Global Change Biology (2004) 10, 350–358, doi: 10.1046/j.1529-8817.2003.00702.x
Natural abundance of 13C and 15N in C3 and C4 vegetation
of southern Africa: patterns and implications
R . J . S W A P, J . N . A R A N I B A R , P. R . D O W T Y, W . P. G I L H O O L Y I I I and S . A . M A C K O
Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA
Abstract
The mean annual rainfall in southern Africa is found to explain over half of the observed
variance in the stable nitrogen (N) isotopic signatures of C3 vegetation in southern Africa
(r2 5 0.54, Po0.01). The inverse relationship between the stable N isotopic signatures of
foliar samples from C3 vegetation and long-term southern African rainfall is found on a
scale larger than previously observed. A modest relationship is found between stable
carbon (C) isotopic signatures of C3 vegetation and rainfall across the region (r2 5 0.20,
Po0.01). No such relationship is found between stable C and N isotopic signatures of C4
vegetation and rainfall. The explanation of the relationship between 15N in C3 vegetation
and the mean annual rainfall presented here is that nutrient availability varies inversely
with water availability. This suggests that water-limited systems in southern Africa are
more open in terms of nutrient cycling and therefore the resulting natural abundance of
foliar 15N in these systems is enriched. The use of this relationship may be of value to
those researchers modeling both the dynamics of vegetation and biogeochemistry across
this region. The use of the isotopic enrichment in C3 vegetation as a function of rainfall
may provide an insight into nutrient cycling across the semi-arid and arid regions of
southern Africa. This finding has implications for the study of global change, especially
as it relates to vegetation responses to changing regional rainfall regimes over time.
Keywords: carbon and nitrogen stable isotopes, C3/C4 vegetation, isotopic gradient, rainfall gradient,
southern Africa
Received 9 January 2002; revised version received and accepted 14 November 2002
Introduction
The question arises as to whether a relationship exists
in the processing of carbon (C) and nitrogen (N) by
vegetation across physical environments on subcontinental scales as evidenced by the natural abundances of
C and N in foliage. Given the strong gradients in
rainfall and vegetation type, southern Africa is an ideal
region to explore this question. This paper examines
whether a pattern exists in the stable C and N isotopic
signatures of southern Africa vegetation and conjectures as to several possible mechanisms for the
observed patterns. The vegetation samples used in this
study were collected during the following campaigns:
the Southern African Fire–Atmosphere Research Initiative (SAFARI 92; Journal of Geophysical Research
Special Issue, Vol 101, No. D19, 1996); the Kalahari
Transect (Scholes & Parsons, 1997; Ringrose & Chanda,
Correspondence: Robert J. Swap, fax 1 1 (434) 924 4761,
e-mail: [email protected]
350
2000); and the recently conducted wet season intensive
campaign of the Southern African Regional Science
Initiative – SAFARI 2000 (Otter et al., 2002). The
relationships between the stable C and N isotopic
signatures and southern African rainfall are evaluated.
All sampled vegetation was classified according to C3
or C4 photosynthetic pathway. Questions addressed
include whether there are coincident physical gradients
and/or systematic patterns across southern Africa in
the distribution of isotopic signatures, and whether
these isotopic gradients are related to differences
between photosynthetic pathways and the associated
processing of C and N.
Stable C and N isotopic signatures of vegetation
samples reveal information about biotic and abiotic
controls on the cycling of C and N. The primary source
of isotopic fractionation of d13C signatures in plants is
related to C assimilation and diffusion of CO2
influenced by water stress. Differences between carboxylation reactions induce the disparate isotopic fractionation between the two primary photosynthetic
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S TA B L E I S O T O P I C PAT T E R N I N A F R I C A N V E G E TAT I O N
pathways of the Calvin cycle (C3) and the Hatch–Slack
cycle (C4) (O’Leary, 1981, 1988). In water-limited areas,
water-use efficiency (WUE) becomes an additional
influence on the C isotopic signature. Increasing aridity
can result in stomatal closure and an associated
decrease in C isotope discrimination by leaves
(Ehleringer & Cooper, 1988; Comstock & Ehleringer,
1992; Ehleringer, 1995; Lin et al., 1996), which suggests
that plants with high WUE should have been enriched
relative to those with lower WUE (Farquhar &
Richards, 1984; Farquhar, 1991). Other species-specific
responses may also affect the isotopic signatures and
dampen or confound the effects of drought on C
isotope discrimination (Schulze et al., 1991, 1996a, b,
1998, 1999).
Plant N isotopic discrimination is related to the
availability of nutrients and water (Tilman, 1988) and
indicative of N cycling on different spatial and
temporal scales (Nadelhoffer & Fry, 1994). Nutrientrich ecosystems tend to be isotopically enriched where
the nitrogen cycle is more open to N loss (Vitousek
et al., 1989). Nutrient-based enrichment has also been
found where N-rich sites supported higher losses of N
through denitrification and leaching than N-poor sites
(Martinelli et al., 1999). Likewise, an enrichment of d15N
signatures in soil, plant and animal samples associated
with arid regions has been demonstrated for sites
within the Atacama desert, South America, and the
deserts of southern Africa and the southwestern US.
Such enrichments suggest different biogeophysical
processing and cycling of N caused by decreased
rainfall. This result produces a more open N cycling
at drier sites, with smaller losses relative to turnover as
the annual precipitation increases (Shearer et al., 1978;
Heaton et al., 1986; Heaton, 1987; Sealy et al., 1987; Vogel
et al., 1990; Schulze et al., 1991; Evans & Ehleringer,
1993, 1994; Austin & Vitousek, 1998).
Southern Africa, a region of strong physical and
biological gradients, provides conditions required to
examine these large-scale patterns in d13C and d15N
distributions across a regional precipitation gradient as
it relates to C3 and C4 photosynthetic pathways. An east
to west and north to south decrease in the mean annual
precipitation exists from the coastal region of southern
Africa east of the great escarpment (o1500 mm), to the
western margins of the subcontinent where the Kalahari and Namib deserts are found (o100 mm). The
vegetation of southern Africa varies along this rainfall
gradient from evergreen to desert-adapted forests and
exhibits decreasing species richness from mesic/humid
to xeric/arid to semi-arid areas (O’Brien, 1993).
Savanna vegetation covers much of this gradient and
is often separated into wet and dry forms. The division
between the wet and dry savanna is the point at which
r 2004 Blackwell Publishing Ltd, Global Change Biology, 10, 350–358
351
the strong linear dependence of productivity asymptotes with the mean annual rainfall. These wet and dry
savannas have been generally classified as broadleaf
and fineleaf savannas, respectively (Scholes & Walker,
1993). The strong dependence of production on rainfall
may obscure the importance of edaphic factors that
contribute to the dry savanna structure and function.
Water and nutrient availability are the primary
factors affecting productivity in dry savannas. Water
controls the duration of grass production and nutrient
availability controls the rate of growth during productive periods (Scholes, 1990). A general model of moist,
dystrophic (nutrient poor) and arid, eutrophic (nutrient
rich) savannas describes the observed trend of relatively infertile soils and higher rainfall in broadleaf
savannas and relatively fertile soils and aridity in
fineleaf savannas (Huntley, 1982). In their survey of
global photosynthesis and stomatal conductance,
Woodward & Smith (1994) present data that suggest
the subtropical arid and semi-arid ecosystems are not N
limited, but rather water limited.
Methods
Foliar samples of southern African vegetation for stable
isotopic analyses were collected at sampling sites that
represent a wide range of precipitation regimes,
different soil types and land use types (e.g. biomass
burning regions, agriculture regions, industrial regions)
(see Fig. 1 for site locations). The location and mean
annual rainfall of the vegetation sampling sites used in
this study are presented in Table 1a and b.
Foliar vegetation was collected from undisturbed
sites, located significantly inland from the southern
African coast, primarily on the southern African highveld, the raised, inland plateau of the subcontinent, and
outside of any immediate industrial activity to control
for the effects of altitude, land use, marine and
industrial contributions. Young and mature sun leaves
sampled from woody trees at the same height in the
canopy, shrubs and grasses were air dried, homogenized to a fine powder (Wiley Mill, mesh number 60;
Thomas Scientific, Swedesboro, NJ, USA) and analyzed
for C and N isotopes on an EA-IRMS Optima
(Micromass Ltd, Manchester, UK), with a precision of
70.5% (Hobbie et al., 2000). All d13C and d15N values
are reported relative to the isotopic standards of PDB
and atmospheric N2, respectively.
The collection of samples occurred over different
years during both wet and dry seasons. Abbadie et al.
(1992) and Garten (1993) found no evidence of a
seasonal effect on the d15N signatures of foliage outside
of the initial growth during the early spring. As none of
the sampling occurred during the period of initial
352 R . J . S W A P et al.
Fig. 1 Mean annual rainfall based on the 1961–1990 mean monthly climatology of New et al. (1999). The order of the vegetation
sampling sites is based on decreasing mean annual rainfall where 1 – Drakensburg, RSA; 2 – Misaka, Zam; 3 – Lukulu, Zam; 4 – Piet
Retief, RSA; 5 – Senanga, Zam; 6 – Maziba, Zam; 7 – South KNP, RSA; 8 – KNP, RSA; 9 - Northern KNP, RSA; 10 - Otavi, Nam; 11 –
Vryburg, RSA; 12 – Maun, Bot; 13 – Sandveld, Nam; 14 – Ghanzi/Okwa, Bot; 15 – Central KNP, RSA; 16 – Karoo-20, RSA; 17 – ENP,
Nam; 18 – Karoo16, RSA; 19 – Karoo17, RSA; 20 – Vastrap, RSA; 21 – Kuiepan, RSA.
growth, it is assumed that such effects are minimal if
any. This study incorporates additional reported values
of d15N signatures for southern African foliar vegetation samples from two other studies, Heaton (1987) and
Högberg & Alexander (1995). Additional d13C data for
South African C3 vegetation reported by Vogel et al.
(1978) were also used (Table 1a and b). Data cited from
these studies conformed to the site selection criteria
stated earlier.
As continuous, long-term rainfall records are spatially sparse in southern Africa, determination of longterm average annual rainfall values for the sites of
interest requires multiple data sets. Values for mean
annual rainfall were either determined from Griffiths
(1972), or in the case of published data were taken from
the original study. The mean annual rainfall for Vastrap
was derived from a 23-year rain gauge data set
collected on an adjacent farm approximately 5 km from
the field site. The Sandveld value was derived from the
27-year dataset collected at the research station immediately adjacent to the field site (Kruger, 1998). Data
provided by the Zambian Department of Agriculture
were used to derive values for the Maziba, Senanga and
Lukulu sites. The Maziba value was linearly interpolated from data collected at Sesheke (43-year record,
85 km distant from site) and Senanga (42-year record,
78 km distant from site). Similarly, the value for the
Senanga site was interpolated using data from Senanga
(28 km from site) and Mongu (44-year record, 74 km
distant from site). The mean annual rainfall used for the
Lukulu site was calculated as an inverse-distance
weighted mean of values from Mongu (44 years),
Kaoma (42 years), Zambezi (38 years) and Kabompo
(31 years) with distances of these stations from the field
site ranging between 93 and 138 km.
Results
Vegetation samples for each site were classified
according to their photosynthetic pathway as either
C3 or C4. The number of isotopic samples used in this
study for the C3–13C, C3–15N, C4–13C and C4–15N
groups were 105, 143, 32 and 58, respectively. Where
applicable, the mean and standard deviations of the
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S TA B L E I S O T O P I C PAT T E R N I N A F R I C A N V E G E TAT I O N
353
Table 1 Number of samples, mean and standard deviation of d13C and d15N presented for C3 and C4 signatures of southern
African vegetation, location and annual precipitation (PPT)
Site
(a) C3 vegetation
Drakensburg
Misaka
Lukulu
Piet Retief
Senanga
Maziba
SKNP
KNP
Rustenburg
NKNP
Otavi
Vryburg
Maun
Sanveld
Okwa
CKNP
Grootfontein
Karoo20
ENP
Karoo16
Vastrap
Kuiepan
(b) C4 vegetation
Drakensburg
Piet Retief
SKNP
KNP
Otavi
Vryburg
Sandveld
CKNP
Karoo16
ENP
Karoo17
Vastrap
Kuiepan
(n)
d13C (%)
C Stddv
27
28.0
1.1
12
17
26.9
27.8
1.9
1.8
7
1
25.7
26.7
1.8
14
4
14
26.8
26.2
27.2
2
26.0
11
24.6
1
23.2
1.9
0.3
1.3
1.3
8
13.7
0.8
3
14.1
0.1
13
13.3
0.7
8
14.0
0.6
(n)
d15N (%)
N Stddv
Longitude
29.33
28.25
23.52
30.75
23.34
23.61
31.00
31.27
27.20
31.00
17.33
22.25
23.59
19.17
21.71
31.10
18.10
25.00
16.00
22.10
21.42
22.25
29.33
30.75
31.00
31.27
17.33
22.25
19.17
31.10
22.10
16.00
22.15
21.42
22.25
3
21
27
1
12
17
3
7
0.7
1.4
0.8
2.4
1.8
2.3
3.9
0.9
1.7
1.1
1.2
3
2
2
14
0.4
14
2
4.2
3.5
4.7
5.1
4.0
6.3
2.8
1
11
1
1
2
3.6
7.2
3.4
4.6
7.4
6
2
3
8
2
5
3
1
2
13
1
8
4
1.7
2.1
1.9
1.5
4.4
1.9
0.6
7.4
2.2
3.2
8.5
3.7
4.0
1.2
1.3
1.7
2.2
2.0
0.7
2.3
5.2
2.1
1.8
0.7
1.0
3.4
4.2
1.8
0.6
Latitude
PPT (mm)
Study
29.25
12.75
14.42
27.00
15.86
16.74
25.00
25.17
25.60
22.66
19.50
27.50
19.92
22.02
22.41
23.10
19.60
32.00
19.00
31.00
27.75
27.66
1300
1200
970
890
810
740
690
650
650
570
560
460
460
410
407
390
390
350
340
260
230
200
4
5
3
4
3
3
4
1
6
4
4
4
3
3
3
4
6
4
1,2
4
3
4
29.25
27.00
25.00
25.17
19.50
27.50
22.02
23.10
31.00
19.00
30.25
27.75
27.66
1300
890
690
650
560
460
410
390
350
340
260
230
200
4
4
4
1
4
4
3
4
4
1,2
4
3
4
The references for the studies are: 1 – Swap (1996); 2 – Dieundonne (1997); 3 – Aranibar et al. (2003); 4 – Heaton (1987); 5 – Högberg
& Alexander (1995); 6-Vogel et al. (1978). Stddv, standard deviation.
isotopic d13C and d15N values at each site are given
(Table 1 a and b). The d15N values of Heaton (1987) and
Högberg & Alexander (1995) are not paired with d13C
values as d13C values were not reported as part of their
original discussion.
Carbon
The mean annual rainfall and stable C isotopic
signatures of C3 vegetation are negatively correlated
(Fig. 2a). The regression analysis results in an r2 of 0.20;
r 2004 Blackwell Publishing Ltd, Global Change Biology, 10, 350–358
Po0.01. This correlation, although significant, is not as
strong as that reported for d13C vs. long-term annual
rainfall for C3 vegetation along a 900- km rainfall
gradient in Queensland, Australia (r2 5 0.78; Stewart
et al., 1995).
No relationship is found between d13C signatures of
C4 vegetation and mean annual rainfall (Fig. 2b). These
findings are consistent with Farquhar et al. (1982, 1989),
Farquhar & Richards (1984) and Farquhar (1991), in
observing both the positive relationship between WUE
and 13C abundance and WUE and water availability.
354 R . J . S W A P et al.
Fig. 2 (a) d13C signatures (%) of C3 southern African vegetation plotted against mean annual rainfall (mm). (b) As for (a) but
for C4 species.
Fig. 3 (a) d15N signatures (%) of C3 southern African vegetation plotted against mean annual rainfall (mm). (b) As for (a) but
for C4 species.
Nitrogen
to changes in the density, richness and composition of
C3, C4 and CAM vegetation species. The pattern
observed for southern Africa is consistent with those
observed by Winter et al. (1982) and Garten & Taylor
(1992). Winter et al. (1982) found depleted values for C3
vegetation under more mesic/high humidity conditions than under drier conditions due to higher
stomatal conductance. Garten & Taylor (1992) found
an enrichment in the d13C signatures in both coniferous
and deciduous foliage at their more xeric study sites.
However, it is important to note that vegetation in xeric
environments responds more strongly to wetter conditions than does vegetation in mesic sites (Garten &
Taylor, 1992).
Schulze et al. (1996b) found no relationship between
d13C in foliage exposed to direct sunlight and rainfall in
Patagonia and attributed this lack of relationship to the
absence of long-term water stress along this aridity
gradient due to the ample supply of ground water.
Schulze et al. (1991) reported no significant relationship
between aridity and d13C in Namibia. Schulze et al.
(1998) found constant community values for those
regions along an aridity gradient in Australia with
annual precipitation greater than 475 mm, and suggested that at low water availability, plants with low
There is a significant negative correlation between the
stable N isotopic signatures of C3 vegetation and mean
annual rainfall (Fig. 3a; r2 value of 0.54, Po0.01). This
linear fit accounts for over half of the observed
variation. This is somewhat surprising given the variety
in the composition of vegetation, the changes in land
use and the history of land use and the differences in
soil types, across the region. With the above-mentioned
sources of variation, the regression analysis of d15N
signatures of C4 vegetation and mean annual rainfall
results in a correlation that is significant (Po0.05), but
that accounts for just 4% of the variation (Fig. 3b).
Discussion
Carbon
Plants adapted to arid environments are expected to
have higher 13C abundance (Farquhar et al., 1989;
Farquhar, 1991). Admundson et al. (1994) found an
isotopic enrichment in the d13C signatures of vegetation
coincident with increasing aridity in the Vizcaino
Desert of Baja California Mexico. They attributed this
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specific leaf area and N, and high discrimination were
selected. Species replacement obscured the effects of
water stress on C discrimination.
Nitrogen
The natural abundance of stable N isotopes in C3
vegetation varies inversely with annual precipitation
across southern Africa with depleted d15N values in the
mesic east and enriched values in the xeric western
regions of the subcontinent. It is interesting to note that
the relationship appears to level off around 1000 mm of
annual precipitation. The regression of C3-d15N against
mean annual rainfall excluding data from sites with
mean annual rainfall above 1000 mm results in an r2 of
0.62; Po0.01. This suggestion of an asymptotic relationship is coincident with the delineation in the literature
between wet and dry savanna/Miombo woodland
ecosystems based on a relationship between precipitation and potential evapotranspiration (Chidumayo,
1987; Chidumayo & Frost, 1996). Again, it is somewhat
surprising that given the subcontinental scale of the
observed isotopic gradient that these results agree with
previous studies, which also found a negative correlation between d15N signatures and mean annual
precipitation using smaller study areas (Shearer et al.,
1978; Heaton et al., 1986; Lajtha & Schlesinger, 1986;
Heaton, 1987; Sealy et al., 1987; Vogel et al., 1990;
Schulze et al., 1991; Evans & Ehleringer, 1993, 1994;
Austin & Vitousek, 1998; Handley et al., 1999).
The relationship between mean annual precipitation
and stable N isotopic signatures of C3 vegetation gives
rise to questions as to what processes, influenced by
climate change, cause isotopic enrichments in the
system, and whether other ecosystem components,
such as soils and aerosols and trace gases deposited
from the regional atmosphere, show similar patterns.
Of special interest are the feedbacks between the
processes and components of the N and C cycles.
Recent papers by Austin & Sala (1999), Schulze et al.
(1999) and Handley et al. (1999) discuss a wide range of
possible mechanisms, some of which may affect d15N
signatures in southern African vegetation. Volatilization
of ammonium, internal recycling of N within the plant
system, and lower microbial activity and reaction rates
may all contribute to the increase of the N isotopic
signatures in C3 plants of dry Southern African
savannas (Hoefs, 1997). Indeed, Handley et al. (1999)
suggest that soil processes play an important role in
plant isotopic signatures.
The hypothesis is posited that water-limited systems
experience increases in their N pools. These pools are
then subject to microbial processes at two levels: (1)
almost constant, background activity at slow rates
r 2004 Blackwell Publishing Ltd, Global Change Biology, 10, 350–358
355
during long, dry spells and (2) intense episodic activity
at high rates during the onset of the southern African
rainy season. However, in the drier end of the study
region, these intense periods of microbial activity
during wet conditions are not sufficiently long enough
to process all the available N, so that the isotopic
signature of the residual N gradually increases over
time. Although much progress has been made on
gaining an insight into the N cycling of these semi-arid
and arid ecosystems (Aranibar et al., 2003), more
information about the isotopic signatures of N in the
soil mineral and organic fractions, as well as microbial
activity, is still needed in order to test these hypotheses.
There is evidence that the natural abundance of 15N is
greater in systems where nutrients are not limiting.
Whatever the mechanism for this increased nutrient
availability (elevation, precipitation, pollution, fire,
etc.), there is a consistent trend towards enriched d15N
of foliage for vegetation occurring in environments that
are not nutrient limited (Shearer et al., 1983; Vitousek
et al., 1989; Högberg, 1990; Garten, 1993; Högberg &
Johannison, 1993; Garten & Van Miegroet, 1994;
Johanisson & Högberg, 1994; Austin & Vitousek, 1998;
Korontzi et al., 2000). It has been argued that the reason
for the elevated abundance of 15N is through the
fractionation associated with enhanced gaseous (through ammonification, denitrification and nitrification)
and liquid exports (Korontzi et al., 2000). Systems that
are not nutrient limited, in essence those that are
nutrient rich, are often referred to as open systems.
They are open to losses of excess N. Systems that are N
limited do not experience losses of this magnitude, and
thus tend to be very tight in terms of nutrient cycling and referred to as closed systems (Nadelhoffer &
Fry, 1994).
In terms of explaining the relationship between the
southern African rainfall gradient and the observed
enrichment in d15N signatures associated with C3
vegetation, the most plausible explanation is that
conditions become drier, and increasingly water limited
rather than nutrient limited. This explanation is
consistent with information on vegetation physiology
and metabolic rates presented by Woodward & Smith
(1994), who found that the subtropical desert bush,
thorn steppe and dry forest ecosystems have greater
rates of maximum photosynthesis and are only limited
by water availability. In essence, the more arid
ecosystems have a greater nutrient availability relative
to those that are wetter in nature. Therefore, these semiarid and arid systems are more open in terms of their N
cycling relative to those that are more humid. The
resulting C3 vegetation in the arid, southern African
ecosystem exhibits enriched d15N signatures relative to
C3 vegetation found in the wetter ecosystems of the
356 R . J . S W A P et al.
region. In areas where water is not a limiting factor, it
appears that nutrients are limiting and the result is a
closed system with more tightly cycled N and less
enriched isotopic signatures of N.
The predominantly anticyclonic atmospheric circulation over southern Africa serves as a mechanism to link
these different ecosystem types physically (Garstang
et al., 1996; Tyson et al., 1996). A plausible source of
excess nutrients, which is nutrients new to the regional
ecosystems, is aeolian transport and deposition of
southern African aerosols. Aerosol and trace gas
emissions from pyrogenic, industrial, biological and
lithogenic sources are transported and redistributed
throughout the region in a re-circulating manner (Tyson
et al., 1996). Atmospheric deposition of these nutrients
has been suggested as a significant source of biogeochemical species for southern Africa (Seely, 1991; Swap,
1996; Garstang et al., 1998). Recent research on the
biogeochemical cycles of the Okavango Delta by
Mubyana et al. (2003) supports the significance of this
atmospheric deposition and finds it to be of the same
order as that suggested by Garstang et al. (1998). There
is evidence that such deposition of aeolian material,
much of which contains biogeochemically important
trace species (Maenhaut et al., 1996; Piketh et al., 1996,
1999, 2000; Swap et al., 1996), has occurred in the semiarid and arid regions of southern Africa for at least the
last 100 kyr bp (Stokes et al., 1997). Given the relatively
recent addition of industrial emissions from the
industrialized areas of South Africa, Zimbabwe, Zambia, Botswana and Mozambique, the importance of such
contributions will most likely increase. The combination of the emission and deposition of aerosols and trace
gases from multiple sources in southern Africa may
contribute to increasing relative nutrient availability in
regions, where water is the limiting factor of vegetation
productivity. This supply of new nutrients to the arid
ecosystems may contribute to the development or enhancement of an open system for the cycling of N that
leads to the enrichment of the system d15N signature.
significant relationship was found for the d13C signatures of C3 vegetation across southern Africa.
The relationship between d15N and mean annual
precipitation is generally consistent with previous,
related studies in that the greater the relative available
N, the more enriched the stable N isotopic signature.
However, there remain several outstanding questions
including: (1) How can a simple relationship between
d15N signatures and mean annual precipitation explain
a significant amount of the observed variance over so
large a region? (2) Why is it that C4 vegetation growing
alongside of C3 vegetation across the subcontinent does
not exhibit such a systematic difference in its d15N
signatures? (3) Why is it that this relationship is found
for vegetation growing in the same soil type and for
vegetation growing across different soil types? (4) What
are the mechanisms for these observations? Our findings suggest a stable N isotopic gradient in C3 vegetation in southern Africa, on a scale larger than previously seen before, which tracks or co-varies with the
long-term southern African rainfall distribution. The
explanation presented is that the relative nutrient availability, which appears to be related to water availability,
dictates the openness of the system and therefore whether
the resulting natural abundance of foliar 15N in the
southern African system is enriched or depleted.
The findings of this paper suggest a relationship, as
evidenced in stable C and N isotopic signatures of
vegetation, which may allow for the conceptualization
of how changes in vegetation dynamics vary with
water availability across regional scales. If this pattern
is indeed robust, global change researchers may have a
means of relating stable N isotopic signatures to
changes in regional rainfall. This implies that it may
be possible to use preserved vegetation samples to gain
an insight into climate dynamics of the past. Such
information should be of use to those researchers
modeling both the dynamics of vegetation and biogeochemistry across this region for present and past
climate conditions.
Conclusions
Acknowledgements
There is a pronounced, inverse relationship at the
subcontinental scale between the mean annual precipitation and the d15N signature of C3 vegetation of
southern African. Over 50% of the variance in the
observed relationship between d15N signatures of C3
vegetation is explained by mean annual precipitation.
This relationship appears to be valid at the subcontinental scale, across regions of large differences in soil
and rainfall characteristics. No relationship was found
between mean annual precipitation and the d13C and
d15N signatures of C4 vegetation, whereas a weak yet
This study was part of the Southern African Regional Science
Initiative (SAFARI 2000). Funding was provided through the
following NASA grants – NAG5-7956, NAG5-7266, NAG5-7862
and NAG5-9357. We thank T. Botha and the Zambian Department of Agriculture for providing rainfall data and P. Rust, J.
Swanepoel, E. Chileshe and A. Mubone for hospitality at the
field sites and K. Nomai for botanical assistance at the Lukulu
site. We thank R. J. Scholes and D. A. Parsons for their valuable
help in the collection of some of the samples, as well as S.
Alleaume, K. Billmark, D. Richardson, T. Smith and P. Desanker
for their critical review and insightful comments on the
manuscript. We also thank the three anonymous reviewers for
their comments, which strengthened this effort.
r 2004 Blackwell Publishing Ltd, Global Change Biology, 10, 350–358
S TA B L E I S O T O P I C PAT T E R N I N A F R I C A N V E G E TAT I O N
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