roberts etal eucalyptus
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Transpiration from Eucalyptus sieberi (L. Johnson)forests of different age
Sandra Robertsa,*, Rob Vertessyb, Rodger Graysona
aDepartment of Civil and Environmental Engineering, CRC for Catchment Hydrology,
The University of Melbourne, Vic. 3010 AustraliabCRC for Catchment Hydrology, CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia
Abstract
We measured transpiration, sapwood areas, and leaf areas of individual trees in three stands of Eucalyptus sieberi, aged 14,
45, and 160 years. Transpiration was calculated from estimates of sap velocity made by the heat-pulse method. Sapwood and
leaf areas were determined by destructive sampling. Leaf area index (LAI), sapwood area and transpiration of the plots in the
three stands were estimated. Mean sap velocity of trees in the three plots in late summer (FebruaryMarch) was not
signicantly different and averaged 9.5 cm h1. Diameter was a good predictor of sapwood area and leaf area of individual
trees. Diameter predicted 94% of the variation in sapwood area and 96% of the variation in leaf area. Stand sapwood area
declined with age from 11 m2 ha1 in the 14 year old forest, to 6.5 m2 ha1 in the 45 year old forest, to 3.1 m2 ha1 in the 160
year old forest. LAI was 3.6, 4.0, and 3.4 for the 14, 45, and 160 year old plots, respectively. Because of the difference insapwood area, plot transpiration declined with age from 2.2 mm per day in 14 year old forest, 1.4 mm per day in 45 year old
forest, to 0.8 mm per day in 160 year old forest. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Transpiration; Eucalyptus sieberi; Sapwood; Leaf area
1. Introduction
In general, when old forest is replaced with
regrowth forest, increases in water yield are observed(Bosch and Hewlett, 1982). However in a few
instances, the replacement of old forest with regrowth
forest following re or harvesting has been shown to
cause yield reductions. This is of concern to watershed
managers when the water supplied from forested
catchments is required for domestic water supply,
irrigation, or the maintenance of environmental
ows.
An example of this yield reduction is found in theEucalyptus regnans (F. Muell.) forests of Victoria,
Australia. E. regnans forests grow in parts of south-
eastern Australia, forming tall open forests with a wetsclerophyll understory in rainfall zones of 900
2000 mm per year. Langford (1976) observed that
after re, stream ows from these catchments initially
increased and then dropped to levels below those
prevailing before the re within 35 years. Kuczera
(1987) constructed curves that described the empirical
relationship between average annual water yield and
forest age in E. regnans forests (Fig. 1). Stream ows
declined by as much as 600 mm per year after burning.
Langford attributed ow reductions to the higher rates
of evapotranspiration of E. regnans regrowth.
Forest Ecology and Management 143 (2001) 153161
* Corresponding author.
0378-1127/01/$ see front matter # 2001 Elsevier Science B.V. All rights reserved.
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Measurements of interception (Haydon et al., 1996;
Vertessy et al., 1998), transpiration (Dunn and Connor,
1993; Vertessy et al., 1998), and descriptions of stand
parameters such as sapwood area (Haydon et al., 1996;
Vertessy et al., 1998), and leaf area index (LAI)
(Haydon et al., 1996; Watson et al., 1997; Vertessyet al., 1998) in the E. regnans forests, point towards
evapotranspiration following an inverse trend to water
yield. Interception and transpiration are at their peak
when water yield is reduced.
Hydrological data from paired catchments in wet
sclerophyll eucalypt forests near Karuah in New South
Wales, Australia has shown a similar trend to that from
E. regnans forests. Fourteen years after harvesting,
treated catchments exhibited water yield depressions
when compared to pre-treatment levels. The yield
depressions were related to regeneration rates andwater availability. They ranged from 70 to 500 mm
less runoff per year than in the old-growth forests. This
forest's structure and rainfall is similar to that of the E.
regnans forests (Cornish and Vertessy, 1999).
It is unclear whether this ageyield effect is evident
in other eucalypt forest types, e.g., uneven-aged or dry
sclerophyll eucalypt forests in Australia. In E. regnans
forests, 48% of the change in runoff is attributable to
differences in transpiration, and 45% is due to rainfall
interception (Vertessy et al., 1998). Therefore, mea-
surements of transpiration and leaf areas in forests of
different age may be useful indicators of potential
changes in water yield. Because there are no catch-
ment studies in dry sclerophyll forests in Australia of
sufcient duration to test for an ageyield relationship,
as a rst step to determine whether there is an age
yield effect in dry sclerophyll eucalypt forests, a sitewas selected in Eucalyptus sieberi (L. Johnson) domi-
nated forest near Eden in New South Wales to measure
transpiration. If stands of various ages in this forest
type exhibit differences in transpiration and leaf area,
then this may be indirect evidence for reductions in
yield following disturbance.
2. Study site
The experiment was conducted in the YambullaState Forest, in southeastern New South Wales
(37829HS, 149835HE) (Fig. 2). The Yambulla State
Forest is dominated by E. sieberi, commonly found
in association with other eucalypts such as Eucalyptus
agglomerata, Eucalyptus muellerana, Eucalyptus
consideniana, Eucalyptus globoidea, Eucalyptus obli-
qua, and Eucalyptus cypellocarpa. The understory is
sparse and xeromorphic, except along drainage lines,
creeks and rivers (Ryan, 1993). Wildres burn this
area with a frequency ranging between 3 and 12 years,
and play an important role in the vegetation dynamics
Fig. 1. Relationship between E. regnans forest age and average annual water yield, Victoria, Australia after Kuczera (1985). Dashed lines
indicate the 95% confidence intervals.
154 S. Roberts et al. / Forest Ecology and Management 143 (2001) 153161
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(Walker, 1981 in State Forests of New South Wales,
1994). Depending on the re's intensity, the unders-
tory and ground vegetation may be destroyed, while
some or all of the eucalypt overstory survive. As a
result, mixed-age stands are common, and the unders-
tory age may differ from the bulk of the overstory.
Mixed-age stands are also created by the integrated
harvesting system used in the Yambulla State Forest.Compartments are harvested using an alternate stand
harvesting system, where half of the stands are cut and
the other half are left for cutting about 20 years later.
Because many stands are of mixed age, about 43% of
trees in stands are retained during harvesting opera-
tions to grow into harvestable logs (State Forests of
New South Wales, 1994).
Rainfall at the experimental site averages 900 mm
per year, but the range is broad (e.g., 4571514 mm
between 1977 and 1983) (Olive and Rieger, 1987).
Rainfall tends to be erratic with poorly dened
seasonality. Much of the rainfall occurs during storms
associated with stationary depressions on the New
South Wales south coast (Moore et al., 1986). The
climate is ``temperate maritime'', with mean tempera-ture minima and maxima varying between 11.5 and
27.38C in summer, and 3.1 and 11.68C in winter (Oliveand Rieger, 1987).
The topography of the experimental area is undu-
lating with slopes generally less than 208. The altitude
is between 370 and 400 m above sea level. The
experimental site's general aspect is NNW.
Soils in the region are derived from adamellite/
granite. They are coarse-sandy in texture, low in clay
content, and low in chemical fertility. The granite is
often only a short distance below the surface (less than1 m in upslope areas, more in lower slope areas), and
forms an impermeable layer. Surface soils are highly
permeable with hydraulic conductivity of up to
450 mm h1 on ridge, upslope and midslope areas.
Surface soils have low bulk densities (0.851.1 g
cm3), well developed macropore structure and 3
10% organic content. In permanently wet zones, bulk
density and hydraulic conductivity are reduced.
Hydraulic conductivity declines to almost zero at
1 m depth, where porosity is low (0.250.35 v/v),
and bulk densities are high (1.752 g cm3). Thesesoil characteristics indicate that rapid subsurface ows
are likely, with overland ows restricted to saturated
zones near drainage lines (Moore et al., 1986). This is
supported by hydrographic data which showed a rapid
runoff response to rainfall which was attributed to
shallow perched water tables (less than 1 m below the
surface) responding quickly to rainfall. Flows peaked
within 2060 min of the maximum rainfall intensity.
On average, 58% of runoff was generated as quick-
ow, depending on the wetness of the catchment
(Moore et al., 1986).This is a distinctly different set of conditions to E.
regnans forests where rainfall generally exceeds
1200 mm, soils are basalt based, res and harvesting
generate even aged stands, and the understory is of the
wet sclerophyll type.
3. Methods
Three adjacent stands ofE. sieberi aged 14, 45, and
160 years were chosen to test for differences in
Fig. 2. Location of the Yambulla State Forest experimental area,
New South Wales, Australia.
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transpiration of stands of different age. The 14 year old
stand regenerated following harvesting and re in
1983, the 45 year old forest resulted from intense re
in 1952, and the 160 year old stand is the result ofwildre ca. 1839. One plot was selected at each site,
following a random inventory of each stand. Areas
where stocking and diameter were closest to the stand
average were selected for the plots. Plot size was
1010 m2 in the 14 year old forest, 2525 m2 in
the 45 year old forest and 5050 m2 in the 160 year
old forest, to include approximately 50 trees. The
diameter of all trees within the transpiration plots
was measured at 1.3 m above ground level.
The heat-pulse method (Swanson et al., 1979;
Cohen et al., 1985; Vertessy et al., 1995) was usedto estimate the heat-pulse velocity of 10 trees in each
plot during FebruaryMarch 1997. The season was
late summer. Rainfall between 1 December 1996 and
31 March 1997 was 262 mm, which is close to the 20
year average of 297 mm during this period. The most
symmetrical trees, greater than 10 cm in diameter and
covering the full range of diameter classes were
selected for heat-pulse velocity estimates. Estimates
of heat-pulse velocity were made at four positions in
each stem at 20 min intervals by sapow sensor probes
(Greenspan, Warwick, Qld) implanted in sapwood at aheight of 1.3 m. Probes were attached to Greenspan
sapow sensor loggers. An increment corer was used
to extract cores from each stem. Staining the cores
with methyl orange identied the width of the sap-
wood band. This combined with bark thickness
allowed the depth of the probe implant to be deter-
mined. Sensors did not run continuously for the whole
period and some units were transferred to other trees
during the experiment to replace failed units. During
February, 30 trees were monitored simultaneously, 10
per plot. Fifteen trees (ve per plot) were monitoredthroughout March. No transpiration measurements
were made in the understory, as the shrubs (Banksia
spp., Hakea spp., and Acacia spp.) were not large
enough to support the sapow sensors. However,
because of their low leaf area, these species were
considered to be unimportant to the water balance
of the site.
To derive sap velocity from the heat-pulse velocity
estimates, corrections are required for sapwood lumen
area and for wound size around the probesets (Swan-
son and Whiteld, 1981). Fractions of wood, water
and air in four sapwood blocks from each tree were
determined gravimetrically. An average wound size of
2.4 mm was estimated (Occhipinti, pers. comm.),
which is greater than the 2.12 mm observed in E.regnans for a similar probe (Dunn and Connor,
1993). There are a number of possible explanationsfor this difference including drilling technique, and
tyloses formation (occlusion formation in wounded
xylem cell lumen), which is rare in E. regnans, while
E. sieberi has sparse to moderately abundant tyloses
formation following injury (Dadswell, 1972). Daily
sap uxes were estimated for each tree using software
supplied with the sapow sensors. Gaps in the daily
sap ux data set were lled using regression relation-
ships between trees in the plots (Table 1). Filling gapseliminate bias in the incomplete data set due to
differences in the weather during different periods
of the experiment (Vertessy et al., 1995).
The Licor PCA 2000 was used to estimate the LAI
of the canopy of each of the plots during February
(Welles and Norman, 1991). Five transects were made
in each of the plots (two running NS, two running E
W, and one diagonally), with readings taken approxi-
mately every metre. The measurements were made
over a 3608 eld of view, at dawn when there was no
direct sunlight on the canopy. Solar radiation under thecanopy was compared to that measured in a clearing
during the same period, and LAI was computed. LAI
was calculated from the four inner rings of the sensor,
conning the eld of view to a 41 m radius, so LAI
was also inuenced by vegetation surrounding the
plots.
In April 1997, the 10 trees monitored in each plot
and all the trees in the 14 year old plot were destruc-
tively sampled. The leaves of each tree were harvested
to obtain leaf mass, and subsamples were collected
to measure leaf area with a Paton Belt Planimeter. Theleaf mass:leaf area ratio was calculated for each
tree. The average ratio of 2.1 m2 kg1 (standard
deviation0.2) was used to calculate the leaf area
of each tree.
Sapwood areas were determined from measure-
ments on discs, by assuming circular tree cross sec-
tions at 1.3 m, and subtracting the heartwood area
from the total wood area. Diameter and sapwood
widths were measured at 416 points on each disc,
depending on its size and regularity. In all but three
cases (trees M47, M60, O47), enough measurements
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year old forest), and 3.6 (14 year old forest). The Licor
PCA 2000 underestimated LAI when compared to the
destructive method. It estimated LAI as 2.1, 1.9, and
2.1 for 160, 45, and 14 year old forests, respectively.
The sapwood areas of individual trees ranged from:
45.2 to 376.5 cm2 in the 160 year old plot, 19.1 to
165.4 cm2 in the 45 year old plot, and 2.8 to 77.9 cm2
in the 14 year old plot (Tables 1 and 2). Mean sapwoodareas were 178.9, 77.7, and 20.8 cm2, respectively.
The relationship between sapwood area and stem
diameter (r20.94) in log domain was used to estimate
the sapwood area for each plot (Fig. 4). Total sapwood
area of each plot was divided by the ground area to
give stand sapwood areas of 3.1 m2 ha1 (160 year old
forest), 6.5 m2 ha1 (45 year old forest), and
11 m2 ha1 (14 year old forest).
The mean daily sap ux of trees aged 160, 45, and
14 years old, respectively, was 49.4, 21.8, and 10.6 l
per day. Mean daily sap ux ranged from 6.5 to 123.4 lin the 160 year old trees, 3.4 to 50.4 l in the 45 year
old trees, and 3.3 to 19.8 l in the 14 year old trees
(Table 1).
Mean sap velocity of individual trees was calculated
by dividing the mean daily ux by sapwood area
(Table 1). For 160, 45, and 14 year old trees, the
averages were 10.8, 9.1, and 8.5 cm h1, respectively.
Two sample t-tests assuming unequal variance show
that the mean velocities of the three sites were not
signicantly different at the 95% level. The overall
average velocity was 9.5 cm h1. Sap velocity mea-surements were not obtained for tree M33, and those
for M43 were regarded as unreliable as there was no
evidence of diurnal variation in the sap velocity
record.
4.2. Transpiration
Plot transpiration was derived from the product of
sapwood area of the plot and average sap velocity of
the trees in the plot. The transpiration of the 160, 45,
and 14 year old plots during the experimental periodwas 0.8, 1.4, and 2.2 mm per day.
Sapwood area varied between the three stands, but
the LAI remained relatively constant, indicating that
in the older plot, the ratio of leaf area/sapwood area
was larger (i.e., more leaf area per unit area of sap-
wood). Ratios for the 160, 45, and 14 year old plots
were 11 000, 6100, and 3000 m2 m2, respectively. As
sap velocity was similar, this implies that water use per
unit leaf area declines with age. Water use per unit leaf
area was 0.22, 0.36, and 0.71 mm m2 per day for the
160, 45, and 14 year old trees, respectively.
Table 2
Descriptive statistics by tree, Yambulla State Forest, New South
Wales, 1997
Age Tree DBH(cm)
Leafmass
(kg)
Estimatedleaf area
(m2)
Sapwoodarea
(cm2)
14 Y2 13.8 4.5 9.5 32.2
14 Y5 11.8 4.7 9.9 27.7
14 Y6 11.9 4.7 9.9 26.5
14 Y8 11.0 1.7 3.6 27.0
14 Y14 13.9 7.5 15.8 31.9
14 Y15 18.4 12.8 27.0 55.7
14 Y16 14.2 4.3 9.1 35.2
14 Y17 14.4 8.6 18.1 45.0
14 Ya 3.9 0.5 1.0 5.0
14 Yb 4.1 0.5 1.0 4.6
14 Yc 3.2 0.2 0.4 3.8
14 Yd 4.0 0.4 0.9 5.7
14 Ye 2.9 0.5 1.0 4.2
14 Yf 4.0 0.4 0.8 4.7
14 Yg 4.6 0.6 1.3 6.2
14 Yh 4.9 0.6 1.4 6.2
14 Yi 2.7 0.2 0.4 2.8
14 Yj 2.7 0.2 0.4 3.2
14 Yk 2.8 0.2 0.5 4.5
14 Yl 4.5 1.1 2.2 6.9
14 Ym 5.7 0.7 1.6 11.1
14 Yn 3.7 0.3 0.7 3.9
14 Yo 5.1 0.9 2.0 10.0
14 Yp 5.5 0.4 0.9 9.1
14 Yq 4.7 1.2 2.5 13.5
14 Yr 2.9 0.4 0.8 4.4
14 Ys 4.2 0.6 1.2 6.8
14 Yt 3.4 0.6 1.3 5.2
14 Yu 5.9 0.5 1.1 15.2
14 Yv 3.6 0.6 1.2 6.6
14 Yw 3.0 0.4 0.9 3.2
14 Yx 6.7 1.0 2.2 8.9
14 Yy 5.1 0.9 1.9 12.8
14 Yz 4.6 0.8 1.7 9.4
14 Yaa 7.6 1.9 4.0 23.5
14 Ybb 5.6 1.4 3.0 10.5
14 Ycc 6.1 1.1 2.4 16.3
14 Tdd 1.4 0.1 0.1 5.0
14 Yee 2.6 0.1 0.1 4.0
14 Yff 5.1 0.6 1.4 11.0
14 Ygg 8.2 2.0 4.2 19.0
14 Yhh 8.8 1.7 3.5 17.6
14 Yii 9.4 2.2 4.6 21.2
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5. Discussion
This study demonstrated strong statistical relation-
ships between stem diameter and sapwood area
(r20.94), and stem diameter and leaf area (r2
0.96). These relationships allow sapwood area and
leaf area to be calculated for individual trees from
diameter measurements, so that plot LAI and sapwood
area can be estimated. Stand sapwood area declined
with age. Sap velocity was shown to be similar
(average 9.5 cm h1) in the plots of different age.
This implies that sapwood area determines transpira-
tion of stands of different ages. LAI measurements did
not demonstrate differences in the three plots of
different age.Daily sap velocity in E. regnans stands aged 50, 90,
150, and 230 years old averaged 11.5, 11.4, 9.9, and
11.8 cm h1, respectively (Dunn and Connor, 1993),
and 11.9 cm h1 at age 15 (Vertessy et al., 1995). Sap
velocity in E. regnans forests of different ages appears
to remain constant. This is also likely to be the case in
E. sieberi forests where there was no signicant
difference in the average daily sap velocities of the
trees of different age.
Because average daily sap velocity does not appear
to be affected by age, this means that sapwood area is a
Fig. 3. Scatterplot of stem diameter and leaf area for the sampled E. sieberi trees, Yambulla State Forest, New South Wales, 1997.
Fig. 4. Scatterplot of stem diameter and sapwood area for the sampled E. sieberi trees, Yambulla State Forest, New South Wales, 1997.
S. Roberts et al. / Forest Ecology and Management 143 (2001) 153161 159
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strong determinant of water use of a forest. If sapwood
area varies with age, water use will also vary. Haydon
et al. (1996), Mencuccini and Grace (1996) and
Vertessy et al. (1998) observed that forest sapwoodarea peaks and then declines with age. In the E.
regnans forest, the overstory sapwood area peaked
to 10.6 m2 ha1 at age 15. It gradually declined to
3.6 m2 ha1 at age 240 (Vertessy et al., 1998). These
values are not dissimilar to those observed in the E.
sieberi forest. The E. sieberi values actually t within
the 95% condence limits of the sapwood-age curve
derived by Haydon et al. (1996).
Estimates of LAI are often used in studies and
modelling of basic ecopyhysiological processes such
as evapotranspiration, transpiration, rainfall intercep-tion, and radiation interception (Gazarini et al., 1990;
Welles and Norman, 1991). Understanding how LAI
varies in forests is important to understand how it may
affect these processes. In studies of LAI changes over
time, researchers have observed that LAI of the forest
canopy rises to a plateau and then gradually declines
with age (Yoder et al., 1994; Haydon et al., 1996;
Mencuccini and Grace, 1996; Watson et al., 1997;
Vertessy et al., 1998). If this is the case, then it could
be expected that changes in evapotranspiration would
be observed as the forests age.The LAI of the E. sieberi overstory (3.44.0) was
quite similar to that observed in E. regnans overstory
(2.83.8), although the total LAI of the E. regnans
forest was greater (3.85.5) (Watson et al., 1997). The
general trend of increasing LAI followed by declines
with age was not contradicted by this study.
The Licor PCA 2000 underestimated LAI in the E.
sieberi forests. Because the canopy of this forest was
clumped rather than randomly distributed, some of the
assumptions of the canopy gap model were breached
so that underestimation of LAI was likely. In addition,the radius of the sensor area assessed was 41 m, for
plots of 1010 m2, 2525 m2 and 5050 m2, so forest
areas outside the plots were also sampled. Battaglia
et al. (1998) found that this instrument tended to
underestimateLAIbyafactorofabout1.5in Eucalyptus
nitens plantations. For this technique to be useful
in open forest, calibration by comparing destruc-
tive estimates or hemispherical photographs of the
canopy with Licor estimates would be necessary.
This study has raised some questions about the
physiology and hydraulic properties of E. sieberi.
The ratio of overstory leaf area to sapwood area
increased as the E. sieberi forest aged, indicating that
older trees are transpiring less water per unit area of
leaf. The reasons for this are unknown, but may berelated to leaf age, the hydraulic properties of the
trees, or water stress. This is an area that needs furtherinvestigation, as the ndings here contrast those of
researchers such as Mencuccini and Grace (1996),
who observed that the sapwood area to leaf area ratio
was higher for larger trees, which is the inverse of the
relationship observed here.
This study has a number of limitations. The number
of trees sampled was small considering the variation in
mean sap velocity between trees (variance was 13.1,
9.5, and 15.7 cm h
1 for the 160, 45 and 14 year oldtrees, respectively). Sap velocity variation within
stems was high, so four sampling points in each tree
may not be adequate to describe the range of velocities
within the stem. Equal weighting was applied to the
heat-pulse velocity estimates when calculating sap
ows, as the irregular thickness of sapwood and bark
and varying radius measurements made it difcult to
pinpoint the exact depth of the thermistors in the
sapwood. In this respect, the software supplied with
the sensors proved to be inadequate as it assumes the
sections to be circular, bark has a single thickness andthe width of the sapwood band is constant.
The results are valid only for the period of the study,
or periods of similar weather and soil moisture con-
ditions. To estimate annual transpiration, long term
measurements of transpiration combined with meteor-
ological data are required.
Components of water balance such as understory
water use, interception, throughfall and groundwater
processes have not been studied in this region. There is
some evidence in other forests that the understory
becomes an increasingly important component offorest water balance as the forest ages. In E. regnans
forests, understory LAI increases as the forest ages
(from 0.4 at age 15 to 2.4 at age 240) (Vertessy et al.,
1998). Mencuccini and Grace (1996) found that
increases in evapotranspiration from bracken and
graminoids in pine forests were likely to counter-
balance decreases in the evapotranspiration of the
canopy with age. So far it has been assumed that
the inuence of the ground and shrub layers on
evapotranspiration in E. sieberi forests is negligible;
but this may not be the case upon closer inspection.
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6. Conclusions
This study has demonstrated that the transpiration
of overstory trees in E. sieberi forests is likely to peakat some time, and then decline with age in a similar
pattern to that observed in E. regnans forests. Unless
other factors, which have not been measured, play a
signicant role in the water balance of this site, this is
likely to provide an evidence for the increase in water
yield from this forest type as it matures.
Acknowledgements
This study was funded by the Cooperative ResearchCentre for Catchment Hydrology. We would like to
thank State Forests of New South Wales for access to
the eld sites and assistance with the destructive
sampling. We also thank Peter Reece and Jamie
Margules from CSIRO for their assistance in collect-
ing transpiration and destructive sampling data, John
Dawson for assistance with site selection, and the
numerous ` leaf pluckers'' for their many hours of
labour.
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