period 14 predominantly dry season fires had
occurred inside the enclosure (1968-1987; mean fire interval: 5.7 years), representing fire
frequencies similar to those outside of the enclosure. Since 1987 the percentage woody cover
has increased inside the enclosure; with a relatively larger number of trees occurring in the
medium and tall height classes (Levick, 2001). During this time period (1988-2007) few,
mainly spring fires, had occurred inside the enclosure (mean fire interval: 16 years; Table 4).
Since similar browsing pressure was applicable in both the earlier and later period, we
interpret the increase in the incidence and size of the woody vegetation inside the enclosure
since 1988 as largely the result of long fire intervals.
Longer fire return intervals within the enclosure were associated with an increased
incidence of relatively large-sized woody vegetation and therewith suitable habitat for
Panicum maximum (congruent with Treydte et al., 2009). The three most frequent grass
species inside of the enclosure (2004; forage factors vide Trollope et al., 2014 in parentheses)
were P. maximum (10), P. coloratum (7) and T. triandra (5); in spite of also being the most
73
preferred grass species grazed inside of the enclosure (Knoop and Owen-Smith 2006).
Outside of the enclosure the most frequent grass species were Schmidtia pappophoroides (3),
T. triandra (5) and Urochloa mosambicensis (3). Furthermore, a shift in grass species
composition towards increased relative densities of species with relatively higher forage
factors had occurred inside of the enclosure during the period with long fire intervals (19882004/2008) in comparison with the preceding period (1968-1987) when higher fire
frequencies had prevailed (Table 4; Spearman rank correlation of relative grass species
densities with forage factor rankings: r = 0.72, n= 13, P < 0.006) This positive correlation
remained significant after exclusion of P. maximum (r = 0.65, n= 12, P < 0.023). No
significant association with relative densities of grass species and forage factors were evident
in grass swards subject to high fire frequencies outside of the enclosure (Table 4).
Discussion
DIVERGENT TYPES OF FORAGE QUALITY AND AVAILABILITY OVER WET AND DRY SEASONS
Forage quality of green grass is generally higher than that of senescent grass material,
especially when available as post-fire regrowth only six months after a fire (Table 1; as
encountered in treatment block O). According to Van de Vijver et al. (1999), this result
regarding post-fire regrowth can be attributed to rejuvenation of plant material and
distribution of nutrients over less above-ground biomass. Improved sward structural
properties (increased leaf/stem ratios) are associated with such post-fire regrowth. This
permits greater foraging efficiency, disproportionately so for short grass bulk grazers, and
grazing herbivores are generally attracted to such regrowth (Van de Vijver et al., 1999). Dry
season green regrowth (green flush), made possible by retained soil moisture or some dry
season rainfall, often crucially supports the population performance of grazing herbivores
(Owen-Smith and Ogutu, 2003; Seydack et al., 2012b). However, green regrowth is most
often not available during the dry season and the forage quality of senescent grass material
may then be of crucial importance for the maintenance of body condition and population
performance of grazing herbivores (especially regarding selective grazer species for which
forage with a high dry season FQI is important: Seydack et al., 2012 a,b).
FORAGE QUALITY SETTINGS OF ENHANCED DRY SEASON FORAGE QUALITY IN RELATION TO
RAINFALL AND FIRE EFFECTS
74
Five settings of relatively enhanced forage quality can be identified. Two of these involve
green grass: wet season post-fire regrowth (Van de Vijver et al. 1999; Table 1) and postfire/rainfall-linked dry season regrowth (Owen-Smith and Ogutu 2003; Parrini and OwenSmith 2010; Seydack et al. 2012b). For dry (senescent) grass, three forage quality settings of
enhanced dry season forage quality were identified (Seydack et al., 2014a,b): dry-year
enhanced forage quality (NADP-w); dry-year elevated forage quality (NAD-w) and
sustained/enhanced forage quality in wetter years of grass swards under low fire frequency
scenarios (this study; Clt-W).
Increased nitrogen, phosphorus and lower fibre levels were encountered during the
early dry season in treatment blocks B, E and F (forage quality setting Clt-W; Figs 2 and 3;
2008). In respect of the late dry season during wet year conditions (2008), relatively
increased grass leaf N, NQ and FQI values were recorded in grass swards subject to low fire
frequencies (forage quality setting Clt-W; fire treatment blocks B and F: Tables 2 and 3).
Considerable patch-scale heterogeneity of forage quality was revealed for grass swards
subject to low fire frequency scenarios (Figs 2 and 4); allowing for pockets of forage items of
relatively high quality. Such patches represent dry season nutrient hotspots selectively
available to selective grazing. Late dry season leaf nitrogen concentrations of Panicum
maximum for grass swards subject to low fire frequencies during the wet year approached
maintenance levels for ruminants (Table 2: forage quality setting Clt-NADP-W) in relation to
the recommended minimum nitrogen concentrations in wild herbivore diets of 0.8 %
(according to Schmidt and Snyman, 2002). Grass sward phosphorus values > 0.29 %
(indicated as probably sufficient for pregnant and lactating roan antelope by Van Rooyen,
2009) of the early dry season applied to 15.7 % of areas subject to long fire intervals with no
such values found outside of the enclosure (Fig. 3b).
The forage quality-reducing effects of accentuated TW metabolic functionality in
respect of dry season forage originating from high-rainfall wet season growth are
counteracted/neutralized, i.e. forage quality is maintained into the dry season in grass swards
at advanced post-fire ages (forage quality setting Clt-W: curbed TW growth functionality:
low fire frequency scenario). Low fire frequency effects were noticeably more pronounced
and effective in sustaining relatively high forage quality values into the dry season under
conditions conducive to NADP-type grass metabolism (represented in fire treatment block F:
Tables 2 and 3). Furthermore, NADP-type metabolic performance, with its more efficient
carbon economy under conditions of variable but relatively high water availability, is growth
75
efficient and more productive (Schulze et al., 1996), often producing taller grasses than
NAD-type metabolic performance (Codron et al., 2009). Also due to this reason, greater
growth-curbing effects may come to be exerted in grass swards associated with low fire
frequency scenarios in grass swards when subject to NADP-type performance than of
predominantly of NAD-type metabolic performance. Prevalence of NADP-type metabolic
performance is usually associated with areas of higher, seasonally more extended rainfall and
linked to long-grass environments. Such environments are typically favoured selective grazer
antelope species.
According to this study, conditions prevalent in grass swards at advanced post-fire
age and low-rainfall wet season growth in general (Seydack et al., 2014a) resulted in growthcurbed metabolic functionality during the growing season, resulting in enhanced forage
quality, notably of brown/senescent grass material, during subsequent dry seasons. The
associated settings of enhanced/elevated/ sustained dry season forage quality represent further
dimensions of functional resource heterogeneity (Fynn, 2012), with profound implications for
the population performance of grazing herbivores and with relevance in the context of
spatially appropriately differentiated fire management in semi-arid savanna systems.
FIRE FREQUENCY AND SHIFTS IN GRASS SPECIES COMPOSITION
Smith et al. (2013), Higgins et al. (2007) and Trollope et al. (2014) analyzed data from longterm (> 44 years) fire experiments, the experimental burn plots (EBPs) located in the Kruger
National Park, South Africa. Treatments involved regular burning (1-3 year fire intervals) in
selected seasons and fire exclusion for 44-47 years. The applicability of vegetation response
patterns observed after 44-47 years of fire exclusion and their importance for fire
management are generally de-emphasized because such long fire intervals are practically
unrealistic and considered ecologically inappropriate. However, results from this study
suggest that grass sward age/crowding effects may already take effect at lower post-fire ages
of ≥ 8 years (when associated with low fire frequency scenarios over multiple past fire
cycles) and vegetation responses observed after 44-47 years of fire exclusion thus represent
information increasingly applicable to grass swards burnt at ≥ 8 year intervals. The pattern of
relatively enhanced population performance of particularly larger-sized woody individuals
(Higgins et al., 2007) and increased occurrence of Panicum maximum (Smith et al., 2013) in
fire exclusion plots in comparison with areas regularly burnt was paralleled inside of the
N’washitshumbe enclosure (Table 4; 1988-2004: long fire intervals) as against conditions of
76
regular burning (1968-1987: short fire intervals). Generally, unburned sites showed relatively
better population performances of productive decreaser grass species (species with relatively
high forage values vide Trollope et al., 2014) in comparison with those burnt at 1-3 year
intervals in all landscapes (represented in the EBP experiment; SW-KNP: Sourveld and
Combretum, SE-KNP: Knobthorn-Marula, NE-KNP: Mopane; Smith et al., 2013; Trollope et
al., 2014). Similarly, inside of the enclosure, relative densities of grass species with higher
forage value had increased during the time period with long fire intervals, resulting in an
increased forage value of the associated grass species assemblage (Table 4). The consistency
and wide-spread applicability of these results provide strong support that long fire intervals
support enhanced forage quality for selective grazers in semi-arid savanna systems.
FIRE FREQUENCY EFFECTS ON DIVERGENT TYPES OF FORAGE AVAILABILITY
Larger-statured grass species (such as Panicum maximum) seem to benefit from fire
exclusion/long fire intervals, whereas regular burning promoted palatable shorter grass
species such as Urochloa mosambicensis and Digitaria eriantha (Smith et al., 2013); both
grass species being favoured by short grass preference bulk grazers such as blue wildebeest
and zebra (Bodenstein et al., 2000); but of relatively low acceptability for selective grazers
(sable antelope: Le Roux, 2010). As has been established (Codron et al., 2008; Codron et al.,
2009), bulk grazers (buffalo, zebra, blue wildebeest), preferentially feed on high quality,
often short NAD/PCK type grasses and selective grazers (sable antelope, roan antelope,
tsessebe, eland) selectively feed on high quality items in taller grass areas (NADP-ME grass
types, often andropogonoids). Comparatively enhanced dry season forage quality (as revealed
for Panicum maximum, Panicum coloratum and Themeda triandra) during wet years in grass
swards at advanced post-fire ages (Tables 2 and 3) provide quality dietary forage items
primarily accessible for selective grazers. Unburned long grass environments are generally
avoided by high nitrogen quality short grass bulk grazers (Gertenbach, 1979); whereas the
dietary tolerance for tall fibrous grasses of selective grazers, such as sable antelope, allows
these to occupy such long-grass environments less exploited by other grazers (upper SWKNP for example; Owen-Smith et al., 2013). Short fire intervals in long grass environments,
habitat preferred by selective grazers, result in the increased landscape-scale occurrence of
highly nutritional post-fire green growth (Table 1: 2009) suitable for bulk foraging. The
resultant influx and increased presence of blue wildebeest (Connochaetes taurinus) and more
so of zebra (Equus burchelli), with their somewhat higher tolerance for relatively taller
77
grasses, and associated predators in such long grass environments is likely to increase
predation pressure on selectively grazing species (sable and roan antelope, tsessebe, eland),
thereby contributing to their poor population performance; with aggravating impacts on the
long-term population declines attributed to climate effects (Seydack et al., 2012 a,b). At low
densities and associated reduced herd sizes selective grazer populations are expected to be
particularly vulnerable to predation (Owen-Smith et al., 2012).
MITIGATION OF NEGATIVE CLIMATE CHANGE EFFECTS ON FORAGE QUALITY
The long-term decline in population performance of selective grazer antelope species (sable
and roan antelope, tsessebe, eland) in the Kruger National Park had been attributed to the
deterioration of forage quality resulting from the acclimation of grass species to progressive
nocturnal warming (Seydack et al., 2012b). In contrast to this general trend outside of the
N’washitshumbe enclosure, good population performance of roan antelope was encountered
inside of the enclosure (Grant et al., 2002). Compromised dry season forage quality
associated with high TW growth during preceding wet seasons (Tables 2 and 3) was
interpreted to have resulted from growth activity at high temperature and water availability
levels causing nitrogen dilution, lowered DMD and allocation shifts from storage towards
growth (increased structural carbon content). Temperature acclimation to progressively
increasing nocturnal warming is expected to accentuate TW growth functionality (An et al.,
2005; Wan et al., 2005; Seydack et al., 2012 a,b). As revealed in this study, forage qualityreducing metabolic functionality at high/surplus availability levels of temperature and water
(wet year accentuated TW functionality) was constrained in grass swards experiencing long
fire intervals (as in fire treatment blocks B and F: curbed TW growth functionality; Fig. 4).
Thus, the associated curbed TW growth functionality, as applicable to treatment blocks B, E
and F inside of the enclosure, would expectedly have neutralized or counteracted any
climatically driven accentuation of TW functionality of grasses; thereby sustaining dry
season forage quality (Tables 2 and 3). In addition to relatively sustained dry season forage
quality, long fire intervals inside of the enclosure had also resulted in increased relative
densities of grass species with comparatively high forage value (Table 4); in particular also of
Panicum maximum, a preferred forage species of roan antelope (Knoop and Owen-Smith,
2006). The forage resource base for selective grazers inside of the enclosure was thus
comparatively favourable and is here considered to explain the good population performance
of roan antelope inside of the enclosure. Accordingly, low fire frequencies inside of the
78
enclosure had created conditions mitigating the negative effects of nocturnal warming on
forage quality implicated to have resulted in the long-term decline of selective grazer
population performance in the Kruger National Park (Seydack et al., 2012b).
IMPLICATIONS FOR FIRE MANAGEMENT
This study was aimed to contribute towards an improved understanding of interactive fireforage quality effects and their ecological consequences. Fires induced post-fire green flush
which is particularly conducive to efficient bulk feeding of highly nutritional forage (Van de
Vijver et al., 1999) and is of special importance, also for selective grazers, when green grass
is made available during the dry season (Magome et al., 2008; Parrini and Owen-Smith,
2010). Long fire intervals were indicated to promote grass species assemblages with
relatively high forage value especially for selective grazers (Table 4). Dry season forage
quality during wet years is relatively enhanced in grass swards of advanced post-fire ages and
the spatiotemporal heterogeneity of forage quality in the landscape is promoted when
considerable areas are subject to long fire intervals (Figs 2 and 3); representing another
dimension of functional heterogeneity (vide Fynn, 2012). Associated high quality forage
items are mainly accessible to selective grazers and long fire intervals accordingly facilitate
the spatial segregation of selectively grazing species and short grass preference bulk grazers
with their associated predators. Results of this study furthermore suggest that the impact of
reduced forage quality linked to progressive nocturnal warming (Seydack et al., 2012 a,b) and
in wetter years in general (accentuated TW functionality) is mitigated by Clt grass metabolic
performance induced by grass sward crowding/age effects. The maintenance of forage
availability with relatively high forage quality indices as in grass swards of advanced postfire ages is considered crucially important for selective grazer population performance
(Seydack et al., 2012 a,b; this study). In general, the combination of high intensity spring
fires and long fire intervals is known to favour the incidence of relatively larger-sized woody
plants (Kennedy and Potgieter, 2003; Higgins et al., 2007; Smit et al., 2010) and the locally
beneficial effects of savanna trees on grass forage quality is well documented (Belsky et al.,
1989; Ludwig et al., 2004; Treydte et al., 2007; Ludwig et al., 2008; Treydte et al., 2008);
furthermore underpinning the ecological significance of long fire intervals in semi-arid
savanna systems.
Based on the results of this study and the identified associated ecological
consequences it is concluded that an ecologically appropriate fire regime in semi-arid
79
savannas is characterized by the predominance of long fire intervals; subject to some
spatiotemporal variation to this basic pattern for natural pyro-heterogeneity. The findings
following from this study represent pertinent and new insights for the identification of
ecologically appropriate fire regimes and fire management systems in alignment with
conservation of biodiversity in all its facets and fluxes (Van Wilgen and Biggs, 2011) for
semi-arid savanna systems.
Acknowledgements
We acknowledge efforts of rangers, field staff and scientists involved in the recording of fire
data inside and outside of the N’washitshumbe enclosure and the effective management of
the enclosure over many decades, without which this study would not have been possible. For
field work collecting grass samples we acknowledge the contributions made by Diba
Rikhotso, Hylton Herd and Lizette Moolman, as well as of the two research assistants who
accompanied us: Jacob Mlangeni and Vilssone Binda. The South African Weather Service
provided temperature and rainfall data. The CAO data were collected and processed by
Gregory Asner, David Knapp, Ty Kennedy-Bowdoin, Roberta Martin and colleagues of the
Department of Global Ecology, Carnegie Institution for Science. The Carnegie Airborne
Observatory is made possible by the Gordon and Betty Moore Foundation, the John D. and
Catherine T. MacArthur Foundation, W. M. Keck Foundation, the Margaret A. Cargill
Foundation, Grantham Foundation for the Protection of the Environment, Mary Anne Nyburg
Baker and G. Leonard Baker Jr., and William R. Hearst III. This study represents an output of
the specialist scientist research program in systems ecology conducted by members of the
Conservation Services Division (Knysna and Skukuza) of South African National Parks. The
main author, in his capacity as honorary research associate at the Department of Biological
Sciences (University of Cape Town), thankfully acknowledges general support received from
this institution.
80
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85
Table 1. Forage quality indicators and forage quality indices for green grass leaf material
during the late wet season 2009 (post-fire regrowth), as differentiated for treatment blocks
(Fig. 1: areas with divergent post-fire ages of grass swards and long-term fire frequencies;
northern plains: Kruger National Park, South Africa)
Values of grass leaf material for
composition (% air-dried mass), in
vitro dry matter digestibility (%) and
forage quality indices
Wet season
(February 2009)
Plant metabolic performance types *
(0.5)
(6.5)
(23.5)
(20.5)
NADP
NAD
NAD
NADP
Nitrogen
Panicum maximum
Panicum coloratum
Themeda triandra
Dry matter digestibility
Panicum maximum
Panicum coloratum
Themeda triandra
Total non-structural carbohydrates
Panicum maximum
Panicum coloratum
Themeda triandra
1.87a
2.14a
1.94a
1.52(a)
56.2
62.2
55.3
51.0
7.50
6.88
7.46
8.18
1.45c
1.58c
1.57c
1.19(c)
55.0
60.7
51.5
52.7
6.66
5.98
6.70
7.32
1.65b
1.92(ab)
1.70abc
1.33(abc)
53.2
61.5
51.1
46.8
6.18
6.17
6.20
6.18
1.68b
1.90(ab)
1.88(b)
1.25(bc)
51.2
58.5
49.5
45.6
6.44
7.03
5.95
6.33
106a
134a
108
77
818a
947
857
651
80b
96b
81
62
519b
565
534
457
90(b)
118ab
88
63
571b
752
559
403
87b
113ab
93
55
546b
734
556
348
Nitrogen quality (NQ): Foliage
Panicum maximum
Panicum coloratum
Themeda triandra
Forage Quality Index (FQI): Foliage
Panicum maximum
Panicum coloratum
Themeda triandra
Comparison of values over
treatment blocks
(post-fire sward age in
parentheses)
O
A
B
SP
F
1
1
2
1
2
2
1
2
3
1
1
2
* Metabolic performance types: Table 1(Seydack et al., 2014a). Multi-factor ANOVA: treatment blocks (O vs A
vs B vs F) x species (3) with 3 replications (n = 4 x 3 x 3 = 36). Statistically significance between fire treatment
blocks for the sampling event of post-fire regrowth during the wet season of 2009: indicated by superscripts a, b
and c for P < 0.05 or (a), (b) and (c) for P ˃ 0.05 and P < 0.10 (Duncan range test comparisons). SP column: 1 >
2 > 3 for sample event mean of grass species: P < 0.05). NQ: nitrogen quality = nitrogen x dry matter
digestibility; FQI: forage quality index = nitrogen x dry matter digestibility x total non-structural carbohydrate
concentrations.
86
Table 2. Forage quality indicators for senescent grass senescent leaf and stem material for
the wet year 2008 (differentiated for treatment blocks; Fig. 1: areas with divergent post-fire
ages of grass swards and long-term fire frequencies; northern basalt plains: Kruger National
Park, South Africa)
Values of grass leaf material for
composition (% air-dried mass), in
vitro dry matter digestibility (%) and
forage quality indices
Late dry season (2008)
(following a high-rainfall wet season )
Plant metabolic response types and
modes *
Leaf
Nitrogen
Panicum maximum
Panicum coloratum
Themeda triandra
Dry matter digestibility
Panicum maximum
Panicum coloratum
Themeda triandra
Total non-structural carbohydrates
Panicum maximum
Panicum coloratum
Themeda triandra
Stem
Nitrogen
Panicum maximum
Panicum coloratum
Themeda triandra
Dry matter digestibility
Panicum maximum
Panicum coloratum
Themeda triandra
Total non-structural carbohydrates
Panicum maximum
Panicum coloratum
Themeda triandra
Comparison of values over treatment blocks
(post-fire sward age in parentheses)
O
F
A
SP
B
(1)
(20)
(6)
(23)
NADP
LT-W
NADP
Clt-W
NAD
LT-W
NAD
Clt-W
0.44c
0.50c
0.42b
0.41b
29.0b
33.4b
27.3(ab)
26.3
8.39a
6.91
9.06
9.19
0.65a
0.78a
0.55a
0.60a
36.7a
48.2a
30.5(a)
31.4
7.43b
5.79
7.85
8.64
0.52b
0.60bc
0.47b
0.51ab
24.4b(c)
30.1b
18.6(b)
24.4
7.95ab
6.75
8.01
9.09
0.58(b)
0.67b
0.53b
0.53ab
28.7b
36.1(b)
22.8(ab)
27.1
8.00ab
6.61
7.86
9.52
0.37a
0.47
0.35
0.34
27.0a
20.9
29.8a
30.4
6.64
5.45
6.19
8.28
0.43a
0.50
0.35
0.45
28.4a
27.8
26.9(a)
30.5
6.12
5.36
5.78
7.23
0.35b
0.40
0.31
0.32
21.9(b)
21.5
16.8b
27.4
6.45
6.05
5.77
7.52
0.40a
0.45
0.36
0.39
24.8a
28.4
25.0(ab)
21.1
6.30
5.73
6.22
6.94
1
2
2
1
2
2
3
2
1
1
2
2
2
2
1
*Metabolic response modes/types (Seydack et al., 2014a): Clt: low fire frequency scenario; LT: high fire
frequency scenario; NAD: intra-specific NAD-ME type metabolism (water deficit: high photosynthesis /growth
ratio; water surplus: high growth/photosynthesis ratio) and NADP: intra-specific NADP-ME type metabolism
(high growth productivity). Multi-factor ANOVA: treatment blocks (O vs A vs B vs F) x species (3) with 3
replications (n = 4 x 3 x 3 = 36). Statistically significance between fire treatment blocks within the late dry
season 2008 sampling event: indicated by superscripts a, b and c for P < 0.05 or (a), (b) and (c) for P ˃ 0.05 and
P < 0.10 (Duncan range test comparisons between fire treatment blocks) and in SP column for sample event
means across species (1 > 2 > 3, P < 0.05).
87
Table 3. Forage quality indices for senescent grass leaf and stem material for the wet year
2008 (differentiated for treatment blocks; Fig. 1: areas with divergent post-fire ages of grass
swards and long-term fire frequencies; northern basalt plains: Kruger National Park, South
Africa)
Values of grass leaf material for
composition (% air-dried mss), in
vitro dry matter digestibility (%) and
forage quality indices
Late dry season (2008)
(following a high-rainfall wet season )
Plant metabolic response types and
modes *
Nitrogen quality (NQ): Foliage
Panicum maximum
Panicum coloratum
Themeda triandra
Forage Quality Index (FQI): Foliage
Panicum maximum
Panicum coloratum
Themeda triandra
Comparison of values over
treatment blocks
(post-fire sward age in parentheses)
(1)
(20)
(6)
(23)
NADP
LT-W
12.8c
15.8(c)
11.4(b)
11.0(b)
105b
110b
105
102
NADP
Clt-W
24.4a
37.6a
16.9(a)
18.8(a)
173a
224a
133
162
NAD
LT-W
13.2(c)
18.5(bc)
8.7(ab)
12.3(ab)
101b
122b
69
112
NAD
Clt-W
16.8b
24.1b
11.8(ab)
14.4(ab)
128b
156(b)
91
137
Nitrogen quality (NQ): Stem
Panicum maximum
Panicum coloratum
Themeda triandra
Forage Quality Index (FQI): Stem
Panicum maximum
Panicum coloratum
Themeda triandra
10.3ab
10.2(ab)
10.2(a)
10.4(b)
68
55.8
62.8
86.7a
12.3(a)
13.8(a)
9.3(ab)
13.8a
76
74.6
54.6
99.7a
7.4(b)
8.2(b)
5.4(b)
8.6(b)
48
49.1
32.1
64.1(b)
10.0a
12.7(ab)
9.2(ab)
7.9b
61
73.0
55.6
54.7(b)
O
F
A
SP
B
1
2
2
1
2
1
1
2
1
(2)
2
1
*Metabolic response modes/types (Seydack et al., 2014a): Clt: low fire frequency scenario; LT: high fire
frequency scenario; NAD: intra-specific NAD-ME type metabolism (water deficit: high photosynthesis /growth
ratio; water surplus: high growth/photosynthesis ratio) and NADP: intra-specific NADP-ME type metabolism
(high growth productivity). Multi-factor ANOVA: fire treatment blocks (O vs A vs B vs F) x species (3) with 3
replications (n = 4 x 3 x 3 = 36). Statistically significance between treatment blocks within the late dry season
2008 sampling event: indicated by superscripts a, b and c for P < 0.05 or (a), (b) and (c) for P ˃ 0.05 and P <
0.10 (Duncan range test comparisons between treatment blocks) and in SP column for sample event means
across species (1 > 2 > 3: P < 0.05 or P ˃ 0.05 and P < 0.10 when in parenthesis). NQ: nitrogen quality =
nitrogen x dry matter digestibility; FQI: forage quality index = nitrogen x dry matter digestibility x total nonstructural carbohydrate concentrations.
88
Table 4. Relative densities of grass species inside (1970 and 2004) and outside (2004) of the
N’washitshumbe enclosure (northern basalt plains, Kruger National Park, South Africa)
Relative densities (RD = % contribution of individual species) and associated rankings (R)
Inside
Outside
Inside
Forage
1970 a
2004
2004
factors b
Mean fire intervals (years):
1968-1987
5.7 (5-6)
1988-2007
3.3 (2-3/7)
16 (6-20+)
RD
34.25
14.36
14.15
9.72
8.52
7.01
3.89
2.78
2.19
1.29
0.95
0.90
0.02
Schmidtia pappophoroides
Panicum coloratum
Digitaria eriantha
Themeda triandra
Aristida species
Heteropogon contortus
Cenchrus ciliaris
Setaria woodii/incrassata
Urochloa mosambicensis
Eragrostis superba
Enneapopgon cenchroides
Bothriochloa radicans
Panicum maximum
Mean forage value of assemblage c
386
R
1
2
3
4
5
6
7
8
9
10
11
12
13
RD
23.32
4.45
1.84
18.25
2.30
4.75
4.83
4.14
14.26
12.27
0.45
0.46
3.69
356
R
1
7
11
2
10
6
5
8
3
4
13
12
9
RD
11.08
13.68
1.23
11.30
0.56
4.56
5.90
8.29
11.27
6.31
0.94
1.13
23.75
R
5
2
10
3
13
9
8
6
4
7
12
11
1
FF
3
7
4
5
2
2
5
6
3
1
1
2
10
561
Spearman rank correlations
Relative density: Inside 1970 / Forage factor ranking:
r = 0.22 ns (n= 13)
Relative density: Outside 2004 / Forage factor ranking:
r = 0.17 ns (n=13)
Relative density: Inside 2004 / Forage factor ranking:
r = 0.72 P < 0.006 (n=13)
a
Joubert (1976); b Trollope et al. (2014); c Mean forage value = ∑ (RD x FF).
RD: relative density %; R: rankings
89
R
7.5
2
6
4.5
10
10
4.5
3
7.5
12.5
12.5
10
1
C
6ha
O (outside)
N
A
63ha
D
B
22ha
65ha
H
E
13ha
F
63ha
62ha
G
6ha
Fig. 1. Layout of the N’washitshumbe enclosure (northern plains, Kruger National Park)
indicating the positions of treatment blocks A, B, E, F and O (outside of the enclosure).
Northern basalt plains, Kruger National Park (South Africa).
90
91
Fig. 2. Early dry season (1 May 2008) nutrient maps generated for the area covering
treatment blocks (O and A: high fire frequency scenario; B, E and F: low fire frequency
scenario; Fig. 1) depicting the spatial variation in (a) nitrogen, (b) phosphorus and (c) fibre
percentage contents of the grass sward. The colour scale bar represents the % DM of the
respective nutrients (Knox et al. 2011).
92
(a)
Percentage area
70
60
50
40
O
30
A
20
B
10
E
0
F
Nitrogen (% DM)
(b)
Percentage area
60
50
40
O
30
A
20
B
10
E
0
F
Phosphorus (% DM)
93
(c)
45
Percentage area
40
35
30
O
25
A
20
B
15
E
10
F
5
0
40-41 41-42 42-43 43-44 44-45 45-46 46-47 47-48 48-49
Fibre (% DM)
Fig. 3. Breakdown of percentages of the areas of treatment blocks O, A, B, E, and F (Fig. 1)
for which indicated levels of nitrogen (a), phosphorus (b) and fibre (c) of the grass sward are
applicable during the early dry season of 2008; as depicted in Figure 2.
94
Dry season foliar nitrogen (% DM)
(a)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
O
A
BE
O
2007
A
BF
2008
δ13C rankings: High-Intermediate-Low
Dry season dry matter digestibility
(b)
50
40
30
20
10
0
O
A
BE
2007
δ13C
O
A
BF
2008
rankings: High-Intermediate-Low
95
Dry season forage quality index
(c)
300
250
200
150
100
50
0
O
A
BE
2007
O
A
BF
2008
δ13C rankings: High-Intermediate-Low
Fig. 4. Forage quality indicators (grass leaf material) for treatment block groupings (O/A/BE
or BF) over two dry season sampling events (2007/2008); differentiated according to δ13C
rankings of the three replicate samples per species in high, intermediate and low. δ13C
rankings index LTW growth activity (metabolic functionality at relatively high levels of light,
temperature and water availability). a) Nitrogen, b) Dry matter digestibility and c) Forage
quality index (Nitrogen x dry matter digestibility x total non-structural carbohydrates). For
2008 (preceding high-rainfall wet season) in AO: Nitrogen (high δ13C): < nitrogen
(intermediate/low δ13C), P < 0.02; Forage quality index (high δ13C): < forage quality index
(intermediate/low δ13C), P < 0.003. Negative δ13C-forage quality associations absent in
treatment block grouping BF (2008).
96