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.

PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 5 1 4 - 4


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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.

S. Roberts et al. / Forest Ecology and Management 143 (2001) 153161 155


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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.



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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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