Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
VOLUMES AND EFFICIENCIES OF WATER-USE WITHIN SELECTED INDIGENOUS
AND INTRODUCED TREE SPECIES IN SOUTH AFRICA: CURRENT RESULTS AND
POTENTIAL APPLICATIONS
M.B. Gush¹, P.J. Dye², C.J. Geldenhuys³ and H.H. Bulcock4
¹CSIR, Stellenbosch
²University of the Witwatersrand, Johannesburg
³FORESTWOOD cc, Pretoria
4
University of KwaZulu-Natal, Pietermaritzburg
Corresponding author: [email protected]
Abstract
South African indigenous forests provide goods and services which are recognised as valuable natural
capital, and are well documented. However, the limited extent of these forests has forced South Africa to
accelerate the expansion of its own plantation forest industry over the last century, using fast-growing
introduced tree species to meet the timber needs of the country. The resultant impacts on streamflow and
water resources have been the subject of considerable research, and first led to regulation of this industry in
1972 due to water-use concerns. Conversely, there is widespread belief that indigenous tree species use
little water and deserve to be planted more widely. However, research and data on the water-use of
indigenous trees and forests has historically been limited. This paper discusses current progress in a Water
Research Commission solicited project on the measurement and modelling of water-use and growth in
selected South African indigenous tree species. Hourly sap flow rates (water use) over a 12-month period
were recorded in a diverse selection of indigenous tree species, while stem circumferences were recorded at
the start and end of the monitoring period, to derive biomass increments. Rates of growth and water-use
were used to calculate water-use efficiency, defined as mass of utilisable wood produced per unit of water
transpired, and were compared to existing data for introduced plantation species. Water-use efficiency in the
indigenous species studied was lower than for introduced plantation species, however overall water-use was
also generally lower in the indigenous species. It was concluded that the relatively lower water-use efficiency
of the indigenous species studied was primarily a consequence of slow growth rates as opposed to high
water-use rates. Implications and potential applications of these findings in alternative forms of indigenous
forestry and sustainable resource use are discussed.
1. Introduction
South Africa is very reliant on its plantations of introduced tree species to meet its pulp and timber needs,
and the benefits of this industry in terms of production, income generation and job provision are undisputed
(Chamberlain et al., 2005). The downside is that these benefits come at some environmental cost, not least
the impact of the industry on water resources (Dye & Versfeld, 2007). Many catchment areas are
consequently now closed to further afforestation, but economic growth and development continue unabated.
Imports and improved productivity are potential solutions to continue meeting the demand for timber and
forest products, but further consideration of the feasibility of expanding indigenous tree resources is also
warranted. With over 1000 species of indigenous trees in the country, South Africa is extremely rich in
natural arboreal diversity (von Breitenbach, 1990). The numerous benefits of indigenous trees and forests, in
terms of the goods and services that they offer, are widely recognised (Lawes et al., 2004; Shackleton et al.,
2007). There are also widespread perceptions that indigenous tree species use less water than introduced
tree plantations, however up to now these have been unsubstantiated. While data from previous studies are
available on the water-use efficiency (WUE) of common introduced plantation species in South Africa
(Olbrich et al., 1996; Dye et al., 2001), information on the water-use of indigenous trees and forests is scarce
1
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
and indirect, and relationships between growth and water-use within indigenous forests have until now not
been investigated in South Africa.
With a growing awareness of the socio-economic and environmental challenges being posed by finite water
supplies in a developing country such as South Africa, there is renewed interest in the possibility of low
water-use forms of forestry. This information is required in order to facilitate sustainable land-use planning
from a hydrological perspective (Dye et al., 2008). New and innovative techniques to quantify the water-use
(transpiration and evapotranspiration) of a range of tree species and forest types are available (Jarmain et
al., 2008), and these may be used to broaden our understanding of forest hydrological processes, and their
associated effects on water resources in this country. Evidence of low and efficient water-use in indigenous
tree species would make them an attractive alternative forestry solution, particularly in catchments that are
water-stressed. The overall efficiency of water-use for biomass production, and the net benefit of the water
used are also important criteria that need to be understood to permit the evaluation of different land use
scenarios. Expanded indigenous forestry systems in South Africa could, under certain conditions, offer
attractive alternative land-use scenarios to plantations of introduced timber species, but the feasibility of their
expansion needs to be thoroughly evaluated from socio-economic and environmental perspectives. A
research project, solicited, managed and funded by the Water Research Commission (WRC) with co-funding
by the Working for Water Programme of the Department of Water Affairs and Forestry, was commissioned to
study the water-use, growth rates and economic value of the biomass of indigenous trees and forests in
South Africa. Research questions posed by the study include:
1. Do indigenous tree species use less water than introduced plantation tree species?
2. Do indigenous tree species use water more efficiently than introduced plantation tree species?
3. Is there scope for the expansion of indigenous tree systems in South Africa?
This is a 6-year project (2009 – 2015), currently nearing the end of its second year (Water Research
Commission, 2010). This research project follows on from an earlier pioneering WRC study, which explored
water-use and growth rates in various indigenous tree systems in South Africa (Dye et al., 2008). This paper
reports on some of the recent findings of the project in terms of comparative growth and water-use studies
between indigenous and introduced tree species, and discusses potential applications of the research.
2. Materials and methods
The objective of the measurement methodology was to determine the water-use efficiency of selected
indigenous and introduced tree species by means of accurate measurements of water-use and growth. To
do that, measurements were conducted for one year to incorporate seasonal variations in, and responses to,
climate. In essence, the sampling strategy employed was to conduct continuous sap flow (transpiration /
water-use) monitoring on an hourly basis, together with annual stem biomass increments at the start and end
of the monitoring period. Hourly measurements of a full suite of climatic variables (solar radiation,
temperature, relative humidity, wind speed and rainfall), together with soil water content measurements in
the A-horizon complemented these.
2.1 Site and species
Two monitoring sites were selected on the Mondi De Magtenburg forestry estate in the Karkloof region of the
KwaZulu-Natal (KZN) midlands in South Africa (S29° 21' 25.2''; E30° 11' 49.3'', alt. 1 148 m.a.m.s.l). The site
has a mean annual precipitation of approximately 1100 mm (Lynch & Schulze, 2006), and is an established
commercial forestry estate, consisting primarily of pine (Pinus patula) and wattle (Acacia mearnsii) stands in
former grassland. The stands selected for this study consisted of:
1. An indigenous Podocarpus henkelii (Stapf ex Dallim.) & Jacks (Yellowwood) plantation.
2. An introduced Pinus patula (Schiede ex Schlechtendal) & Chamisso (Mexican weeping pine)
plantation.
The small (<1ha) P. henkelii stand is situated in a riparian area close to a small stream on the estate, and is
one of very few formally established indigenous tree plantations in South Africa. Due to changes in
ownership of this particular farm details on the stand are limited, and the trees are of an unknown age.
However, by virtue of their size (average tree height of 8 m) and stem diameters (DBH: 15-30 cm) the trees
2
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
are estimated to be roughly 40 years old. The stand is relatively weed free, having achieved canopy closure,
but a few invasive A. mearnsii and Solanum mauritianum (Bugweed) plants are present in the stand. The
trees have not actively been pruned and planting espacement is somewhat irregular but an average distance
between trees (3 m X 3 m) translates into a planting density of approximately 1111 trees per hectare. Sap
flow (water-use) and stem increment (growth) rates were measured in three trees at this site (Table 1). The
study trees were selected after first conducting a survey of the range of stem sizes present in the plantation.
All trees were subsequently assigned to one of 3 stem circumference size classes, with each size class
being represented by one sample tree on which sap flow measurements were conducted. The annual
transpiration totals for the 3 study trees were weighted according to the number of trees in the size class that
they represented, and a single transpiration rate for the plantation was determined, accounting for variations
in stem circumference and water-use within the Yellowwood plantation.
The second site is situated within 50 m of the Yellowwood stand and consists of a Pinus patula stand with
trees of comparable size to the Yellowwood trees (8-10 m high, DBH: 18-25 cm). Tree spacing within this
non-riparian stand is also somewhat irregular but an assumed planting distance of 3.5 m X 3.5 m (a common
espacement for production of saw timber) translated into a planting density of approximately 816 trees per
hectare. Sap flow (water-use) and stem increment (growth) rates were measured in two trees at this site
(Table 1).
Bark thickness of the sample trees were determined by excising bark sections from the stems.
Measurements of sapwood depth, required to determine the insertion depths of thermocouple probes for
water-use measurements, were obtained using a 5 mm inside-diameter increment corer (Haglöf, Sweden).
Cores were subsequently analysed for sapwood depth using measurements of the visual distinction between
lighter coloured sapwood and darker coloured heartwood. Wood density for the two tree species was
determined using mass and volume measurements (Archimedes Principle) on stem-wood samples chiselled
from the trees. Monitoring at both sites commenced on 13 August 2009 and continued for an entire year.
Simultaneous measurements of certain meteorological variables (rainfall, solar radiation, air temperature and
relative humidity) and soil water content in the top 10 cm of the soil profile, took place hourly at the site for
the corresponding period.
Table 1.
Sample tree details
Species
P.henkelii 1
P.henkelii 2
P.henkelii 3
P. patula 1
P. patula 2
Diameter at
breast height
(cm)
14
23
18
20
24
Tree
height
(m)
6.34
7.33
7.00
8.77
10.79
Sapwood
depth
(cm)
5.5
9.5
7.5
8.5
10.0
Wound
width (mm)
Bark width
(mm)
3
3
3
4
4
7
7
7
10
10
Wood
density
-3
(g.cm )
0.468
0.468
0.468
0.380
0.380
2.2 Sap flow measurements
The Heat Pulse Velocity (HPV) technique is an internationally accepted method for the measurement of sap
flow (water-use) in woody plants and has been extensively applied in South Africa (Dye & Olbrich, 1993;
Dye, 1996; Dye, Soko & Poulter, 1996; Dye et al., 1996; Gush, 2008; Gush & Dye, 2009). The heat ratio
method (HRM) of the HPV technique (Burgess et al., 2001) was selected for sap flow measurements in this
study because of its ability to accurately measure low rates of sap flow, expected to be the case in
indigenous tree species. The HRM requires a line-heater to be inserted in the xylem at the vertical midpoint
(commonly 5 mm) between two temperature sensors (thermocouples). Heat pulses are used as a tracer,
carried by the flow of sap up the stem. This allows the velocity of individual heat pulses to be determined by
recording the ratio of the increase in temperature measured by the thermocouples (TCs), following the
release of a pulse of heat by the line heater. For these measurements TC pairs and heater probes were
positioned 80 cm up the main stem of each tree, below the first branches. TCs were inserted to four different
depths within the sapwood to determine radial variations in sap flow. Insertion depths of the TC’s were
calculated after first determining the total sapwood depth for each species, and then spacing the probes
evenly throughout. All drilling was performed with a battery-operated drill using a drill guide strapped to the
tree, to ensure that the holes were as close to parallel as possible. CR1000 data loggers connected to
AM16/32 multiplexers (Campbell Scientific, Logan, UT) were programmed to initiate the heat pulses and
3
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
record hourly data from the respective TC pairs. Cellular phone modems connected to the loggers allowed
remote downloading of data.
Heat pulse velocities derived using the HRM were corrected for sapwood wounding caused by the drilling
procedure, using wound correction coefficients described by Swanson & Whitfield (1981). The corrected heat
pulse velocities were then converted to sap flux densities according to the method described by Marshall
(1958). Finally, the sap flux densities were converted to whole-tree total sap flow by calculating the sum of
the products of sap flux density and cross-sectional area for individual tree stem annuli (determined by
below-bark individual probe insertion depths and sapwood depth). Hourly sap flow values were recorded
from all the trees. Periods of missing data were patched and the complete record was aggregated into daily,
monthly and annual totals. Individual-tree sap-flow volumes (L.annum¯¹) were scaled up to a hectare using
the planting density to also derive sap flow (transpiration) totals in mm-equivalent volumes for the year.
2.3 Stem growth measurements and water-use efficiency
Stem biomass increment surveys were conducted on all the sample trees in conjunction with sap flow
measurements. Initial biomass determination was carried out shortly after the individual trees had been
instrumented with the HPV systems, and the final surveys were performed one year thereafter to incorporate
1-year seasonal variation in both water-use and growth. Biomass increments were calculated for all sample
trees as the difference between the initial and final surveys. Stem circumferences at increasing heights up
the tree were measured. These measurements were converted to volumes by assuming that the stem
consisted of a series of truncated cones with a complete cone at the top. The volumes of individual cones V
(m³) were calculated using (Eq. 1):
V =  (πr2h) / 3.
(Eq. 1.)
where r is radius of the base of the cone (m), and h is height of the cone (m). The volumes of the truncated
cones were calculated using (Eq. 2):
V =  (πh (r12 + r1 r2 + r22)) / 3
(Eq. 2.)
where r1 is radius of the base of the truncated cone (m), r2 is radius of the top of the truncated cone (m), and
h is height of the truncated cone (m).
The individual stem section volumes were totalled for each tree, which allowed for the calculation of stem
volume increase in the year. Stem biomass increments were converted from volume to mass using wood
densities determined for each species (Table 1). In conjunction with the sap flow (water-use) results this
allowed the calculation of WUE, defined as mass of woody biomass produced (g) per unit of water transpired
(L), and results were compared against existing data for indigenous and introduced tree species available
from previous studies (Olbrich et al., 1996; Dye et al., 2001; Gush & Dye, 2009).
3. Results
3.1 Weather
In general, the weather conditions at the site exhibited the seasonal pattern typical of this area (Table 2). The
site experienced an extremely wet spring season initially. The start of the rainy season in this area is usually
in October; however the rains commenced early, with significant amounts falling in August (51 mm) and
September (75 mm) already. The rainfall season was also prolonged, with rainfall of 53 mm as late as April,
and well distributed over the season. A high percentage of rain-days (irrespective of amounts) were recorded
during the year (there were 197 rain days, or 54% of the days, with rainfall). No drought periods were
experienced; however the total precipitation for the year was only 1025 mm, being slightly less than the longterm mean. The high percentage of rain days influenced the other weather variables, so while daily
maximum temperatures regularly peaked above 30°C (with the highest recorded temperature being 41.1°C
on 17 December 2009), monthly means of maximum temperature were mild initially, peaking in February and
March. The frequent overcast conditions also limited the extent to which temperatures cooled at night, with
the lowest recorded temperature being 0.8°C on 12 July 2010. Daily average solar radiation values were
also very consistent initially, increasing gradually to 22.3 MJ.m‫־‬².day‫־‬¹ in February 2010, but dropped
4
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
dramatically in June and July 2010 to daily averages of approximately 7.7 MJ.m‫־‬².day‫־‬¹. Wind speeds were
generally low (±0.5m.s‫־‬¹) peaking at a daily average of 1.4m.s‫־‬¹ in August which is normal.
Table 2.
Monthly values of meteorological variables recorded at the site
Variable / Month
Aug
‘09
Sep
‘09
Oct
‘09
Nov
’09
Dec
’09
Jan
‘10
Feb
‘10
Mar
‘10
Apr
‘10
May
‘10
Jun
‘10
Jul
‘10
Rainfall Totals (mm)
51.0
75.3
179.6
123.6
167.2
190.6
93.1
98.3
53.0
6.1
11.6
11.5
Ave. Daily Max T (°C)
19.6
21.6
21.2
22.7
26.3
27.2
31.0
29.6
27.7
27.1
22.6
21.6
Ave. Daily Min T (°C)
8.9
10.5
11.5
11.8
13.6
14.8
16.2
14.5
12.9
11.5
7.1
7.6
Ave. Daily Solar. Rad.
(MJ.m‫־‬².day‫־‬¹)
16.3
16.7
16.2
17.4
17.9
17.4
22.3
18.2
14.4
12.8
7.7
7.8
Ave. Daily Wind
Speed (m.s‫־‬¹)
1.4
1.3
1.0
1.2
1.1
0.7
0.6
0.4
0.4
0.5
0.8
0.5
3.2 Sap flow
3.2.1 Podocarpus henkelii
Sap flow (water-use) volumes recorded in the Yellowwood trees illustrate consistent transpiration rates yearround (Figure 1). This is attributable to the evergreen nature of this species and the lack of seasonal water
stress due to the riparian location. A slight decline in sap flow rates is evident during the dry season,
particularly around September when leaf exchange takes place. Thereafter, sap flow rates increase towards
mid-summer, as leaf areas, temperatures and available water from rainfall increase. Towards the end of
March sap flow rates gradually decline once more and the cycle is repeated. Individual days of wet, overcast
weather in the summer (when available energy from solar radiation is limited) are characterised by very low
sap flow rates. Daily volumes of water-use in these trees peaked at between 10 and 20 L.day¯¹ during the
summer, declining marginally to between 5 and 15 L.day¯¹ in winter. Annual transpiration totals for the 3
study trees were weighted according to the number of trees in the size class that they represented, and a
single transpiration rate for the plantation was determined, accounting for variations in stem circumference
and water-use, within the Yellowwood plantation (Table 3). Individual tree water-use volumes (L.annum¯¹)
were also scaled up to weighted plantation equivalent depths of water-use (mm) using planting density in the
stand.
Table 3.
Weighting of observed transpiration volumes in 3 Podocarpus henkelii trees relative
to stem circumference variation within the plantation, to determine a representative
total water-use
Stem circ.
size classes
(cm)
No. of trees
in subsample
HPV
tree
no.
HPV tree
stem
circ. (cm)
1-yr Wateruse
(L.tree¯¹)
1-yr Wateruse (mm)
Weighting
≤45 cm
10
1
44.17
1755
194.98
29.41%
45-65 cm
10
3
60.00
3554
394.85
29.41%
≥65 cm
14
2
73.48
5033
559.17
41.18%
3634
403.7
100.0%
Weighted Total for Plantation
5
Figure 1.
6
12-Aug-10
29-Jul-10
15-Jul-10
01-Jul-10
17-Jun-10
03-Jun-10
20-May-10
06-May-10
22-Apr-10
08-Apr-10
25-Mar-10
11-Mar-10
25-Feb-10
11-Feb-10
28-Jan-10
14-Jan-10
31-Dec-09
17-Dec-09
03-Dec-09
50
19-Nov-09
60
05-Nov-09
110
10
100
20
90
30
80
40
70
50
60
60
Daily Rainfall (mm)
70
Transpiration - Podocarpus henkelii Tree1 (L/day)
80
30
90
20
100
10
110
0
120
120
0
110
10
100
20
90
30
80
40
70
50
Daily Rainfall (mm)
60
Transpiration - Podocarpus henkelii Tree 2 (L/day)
70
40
80
30
90
20
100
10
110
0
120
120
0
110
10
100
20
90
30
80
40
70
50
Daily Rainfall (mm)
60
40
Transpiration - Podocarpus henkelii Tree 3 (L/day)
70
80
30
90
20
100
10
110
0
120
Daily Rainfall (mm)
0
Daily Rainfall (mm)
12-Aug-10
29-Jul-10
15-Jul-10
01-Jul-10
17-Jun-10
03-Jun-10
20-May-10
06-May-10
22-Apr-10
08-Apr-10
25-Mar-10
11-Mar-10
25-Feb-10
11-Feb-10
28-Jan-10
14-Jan-10
31-Dec-09
17-Dec-09
03-Dec-09
19-Nov-09
05-Nov-09
22-Oct-09
08-Oct-09
24-Sep-09
120
Daily Rainfall (mm)
12-Aug-10
29-Jul-10
15-Jul-10
01-Jul-10
17-Jun-10
03-Jun-10
20-May-10
06-May-10
22-Apr-10
08-Apr-10
25-Mar-10
11-Mar-10
25-Feb-10
11-Feb-10
28-Jan-10
14-Jan-10
31-Dec-09
17-Dec-09
03-Dec-09
19-Nov-09
05-Nov-09
50
22-Oct-09
08-Oct-09
60
22-Oct-09
08-Oct-09
10-Sep-09
27-Aug-09
13-Aug-09
Daily Sap Flow (Transpiration) (l.day ¹)
40
24-Sep-09
10-Sep-09
27-Aug-09
13-Aug-09
Daily Sap Flow (Transpiration) (l.day ¹)
50
24-Sep-09
10-Sep-09
27-Aug-09
13-Aug-09
Daily Sap Flow (Transpiration) (l.day ¹)
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
Daily sap flow (water-use) volumes (L.day¯¹) recorded in 3 indigenous Podocarpus
henkelii trees
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
3.2.2 Pinus patula
0
110
10
100
20
90
30
80
Daily Rainfall (mm)
40
70
Transpiration - Pinus patula
Tree 1 (L/day)
50
60
60
12-Aug-10
29-Jul-10
15-Jul-10
01-Jul-10
17-Jun-10
03-Jun-10
20-May-10
22-Apr-10
06-May-10
08-Apr-10
25-Mar-10
11-Mar-10
25-Feb-10
28-Jan-10
11-Feb-10
120
14-Jan-10
0
31-Dec-09
110
17-Dec-09
10
03-Dec-09
100
19-Nov-09
20
22-Oct-09
90
05-Nov-09
30
08-Oct-09
80
24-Sep-09
40
10-Sep-09
70
27-Aug-09
50
Daily Rainfall (mm)
120
13-Aug-09
Daily Sap Flow (Transpiration) (l.day ¹)
Sap flow (water-use) volumes recorded in the P. patula trees illustrate a more distinct seasonal transpiration
pattern (Figure 2). The trees are evergreen and clearly transpire throughout the year, however peak flow
rates were recorded in February and March. These are the warmest months of the year, towards the end of
the rainy season when water availability is good. In the middle of the growing season peak sap flow volumes
of 50-100 L.day¯¹ were recorded in these trees, which is five times greater than the volumes recorded in the
yellowwood trees. These dropped to approximately 30 L.day¯¹ in the late winter months.
120
0
110
10
Daily Rainfall (mm)
20
Transpiration - Pinus patula
Tree 2 (L/day)
Figure 2.
30
12-Aug-10
29-Jul-10
15-Jul-10
01-Jul-10
17-Jun-10
03-Jun-10
20-May-10
22-Apr-10
06-May-10
120
08-Apr-10
0
25-Mar-10
110
11-Mar-10
10
25-Feb-10
100
11-Feb-10
20
28-Jan-10
90
14-Jan-10
30
31-Dec-09
80
17-Dec-09
40
03-Dec-09
70
19-Nov-09
50
05-Nov-09
60
22-Oct-09
60
08-Oct-09
50
24-Sep-09
70
10-Sep-09
40
27-Aug-09
80
Daily Rainfall (mm)
90
13-Aug-09
Daily Sap Flow (Transpiration) (l.day ¹)
100
Daily sap flow (water-use) volumes (L.day¯¹) recorded in 2 Pinus patula trees
7
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
Annual transpiration totals for the 2 study trees were averaged to determine a single transpiration rate for the
Pinus patula plantation (Table 4).
Table 4.
Weighting of observed transpiration volumes in 2 Pinus patula trees, to determine a
representative total water-use
HPV tree no.
DBH (cm)
1-yr Wateruse
(L.tree¯¹)
1-yr Wateruse (mm)
Weighting
Pinus patula 1
20.0
9849
804
50%
Pinus patula 2
25.6
16067
1312
50%
12958
1058
100.0%
Weighted Total for plantation
3.3 Stem growth increments and water-use efficiency
Using observed stem growth increments from the respective sample trees, water-use efficiency (defined as
amount of stem biomass produced (g) per unit of water transpired (L)) was calculated for the respective
study trees (Table 5).
Table 5.
Summary of WUE data for indigenous Podocarpus henkelii trees and introduced
Pinus patula trees, as calculated from a mass-based ratio of biomass increment (stem
wood) over water-use
1-yr
Wood
WUE
Stem Volume
Stem Mass
Tree
WaterDensity
(g stem wood / L
Increment (m³)
Increment (g)
use (L)
(g cm¯³)
water transpired)
1755
0.00215
1006.20
0.5733
P. henkelii 1
0.468
P. henkelii 2
5033
0.01088
0.468
5091.84
1.0117
P. henkelii 3
3554
0.00524
0.468
2452.32
0.6900
P. patula 1
9849
0.05157
0.38
19596.60
1.9897
P. patula 2
16067
0.09035
0.38
34333
2.1369
4. Discussion
The water-use efficiency results from this study show the introduced P. patula trees to be 2-4 times more
water-use efficient than the indigenous P. henkelii trees. This trend is consistent with results from earlier
studies (Olbrich et al., 1996; Dye et al., 2001; Gush & Dye, 2009), and supports the hypothesis that
introduced species such as pines and eucalypts use water more efficiently than indigenous species (Figure
3). Evidence is emerging that the growth rate of plants is linked to WUE. Efficiency of resource use within
forests (including water-use efficiency) has been shown to increase as forests increase their productivity and
rate of resource use (Binkley at al., 2004). This hypothesis has been tested and supported by various studies
including Gyenge et al. (2008), who showed that the water-use efficiency (WUE) of a mixed species native
forest (less productive system) was below that of an introduced Pseudotsuga menziesii (Mirb.) Franco
(Douglas-fir) plantation (more productive system), both growing in the same area in Patagonia, Argentina.
Using case studies from Eucalyptus plantations, Binkley et al., (2004) and Stape et al., (2004) have both
demonstrated that more productive sites tend to have higher efficiencies of resource use than less
productive sites, and silvicultural treatments may increase both resource supply and efficiencies of resource
use. This is consistent with results from other studies, which have also shown that WUE is often well
correlated with growth rate (Almeida et al., 2007, Forrester et al., 2010). So more productive systems
(introduced plantations) appear to use water more efficiently than less productive systems (indigenous trees
8
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
and natural forests).This suggests that historical silvicultural practices and tree breeding programmes in
South Africa, which aimed at improving productivity of commercial plantations of introduced tree species
(Verryn, 2000), may have inadvertently already improved WUE in those plantations, and could arguably be
used to improve WUE in indigenous tree species as well.
6
Indigenous Tree Species
Exotic Plantation Species
Mean
Standard Deviation
Water-use Efficiency (g wood. L water -1)
5
4
3
2
1
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
P. patula
P. patula
P. patula
P. patula
P. patula
P. patula
P. patula
P. patula 1 - this study
P. patula 2 - this study
T. orientalis
C. africana
P. falcatus
P. obliquum
O. europaea
B. zeyheri
P. falcatus
P. henkelii 1 - this study
P. henkelii 2 - this study
P. henkelii 3 - this study
0
Species
Figure 3.
A comparison of water-use efficiency data (using 1-year transpiration and stem mass
increment measurements) from Eucalyptus clones, Eucalyptus grandis and Pinus
patula trees (Olbrich et al., 1996; Dye et al., 2001; this study), and selected South
African indigenous tree species (Gush & Dye, 2009; this study)
In terms of volumes of water-use on the other hand, it is noteworthy that compared to existing data, the
indigenous species appear to use consistently less water (on average) than introduced species (Figure 4).
Similar findings have been reported by Kagawa et al. (2009) and Little et al. (2009), who found that the
water-use of native tree species, in Hawaii and Chile respectively, was considerably lower than that of
introduced timber species. This finding is corroborated by similar results obtained by Licata et al. (2008)
amongst gymnosperms in Patagonia, Argentina. They found that an introduced pine species (Pinus
ponderosa Doug. ex. Laws) used significantly more water than a native cypress specie (Austrocedrus
chilensis (D. Don) Pic. Serm. et Bizzarri).
The relatively lower water-use of the indigenous trees used in this study compared to introduced plantation
species has some important implications. One potential application of this benefit could be the planting of
indigenous tree species in riparian zones within commercially afforested areas. These zones are difficult to
manage from grassland conservation and weed control perspectives as they are often narrow riparian
corridors, in which it is dangerous to perform bi-annual burning regimes due to fire-risk within the plantations,
and which are thus often heavily infested with alien invasive plants. The observed low water-use rates of
indigenous tree species relative to introduced timber species make them a viable alternative land-use in
these areas from a hydrological perspective. Other sectors in which the low water-use of indigenous tree
species could play an important role include reforestation programmes, urban greening (e.g. gardens and
street trees) and woodlots in support of rural livelihoods.
9
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
25000
Indigenous Tree Species
Exotic Plantation Species
Mean
Standard Deviation
Transpiration (kg tree¯¹ year¯¹)
20000
15000
10000
5000
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
Eucalyptus clone
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
E. grandis
P. patula
P. patula
P. patula
P. patula
P. patula
P. patula
P. patula
P. patula1 - this study
P. patula2 - this study
T. orientalis
C. africana
P. falcatus
P. obliquum
O. europaea
B. zeyheri
P. falcatus
P. henkelii1 - this study
P. henkelii2 - this study
P. henkelii3 - this study
0
Species
Figure 4
A comparison between 1-yr total sap flow (transpiration), for Eucalyptus clones,
Eucalyptus grandis and Pinus patula trees (Olbrich et al., 1996; Dye et al., 2001; this
study), and selected South African indigenous tree species (Gush & Dye, 2009; this
study – last three bars)
A number of issues are worthy of consideration regarding the potential expansion of indigenous tree
systems. Firstly, the conversion of natural vegetation types to artificial vegetation types, e.g. grassland to
indigenous tree plantations or even "forest" should be cautioned (Theo Stehle, Pers. Comm. Feb 2010).
Plantation owners have to accept responsibility for the natural areas in-between plantations and to manage
these as natural corridors, including natural forests, grasslands, fynbos and wetlands. These remnant natural
ecosystems are regarded as very important to conserve natural habitats and species (fauna & flora). In terms
of the CARA and other laws, riparian zones and wetlands are now managed as natural habitats and it is also
a requirement for FSC certification. Converting these “open” areas into artificial ecosystems would defeat
these objectives. It is wrong to think that all riparian zones need to be treed (forested). In nature this is only
the case where habitat is suitable for forest. Arguments may be made about plantations creating unnatural
habitats through absence of fire and micro-climate effects, however very well managed riparian zones and
wetlands in between introduced plantations do exist, which demonstrates that these can in fact be
successfully managed as natural ecosystems. However, if the riparian zone is continuously weed-infested
and has limited bio-diversity value then an indigenous tree cover could be considered as an alternative
option.
It is worth remembering that indigenous forests are not fire retardants, they just persist in fire ‘refugia’ areas,
and it is still possible for a fire to run through an indigenous forest. This is important when considering
potential planting sites – i.e. outside the fire zone and inside the fire ‘refugia’ areas. Planted and protected
stands of introduced trees, intensive agriculture, road networks, urban development and settlements all
create new fire ‘refugia’ areas, where the planting of indigenous tree production systems could be
considered. However, distinction needs to be made between artificial and natural fire ‘refugia’. Artificial fire
‘refugia’ are ephemeral, such as within highly flammable pine stands, with greater inherent risks associated
with those areas.
10
Gush, M.B., Dye, P.J., Geldenhuys, C.J. and Bulcock, H.H., 2011. Volumes and efficiencies of water-use within selected indigenous and
introduced tree species in South Africa: Current results and potential applications. In: Proceedings of the 5th Natural Forests
and Woodlands Symposium, Richards Bay, 11-14 April.
Nevertheless, there is certainly scope for the expansion of indigenous forest resources in the country (taking
into account habitat requirements and fire risks) in terms of restoration of previously degraded forest areas.
The rehabilitation of forests where they once occurred naturally and then disappeared by some or other
cause, natural or man-made, is highly desirable. In those instances it would be beneficial to limit the water
impacts and so the water-use of the species or combination of species to be re-established would be an
important consideration. Furthermore, in the case of erosion control, appropriate measures to counteract
erosion and rehabilitate eroded lands is a priority, even if suitable tree species have to be planted. In this
case it would be important to understand the relative water use of different component tree species, for use
in those areas to be planted up.
5. Conclusions
Results from the project so far indicate that indigenous tree species exhibit both lower water-use efficiency
and lower overall water-use rates compared to introduced tree species. In other words, the relatively lower
WUE of the indigenous species studied is more a consequence of slow growth rates as opposed to high
water-use rates. As has been the experience with introduced plantation genera, genetic breeding for fast
growth rates and the application of silvicultural practices employed in commercial plantations (e.g. pruning
and thinning) could increase growth rates and improve the water-use efficiencies of indigenous species.
However, the paradox of this is that increased growth rates will in turn lead to increased water-use rates,
thereby negating the benefits of low water-use.
Given the wide range of climatic and site conditions around South Africa, the large number of indigenous
species that are found in this country, and our dwindling water resources, it is important to identify new and
sustainable indigenous forest and woodland production systems. Considering that on the one hand further
afforestation with commercial forest species is now severely restricted due to concerns about reductions in
catchment water yields, while on the other hand significant potential exists to expand indigenous tree
systems, the possibility of low water-use forms of forestry is an attractive proposition. There have already
been some pioneering attempts to increase the area under indigenous trees in the form of experimental
plantations of indigenous trees (Hans Merensky, Komatiland forests), as well as indigenous tree planting /
livelihood programmes such as the Wildlands Conservation Trust “Indigenous Trees for Life” and Sappi
Sandisa Imvelo initiatives. While growth rates are still slow, these initiatives together with other financial
incentives for planting indigenous trees such as carbon sequestration credits and payment for ecosystem
services (PES) schemes look set to expand the area under indigenous trees in South Africa. As this happens
it will be useful to know more about the water-use requirements, growth rates and economic potential of
South Africa’s indigenous tree resources.
Acknowledgements
The research reported on here formed part of a project solicited and initiated by the Water Research
Commission of South Africa (WRC), and was co-funded by the Working for Water Programme of the South
African Dept. of Water Affairs. Their support is gratefully acknowledged. Mondi (particularly Doug Burden) is
thanked for allowing the monitoring of trees on their estate. Technical assistance in the field by CSIR
colleagues Mr. Alistair Clulow and Mr. Vivek Naiken is appreciated.
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