Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:

Tree Physiology 17, 747--756
© 1997 Heron Publishing----Victoria, Canada

Estimating stand water use of large mountain ash trees and validation
of the sap flow measurement technique
R. A. VERTESSY,1,4 T. J. HATTON,1 P. REECE,1,4 S. K. O’SULLIVAN2,4 and R. G. BENYON3,4
1

CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia

2

Monash University, Wellington Road, Clayton, Victoria 3168, Australia

3

Melbourne Water, Box 4342, Melbourne, Victoria 3001, Australia

4

Cooperative Research Center for Catchment Hydrology, GPO Box 1666, Canberra, ACT 2601, Australia

Received November 13, 1996

Summary Mountain ash (Eucalyptus regnans F.J. Muell.)
forest catchments exhibit a strong relationship between stand
age and runoff, attributed inter alia to differences in tree water
use. However, the tree water use component of the mountain
ash forest water balance is poorly quantified. We have used the
sap flow technique to obtain estimates of daily water use in
large mountain ash trees. First, the sap flow technique was
validated by means of an in situ cut tree experiment. Close
agreement was obtained between the sap flow estimate of water
use and the actual uptake of water by the tree from a reservoir.
Second, we compared the variability in sap velocity between a
symmetric and an asymmetric tree by using multiple sap flow
loggers. In the symmetric tree, velocity was fairly uniform
throughout the xylem during the day, indicating that accurate
sap flow estimates can be obtained with a minimal number of
sampling points. However, large variations in sap velocity were
observed in the asymmetric tree, indicating that much larger

sampling sizes are required in asymmetric stems for an accurate
determination of mean sap velocity. Finally, we compared two
procedures for scaling individual tree sap flow estimates to the
stand level based on stem diameter and leaf area index measurements. The first procedure was based on a regression between stem diameter and tree water use, developed on a small
sample of trees and applied to a stand-level census of stem
diameter values. Inputs to the second procedure were tree water
use and leaf area of a single tree and the leaf area index of the
stand. The two procedures yielded similar results; however, the
first procedure was more robust but it required more sampling
effort than the second procedure.
Keywords: Eucalyptus regnans, flow variability, forest catchment, runoff, sap velocity, stand age.

Introduction
The city of Melbourne’s water is harvested from 1550 km2 of
forested catchments in the Central Highlands of Victoria, Australia. Mountain ash (Eucalyptus regnans F.J. Muell.) trees
cover about half this area, but the area that they occupy yields
80% of the stream flow because it receives higher rainfall. It

has been established that re-growth mountain ash forests yield
significantly less water than old-growth stands, partly because

of differences in forest density and structure (Langford 1976,
Kuczera 1987, Jayasuriya et al. 1993, Vertessy et al. 1994).
However, the tree water use component of the mountain ash
forest water balance remains poorly quantified. The main reason for this is that the mountain ash forests are very tall (up to
90 m) and grow in steep and variable terrain, thereby making
micrometeorological techniques difficult to apply. The recent
development of the sap flow measurement technique has
helped overcome this problem (cf. Dunn and Connor 1993,
Vertessy et al. 1995).
To date, two major sap flow measurement experiments have
been carried out in mountain ash forest. Dunn and Connor
(1993) found that the mean daily sap velocity of multiple trees
within mountain ash stands of different ages did not vary
significantly with respect to age when averaged over a 5--6month period. They estimated mean daily sap velocities of
0.032, 0.032, 0.027 and 0.033 mm s −1 for 50-, 90-, 150- and
230-year-old mountain ash stands, respectively, over spring-summer, and attributed differences in water use amongst the
different aged stands to differences in sapwood area. The
sapwood values for the four age classes were 6.74, 6.09, 4.23
and 4.04 m2 ha −1, respectively. Vertessy et al. (1995) demonstrated that mean daily sap flow in nineteen 15-year-old mountain ash trees was closely related to the stem diameters and
canopy leaf areas of the trees. They argued that similar relationships should be derived for older stands to help elucidate

the factors underlying the stand age--water yield relationship
in mountain ash forests. Vertessy et al. (1995) also found that
mean daily sap velocity (measured over a month in spring)
varied considerably amongst trees in the stand, whereas the
stand mean was 0.033 mm s −1. This finding supports the
contention of Dunn and Connor (1993) that stand mean sap
velocity does not vary significantly between mountain ash
forests of different ages when measured over a suitably long
period.
Although these two studies have shown that the sap flow
measurement method can be used to quantify the mountain ash
forest water balance more accurately than before, there are

748

VERTESSY ET AL.

several aspects of the technique that require further evaluation.
The specific objectives of this study were: (1) to provide a
comprehensive validation of the sap flow measurement

method in mountain ash trees; (2) to determine sap velocity
variability in mountain ash trees for use in the development of
an optimal strategy for estimating mountain ash stand water
use with the sap flow method in these forests; (3) to evaluate
the relationships between sap flow and scalar variables such as
stem diameter and leaf area for older stands of mountain ash;
and (4) to optimize the method for scaling sap flow estimates
for individual trees to an areal basis. Vertessy et al. (1995) used
stem diameter measurements to scale up individual tree water
use to the stand level, but it is unclear whether this is the
optimum scaling method for mountain ash forests. We also
discuss the contribution of our findings to the elucidation of the
mountain ash forest age--water use relationship.

Materials and methods
Site description
All field measurements were made in an even-aged 56-yearold mountain ash stand that had regenerated naturally from
seed after a wildfire destroyed the old-growth mountain ash
forest in 1939. The stand is located at Black Spur in the North
Maroondah Experimental Area, now part of the Yarra Ranges

National Park, approximately 100 km NE of the city of Melbourne, Australia (145°38′ E, 37°34′ S), at an elevation of
750 m. Mean annual rainfall is 1660 mm, evenly distributed
throughout the year with a slight spring maximum. Mean
monthly temperatures vary between 3--5 °C in winter and
16--17 °C in summer. Mean monthly solar radiation for the site
varies between 120 MJ m −2 in winter and 630 MJ m −2 in
summer. Soils in the area are deep krasnozems of high permeability (up to 10 m day −1) and high water-holding capacity
(soil water storage exceeding 5000 mm) (Langford and
O’Shaughnessy 1977, Davis et al. 1996).

A triangular wooden stand was constructed around the tree
to support a water reservoir at 1.5 m above ground level. The
reservoir was fabricated from a Class-A evaporation pan; we
cut an 80-cm diameter hole in its center and fitted the inner
hole with a rubber membrane (Figure 1). The pan was cut into
two halves; these were placed on the stand around the stem of
the tree, riveted back together and sealed. The rubber membrane was then strapped to the trunk of the tree and coated with
silicon sealant. The whole internal surface of the pan and the
tree trunk (up to the water line) were then covered with a thick
coat of bituminous waterproof paint. When fitted around the

tree, the reservoir had a storage capacity of 210 l.
Once the bituminous paint had dried, the reservoir was filled
with water and shown to be water-tight. We used a chain saw
to cut a 5-cm-deep groove around the entire circumference of
the tree stem, below the reservoir water line. In so doing, we
cut through the xylem into the heartwood, ensuring that all
subsequent water uptake would occur from the reservoir. The
cutting of the stem was done under water to avoid air entry to
the xylem, and was carried out in the early morning at the time
of maximum daily xylem water potential. The water in the
reservoir was replenished hourly (except during the late eve-

Sap flow nomenclature
We have adopted the unified nomenclature for sap flow measurements proposed by Edwards et al. (1997). The term sap
velocity is used to denote the speed of water movement through
the stem, expressed on a total sapwood area basis. The term sap
flow denotes the volume of water moving through the stem per
unit time (i.e., the product of sap velocity and sapwood area).
Sap velocity (vs) is expressed in units of mm s −1 and sap flow
(Q) is expressed in units of m3 day −1.

Cut tree experiment
We tested the sap flow measurement method by means of a cut
tree experiment (sensu Landsberg et al. 1976, Roberts 1977,
Olbrich 1991), performed on a radially uniform tree of 69 cm
dbh (diameter at breast height over bark) and 65 m height. The
average sapwood thickness of the tree was 32 mm, resulting in
a total sapwood area of 396 cm2. The crown leaf area, determined by destructive sampling after the experiment, was
163 m2.

Figure 1. Schematic of the water reservoir, mounted around the stem
of a tree. Also shown is the installation of sap flow sensors above and
below the cut.

TREE PHYSIOLOGY VOLUME 17, 1997

ESTIMATING STAND WATER USE

ning and early morning when water uptake was negligible) and
the volume of water used was recorded. A steel-point datum
was used to ensure that the reservoir was always refilled to the

same level.
Tree water use was estimated by means of four sap flow
loggers (Greenspan Technology, Warwick, Queensland), each
equipped with four sensors. Each logger sampled sap velocity
at four points in the xylem at breast height level, at 20-min
intervals. A fifth logger was installed beneath the reservoir in
an upside-down configuration to check for the possibility that
water from the pan was moving down the ring-barked xylem;
this was shown not to occur. Monitoring of sap flow commenced at 1000 h on April 4, 1995, and the cut was made at
0800 h on April 10, 1995. Monitoring concluded at 1200 h on
April 13, 1995.
Flow variability experiment
We compared variability in sap velocity in a symmetric and an
asymmetric mountain ash tree by installing four sap flow
loggers in each tree and examining the sap velocity fields in
detail. The variance of mean sapwood thickness at breast
height was estimated in a sample of 41 trees by taking 4--6
increment cores from each tree, spaced equidistantly around
the bole. Based on a χ2 distribution with 40 degrees of freedom, the mean variance was 5.2 mm2 with 99.5% confidence
limits of 3.2 and 10.3 mm2. Tree 12 was the most symmetric

tree, with sapwood thicknesses of 31, 32, 32 and 34 mm and a
variance of 1.6 mm2. The most asymmetric tree (Tree 8) had
sapwood thicknesses of 12, 20, 31 and 43 mm and a variance
of 181.6 mm2. The distribution of ranked variance values for
the 41 sample trees is shown in Figure 2.
For Trees 8 and 12, we examined daytime sap velocities
gathered over several days of sampling. For each tree, sap
velocity was determined at 20-min intervals by 16 sensors,
distributed among four sap flow loggers, installed at breast
height. We defined daytime values as those recorded during the
diurnal period when heat pulse velocities exceeded the threshold sensitivity of the device (0.0155 mm s −1). In the symmetric
tree, the four sensors of each of the four loggers were im-

Figure 2. Distribution of ranked values of variance in sapwood thickness for 41 sample trees in the experimental plot.

749

planted at depths of 8, 13, 20 and 25 mm into the sapwood, so
that the four logger estimates could be compared directly. The
temporal linear correlations among the 16 sets of sap velocities

were determined, resulting in a correlation matrix relating sap
velocity measured by each sensor to that measured by each
other sensor. Because sapwood thickness varied significantly
in the asymmetric tree, the four sensors of each of the four
loggers were implanted at varying depths. Because of limited
availability of loggers, the sap flow measurements in the two
trees were not concurrent.
The sap velocity variability within both trees was examined
for the time of peak flow when sap velocity profiles across the
xylem are most developed (Hatton and Vertessy 1989). For
each tree, the coefficient of variation in sap velocity was
estimated by Monte Carlo sampling with replacement from the
original data set of 16 point estimates of sap velocity. This
involved randomly sampling n = 2...15 values from the pool of
16 point estimates, and computing the coefficient of variation
in the sample; whenever a value was sampled it was immediately returned to the pool so each value could potentially be
chosen multiple times in any given sampling run. For each
sample n, the procedure was repeated 10,000 times, resulting
in a statistically robust estimate of the mean coefficient of
variation in sap velocity for a range of sample sizes. In the
symmetric tree, we sampled sap velocities at the time of peak
flow during both the pre- and post-cut periods. An analysis of
variance was employed to test the null hypothesis that there
were no significant differences in sap velocity among the four
sampling depths in each tree or among sap flow loggers at the
time of sampling.
Stand water use measurements
To determine stand water use, we related mean daily sap flow
of 10 trees to scalar variables such as stem diameter and leaf
area, replicating the study of Vertessy et al. (1995) that was
carried out in a younger mountain ash stand. A 70 × 70 m plot,
containing 94 mountain ash trees with dbh values ranging
between 26.6 and 97.1 cm, with a median of 57.6 cm, was
marked out. Ten sample trees were selected from the plot for
detailed examination; these had dbh values ranging between
39.5 and 89.3 cm, with a median of 62.8 cm. The sample trees
were thus slightly skewed toward the large end of the tree
population in the plot.
Sap flow was measured over a 50-day period in the 10 sample trees with five sap flow loggers. Two of the loggers were
assigned to a ‘control’ tree (Tree 9) for the whole sampling
period, whereas the remaining three loggers were ‘roamed’
amongst the other nine trees, usually for a 10-day period at a
time. In every case, the sap flow loggers were installed at breast
height. Daily sap flow totals in the control tree were regressed
against simultaneously gathered values for the roaming loggers (cf. Vertessy et al. 1995), enabling us to estimate the water
use of all 10 sample trees over the entire 50-day measurement
period.

TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.com

750

VERTESSY ET AL.

Destructive sampling
After sap flow measurements were completed, each of the
10 trees was felled to permit accurate determination of height,
sapwood area at breast height, and canopy leaf weight. The
height and leaf mass of Tree 10 could not be sampled because
it became caught up in the canopy of a neighboring tree after
felling. As a replacement, Tree 11 was felled and sampled in
the same manner as the other trees, though sap flow was not
monitored in this tree. Tree 12 (the cut tree) was also sampled
in a similar manner but the sap flow data obtained from this
tree were not synchronous with the data collected for the other
nine trees. Hence, a full data set is only available for nine trees.
A stem disc was taken at breast height in each tree to enable
accurate determination of sapwood area, based on color differences. After the stem disc had been exposed to air for several
days, the heartwood became brown and the sapwood remained
straw colored. Sapwood thickness was measured at several
points around the bole with a ruler.
Leaf weight was converted to leaf area by measuring the leaf
area/leaf weight ratio of a subset of leaves from each tree. A
total fresh leaf mass of 2.56 kg (sampled from 11 trees) was
fed through a planimeter and yielded a leaf area of 6.25 m2.
This equates to a fresh leaf area to weight ratio of 2.44 m2 kg −1,
which is similar to the value of 2.41 m2 kg −1 determined by
Vertessy et al. (1995) for a 15-year-old mountain ash stand.

Results

Figure 3. Results of the 3-day cut tree experiment, commencing at
1000 h, April 10, 1995. (a) Comparison of hourly estimates of sap flow
and observed water loss from the reservoir. (b) Hourly variation in
solar radiation (Rs). (c) Hourly variation in air temperature (T).

Cut tree experiment
Hourly values of water loss from the reservoir over the 3-day
experiment are plotted in Figure 3 along with the mean of the
four hourly sap flow estimates. Hourly values of solar radiation
and temperature during the experiment are also shown; unfortunately, the humidity sensor malfunctioned during this period.
The first half of the experiment was characterized by cool and
cloudy weather, whereas the second half was characterized by
warm, sunny conditions. A total of 0.167 m3 of water was taken
up from the reservoir, whereas the estimates from the four sap
flow loggers were 0.156, 0.164, 0.162 and 0.162 m3, with a
mean of 0.161 m3. All four loggers exhibited almost identical
temporal patterns of sap flow. Throughout the 3-day measurement period, the mean hourly sap flow estimates of water
uptake were virtually identical to the directly observed values
of water uptake. It was only during times of low flow (at night)
that the sap flow estimates were in error. Because of a lack of
sensitivity in the loggers, low flows (< 0.05 m3 day −1) were
recorded as no flow. This explains why the total sap flow
estimates are slightly lower than the observed water uptake by
the tree.
Flow variability experiment
At 1300 h on April 12, the time of peak flow in the 3-day
post-cut period, sap velocity recorded by the 16 sensors in the
symmetric tree (Tree 12) varied between 0.200 and 0.231 mm
s −1 (Table 1). At the time of peak flow, there was no significant
difference in sap velocity among sapwood depths sampled or

among the loggers (ANOVA, P > 0.05). Sap velocity variability in the symmetric tree was also statistically nonsignificant
(ANOVA, P > 0.05) before the cut, varying between 0.018 and
0.021 mm s −1 at the time of peak flow at 1200 h on March 21
(Table 1). Absolute sap velocity values in the symmetric tree
were much lower before the cut than after the cut, partly
because of weather conditions prevailing during the sampling
period, but mainly because the effect of flow resistance
through the root system was eliminated after the cut. In the
asymmetric tree (Tree 8), there was significant variability in
sap velocity amongst the 16 sensors with velocities ranging
between 0.012 and 0.099 mm s −1 at the time of peak flow,
1400 h on March 16 (Table 1).
Figure 4 shows the relationships between sample size and
the coefficient of variation of mean sap velocity, estimated by
the Monte Carlo sampling approach, for the asymmetric tree
and for the symmetric tree both before and after the cut. We
compared the three curves with the relationship determined by
Hatton et al. (1995) for E. populnea F.J. Muell., a semi-arid
woodland species noted for its high sap velocity variability.
The cut had a minimal effect on sap flow variability within the
symmetric tree. Based on four sensors, the normal configuration per tree in our study, the coefficient of variation in the
symmetric tree was 2.0%, compared to 25.4% in the asymmetric tree. By contrast, Hatton et al. (1995) determined a coefficient of variation of 31.4% (based on four sensors) in a typical
E. populnea tree.

TREE PHYSIOLOGY VOLUME 17, 1997

ESTIMATING STAND WATER USE

751

Table 1. Variation in peak sap velocities within the most symmetric tree (Tree 12) before and after the cut, and within the most asymmetric tree
(Tree 8).
Tree no./

Sensor

Sampling date/
Time

Logger 1

Logger 2

Logger 3

Logger 4

Depth
(mm)

Velocity
(mm s −1)

Depth
(mm)

Velocity
(mm s −1)

Depth
(mm)

Velocity
(mm s −1)

Depth
(mm)

Velocity
(mm s −1)

Tree 12
(after cut)
12/4/95
1300 h

1
2
3
4

8
13
20
25

0.213
0.205
0.200
0.222

8
13
20
25

0.223
0.226
0.214
0.231

8
13
20
25

0.210
0.219
0.207
0.225

8
13
20
25

0.212
0.216
0.204
0.211

Tree 12
(before cut)
21/3/95
1200 h

1
2
3
4

8
13
20
25

0.019
0.020
0.018
0.019

8
13
20
25

0.021
0.020
0.019
0.021

8
13
20
25

0.018
0.020
0.019
0.020

8
13
20
25

0.021
0.019
0.018
0.020

Tree 8
16/3/95
1400 h

1
2
3
4

10
15
25
30

0.038
0.059
0.066
0.043

3
8
13
18

0.018
0.042
0.012
0.016

7
12
18
23

0.099
0.093
0.093
0.099

2
7
10
15

0.038
0.078
0.080
0.041

Figure 4. Relationship between sample size and the coefficient of
variation in sap velocity.

For the symmetric tree, the 128 pair-wise coefficients of
determination (r 2) among the 16 point estimates of sap velocity (collected over 3 days after the cut) ranged between 0.83
and 0.98, with a mean of 0.94 (n = 72). In combination with
the results from the sampling variability analysis, which demonstrated that, on average, sensors placed throughout the sapwood yielded similar estimates of sap velocity at peak flow,
these results indicate that sap velocity was uniform throughout
the xylem of the symmetric tree during the day. This was
clearly not the case in the asymmetric tree.
Destructive sampling
Measurements of tree diameter, height, sapwood area and leaf
area for each of the sample trees are listed in Table 2. Tree
heights varied between 44 and 65 m, leaf areas varied between
45.1 and 404.8 m2, and sapwood areas varied between 209 and
843 cm2.
Observed relationships between stem diameter, leaf area and
sapwood area are shown in Figure 5, relative to similar rela-

tionships reported for 15-year-old mountain ash trees by
Vertessy et al. (1995). Power function-based regressions indicated that stem diameter explained 66% and 69% of the variation in leaf area and sapwood area, respectively. A simple
linear regression indicated that sapwood area explained 84%
of the variation in leaf area. Although these correlations were
not as strong as those reported by Vertessy et al. (1995) for
15-year-old mountain ash trees, they were all statistically significant at the 0.001 level, according to the two-tailed significance test for correlation coefficients (Rosner 1990).
Application of the equation shown in Figure 5a to the stem
diameter census for all 94 trees in the 70 × 70 m plot yielded
an estimate of the total leaf area of the plot of 13,980 m2, which
is equivalent to an LAI of 2.85. This LAI is lower than normal
for mountain ash forests of this age class (Watson and Vertessy
1996), probably because of the ridge-top position of the site,
which is characterized by higher insolation and drier soil
conditions than is typical for such forests. We also estimated
the plot LAI independently with an LAI-2000 Plant Canopy
Analyzer (Li-Cor Inc., Lincoln, NE) (Welles and Norman
1991), and obtained a value of 2.6.
Sap flow measurements
Sap flow in the control tree (Tree 9) varied considerably over
the 50-day measurement period, fluctuating between 0.061
and 0.323 m3 day −1, averaging 0.172 m3 day −1, and totaling
8.601 m3 (Figure 6). Over the first 15 days of the experiment,
because conditions were cool and overcast, sap flow rates were
low, averaging about 0.1 m3 day −1. Warmer and clearer conditions prevailed over the next 20 days and sap flows of 0.3 m3
day −1 were attained on several days. The last 15 days of the
experiment were characterized by mainly overcast and cooler
conditions, and sap flow declined accordingly.
Daily sap flow values in the control tree (Tree 9) correlated
well with simultaneously measured values in Trees 3, 6, 7, 8
and 11; the coefficients of determination (r 2) varied between
0.66 and 0.99 (Table 3). Daily sap flow in Tree 2 was weakly

TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.com

752

VERTESSY ET AL.

Table 2. Stem diameter, height, sapwood area, leaf area, sap velocity and sap flow for the 12 sample trees at Black Spur. A dash denotes a value
that could not be computed.
Tree

Diameter
(cm)

Height
(m)

Sapwood
area
(cm2)

Leaf
area
(m2)

Mean daily
sap velocity
(mm s −1)

Mean daily
sap flow
(m3 day −1)

Min. daily
sap flow
(m3 day −1)

Max. daily
sap flow
(m3 day −1)

Total sap
flow
(m3)

1
2
3
4
5
6
7
8
9
10
11
12

60.6
39.5
79.2
62.8
64.3
56.0
89.3
73.0
83.3
46.2
59.5
69.0

55.8
44.1
60.0
57.7
56.7
54.3
57.6
51.8
54.9
49.6
-65.0

357
213
843
534
579
390
618
452
695
209
403
396

197.8
45.1
404.8
296.5
195.8
111.3
330.4
206.8
263.5
66.1
-163.0

--0.026
--0.022
0.053
0.041
0.029
-0.029
--

--0.191
--0.075
0.285
0.159
0.172
-0.103
--

--0.077
--0.042
0.091
0.060
0.061
-0.042
--

--0.328
--0.123
0.553
0.263
0.323
-0.160
--

--9.54
--3.77
14.25
7.96
8.60
-5.13
--

Figure 5. Relationships between stem diameter (DBH), leaf area, sapwood area and mean daily sap flow among 11 mountain ash trees at Black
Spur (denoted by dots) and nineteen 15-year-old mountain ash trees (denoted by plus signs) (after Vertessy et al. 1995). (a) Stem diameter versus
leaf area. (b) Stem diameter versus sapwood area. (c) Sapwood area versus leaf area. (d) Stem diameter versus mean daily sap flow.

but significantly (P = 0.05) correlated with that in the control
tree; however, we rejected the data from Tree 2 because the sap
flow rates appeared anomalously high in relation to the leaf
area of the tree (i.e., more than twice that of the other trees).
Correlations between daily sap flow in the control tree (Tree 9)

and Trees 1, 4 and 5 were poor and not statistically significant
at the 0.05 level, probably because of poor probe implantation
in these three trees. For this reason we did not estimate 50-day
sap flow totals for these three trees. Water use was not measured in Tree 10, and measurements of water use of the cut tree

TREE PHYSIOLOGY VOLUME 17, 1997

ESTIMATING STAND WATER USE

Figure 6. Variation in total daily sap flow in the control tree (Tree 9)
over the 50-day sampling period.

Table 3. Coefficients of determination (r 2) for daily sap flow in nine
mountain ash trees, relative to the control tree (Tree 9). The letter n
denotes the number of simultaneous days of recording. No simultaneous water use data were available for Trees 10 and 12.
Tree

r2

n

F statistic

1
2
3
4
5
6
7
8
11

0.24
0.39
0.66
0.31
0.06
0.98
0.99
0.83
0.93

15
13
15
12
6
9
9
4
24

0.05 < P < 0.1
0.01 < P < 0.05
P < 0.001
0.05 < P < 0.1
P > 0.01
P < 0.001
P < 0.001
0.05 < P < 0.1
P < 0.001

(Tree 12) was made over a different period than those of the
other trees. Hence, 50-day sap flow statistics were only compiled for six of the 12 sample trees.
Mean daily sap flow over the 50-day period varied between
0.075 m3 day −1 (Tree 6) and 0.285 m3 day −1 (Tree 7) (Table 2).
Stem diameter explained 89% of the variation in mean daily
sap flow values in Trees 3, 6, 7, 8, 9 and 11 (Figure 5d), which
is slightly higher than observed for young mountain ash trees
by Vertessy et al. (1995). For n = 6, an r 2 value of 0.89 is
statistically significant at the 0.001 level (Rosner 1990). Mean
sap velocity in these trees varied between 0.022 and 0.053 mm
s −1, and averaged 0.033 mm s −1.

753

area of the plot by the leaf area of the reference tree (263.5 m2)
gave a value of 48.349, which we refer to as the ‘leaf area ratio.’
The mean, minimum and maximum daily sap flow values for
the ‘reference tree’ were multiplied by the leaf area ratio to
calculate the mean, minimum and maximum transpiration
rates for the whole plot. This method assumes that there is a
linear correlation between leaf area and sap flow in the stand.
This is probably valid for stands with LAI values of 3 or less;
beyond this value, self-shading and shading by adjacent trees
would tend to break down the relationship (Waring and Silvester 1994). Although Yoder et al. (1994) and Mencuccini and
Grace (1996) showed that the relationship was not always
linear across age gradients of particular forest types, Vertessy
et al. (1995) demonstrated linearity across a broad tree size
gradient in a young even-aged mountain ash forest. Linearity
was also observed in Pinus radiata D. Don trees of various
sizes by Teskey and Sheriff (1996).
Estimates, by the two methods, of minimum, mean and
maximum daily water use of all 94 trees in the plot are compared in Figure 7. Based on stem diameter as the scalar
(Method 1), the estimated values were 0.8, 1.9 and 3.1 mm
day −1, respectively. Based on leaf area ratio as the scalar
(Method 2), the estimated values were 0.6, 1.7 and 3.2 mm
day −1, respectively. If a plot LAI of 2.85 was assumed (as
computed by the equation in Figure 5a), the leaf area ratio
increased to 53.055, and the minimum, mean and maximum
daily water use estimates increased to 0.7, 1.9 and 3.5 mm
day −1, respectively. Total water use by the mountain ash trees
in the plot over the 50 days of sampling was estimated to be
95 mm by Method 1, and 85 or 95 mm by Method 2, depending
on which leaf area ratio value was used.

Discussion
Cut tree experiments have been used by several workers to
validate sap flow estimates of eucalypt tree water use. Doley
and Grieve (1966) applied an early version of the sap flow
method (based on surface heating of the stem) to several small
E. marginata J. Donn. ex Sm. Jarrah. trees and found that sap
flow estimates equated to about half the actual tree water
uptake. In an application of the technique to several E. regnans

Stand water use estimates
Two methods were used to extrapolate our estimates of daily
sap flow in individual trees to an areal basis. In Method 1, we
took the relationship between stem diameter and mean daily
sap flow (Figure 5d) and applied this to the measured dbh of
all 94 trees in the 70 × 70 m plot. This calculation was repeated
for minimum and maximum sap flow values; earlier analyses
indicated that stem diameter explained 84 and 91% of the
variation in minimum and maximum sap flow rates, respectively. In Method 2, we related the leaf area of a ‘reference tree’
(Tree 9) to the Plant Canopy Analyzer estimate of the plot LAI
(2.6). Multiplying the LAI by the plot area of 4,900 m2 yielded
a total leaf area for the plot of 12,740 m2. Dividing the total leaf

Figure 7. Various estimates of minimum, mean and maximum daily
stand water use over the 50-day sampling period.

TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.com

754

VERTESSY ET AL.

saplings, Dunn and Connor (1993) obtained sap flow estimates
that were within 5% of actual tree water use. Hatton et al.
(1995) reported a 15% difference between estimated and actual water use in an E. populnea tree, and Barrett et al. (1995)
reported differences of 1--11% in their study examining sap
flow behavior in two small E. maculata Hook. trees. Each of
these studies was performed on small saplings and lasted less
than 24 h in duration.
Olbrich (1991) applied the in situ cut tree technique to a
56-m-tall E. grandis W. Hill ex Maiden. with similar leaf area
(219 m2) and sapwood area (371 cm2) to our cut tree and found
that the sap flow estimate of water uptake was about 12%
greater than the actual value. Because Olbrich (1991) attributed part of the discrepancy to the use of a flexible reservoir to
hold water around the stem, we used a rigid storage system in
our study. We found that the sap flow method underestimated
cumulative water uptake over 3 days by less than 4%. Estimated sap flow closely tracked actual water uptake throughout
the measurement period, except for periods of very low uptake
(< 0.05 m3 day −1) when the sap flow logger recorded no flow.
We conclude that the sap flow method is a valid means of
determining the water use behavior of a single mountain ash
tree, although the sensitivity of the sap flow sensor needs to be
improved if accurate estimates of low sap flows are required.
We sampled the variability in sap velocities in one symmetrical and one asymmetrical tree and found that the degree of
variability in sap velocity within mountain ash trees appears to
be a reflection of the degree of symmetry in sapwood thickness. In the symmetrical tree, sap velocity was fairly uniform
throughout the xylem (both before and after the cut), indicating
that sap flow could be estimated accurately with a single logger
equipped with four sensors. By contrast, Olbrich (1991) concluded that at least eight sampling points were needed in
E. grandis trees larger than 20 cm in diameter, and Hatton et al.
(1995) argued that even greater sampling resources were required to estimate sap flow accurately in a single E. populnea
tree. The high degree of variability in sap velocity in our
asymmetrical mountain ash tree would also necessitate more
than four sampling points. However, mountain ash trees with
highly asymmetrical sapwood cross sections make up only a
small part of the mountain ash forest resource (Figure 2) and
can be easily avoided in field sampling programs. We suggest
that it is desirable to allocate sap flow logger resources across
as many trees as possible because sap velocity variability was
much greater among trees than within trees. For example, there
was less variation within the symmetrical tree than among the
six trees for which we have measurements of mean sap velocity (cf. Tables 1 and 2). Granier et al. (1996) also concluded
that sap velocity variability among trees in a stand was normally much greater than sap velocity variability within individual trees.
Several studies have documented strong relationships between allometric properties of trees and tree water use (Hatton
and Wu 1995, Teskey and Sheriff 1996). We confirmed the
earlier observations of Vertessy et al. (1995) that stem diameter
is a good predictor of sapwood area, leaf area and mean daily
sap flow in mountain ash trees. Although the associations

between these variables were not as strong as previously noted
for young (15-year-old) stands, they were statistically significant at the 0.001 level. As the forest ages, sapwood area
becomes a progressively smaller and more variable fraction of
basal area; hence, several workers have advocated using sapwood area rather than stem diameter as the prime scalar for leaf
area and therefore tree water use prediction (Waring et al.
1977, Whitehead 1978, Waring 1983). However, it is more
practical to use stem diameter than sapwood area as the prime
scalar for stand-level predictions of leaf area and tree water use
in mature mountain ash forests because sapwood area is difficult to measure. For instance, Vertessy et al. (1995) determined
that three or more cores (sometimes as many as 10) were
required to estimate sapwood thickness within 5 mm of the
true mean with 95% confidence in young mountain ash trees.
It is probable that even more estimates would be needed for
older mountain ash trees to obtain similar accuracy. Waring
(1983) has pointed out that, for older trees, sapwood area at the
base of the live crown is a better scalar than sapwood area at
breast height, necessitating the calculation of taper coefficients
for older trees, and hence more work and possibly greater
uncertainty. Another advantage of using stem diameter as the
primary scalar for leaf area and sap flow estimations is that
many stem diameter and stocking rate data are available in
forest inventories gathered by forest agencies.
A major obstacle to overcome with respect to scaling individual tree water use to the stand level is that separate stem
diameter--leaf area relationships are needed for mountain ash
stands of different ages. For instance, based on the equation
reported by Vertessy et al. (1995) for 15-year-old mountain ash
(shown in Figure 5a), the predicted LAI of the Black Spur site
would be 12.9, whereas a value of 2.85 was obtained based on
the equation derived in this study (also shown in Figure 5a),
and the Plant Canopy Analyzer-based estimate was 2.6. A
composite equation fitted to both the Vertessy et al. (1995) data
and the data from this study (leaf area = 0.127dbh1.797, n = 29,
r 2 = 0.82) yielded a plot LAI of 3.7. Watson and Vertessy
(1996) have recently developed a general empirical model that
uses stem diameter data to predict leaf area values for mountain ash stands of any age; however, similar, generalized relationships that relate stem diameter and sapwood area and sap
flow have yet to be developed for mountain ash stands.
Scaling individual tree water use measurements to an areal
basis in mountain ash forest appears to be tractable because the
stands are even-aged and largely monospecific. Scaling based
on stem diameter census (Method 1) requires either a large
number of sap flow loggers, or enough time to roam a few
loggers around a population of trees. Despite the need to
sample a tree destructively, the leaf area ratio method
(Method 2) is easier to apply in the field than Method 1,
because it simply requires an accurate estimate of water use
and leaf area of one tree, coupled with an accurate estimate of
the plot LAI. We have shown that water use of a single tree can
be obtained with a high degree of accuracy by the sap flow
method, and Vertessy et al. (1995) demonstrated that LAI can
be measured in mountain ash forest with a fair degree of

TREE PHYSIOLOGY VOLUME 17, 1997

ESTIMATING STAND WATER USE

accuracy by the Plant Canopy Analyzer measurement technique.
Although Method 1 is more onerous to apply, it yields more
robust estimates of stand water use than Method 2. The mean
daily plot water use calculated by Method 2 was 11% lower
than the value computed by Method 1. However, there was no
difference between the two methods when we applied the leaf
area ratio based on the destructive sampling estimate of plot
LAI (2.85). Method 2 is strongly influenced by the ‘typicality’
of the reference tree and the accuracy of the plot LAI estimate.
For Method 2 to yield an estimate close to Method 1, the
reference tree needs to lie close to the line of best fit in the sap
flow--leaf area relationship (Figure 8). Tree 9 was used as the
reference tree in the leaf area ratio scaling method (Method 2)
because it was the sap flow measurement control tree. Had
Tree 3 been used as the reference tree then the estimates of the
two methods would not have been so close.
The mean daily sap velocity and stand water use estimates
obtained for the Black Spur site compare closely with those
observed in other studies of mountain ash forest water use over
spring--summer periods. Our mean daily sap velocity of
0.033 mm s −1 is similar to that reported by Vertessy et al.
(1995) for a 15-year-old mountain ash forest, and similar to the
value of 0.032 mm s −1 obtained for a 50-year-old mountain
ash stand by Dunn and Connor (1993). These data support the
notion that the stand mean of daily sap velocity (averaged for
a month or more over spring--summer) does not vary significantly among mountain ash stands of different ages. We conclude that reasonable calculations of stand water use can be
made for different age classes of mountain ash if a reliable
stand age--sapwood area relationship can be developed. Our
mean daily stand water use estimate for the Black Spur site
(1.9 mm; based on scaling Method 1) is slightly lower than the
2.3 mm observed by Vertessy et al. (1995) for a 39-day spring
sampling period in a young mountain ash forest. We conclude
that long-term sap flow measurements in mountain ash forests
are needed to determine how climate factors affect stand water
use throughout the year.

Figure 8. Observed relationship between leaf area and mean daily sap
flow in five mountain ash trees at Black Spur, indicating line of best
fit, forced through the origin. Tree number is next to each data point.

755

Acknowledgments
This study was funded by the Cooperative Research Center for Catchment Hydrology. Jim Brophy and Jamie Margules helped out with
many aspects of the field experiments. We also thank Melbourne Water
and the Victorian Department of Conservation and Environment for
valuable field assistance. Drs. J. Landsberg and R. Waring kindly
provided valuable suggestions on a draft of this paper.
References
Barrett, D.J., T.J. Hatton, J.E. Ash and M.C. Ball. 1995. Evaluation of
the heat pulse velocity technique for measurement of sap flow in
rainforest and eucalypt forest species of south-eastern Australia.
Plant Cell Environ. 18:463--469.
Davis, S., R.A. Vertessy, D.L. Dunkerley and R.G. Mein. 1996. The
influence of scale on the measurement of saturated hydraulic conductivity in a forest soil. Australian Institute of Engineers, Hydrology and Water Resources Symp. National Conference Publication
No. 96/05, Canberra, pp 103--108.
Doley, D. and B.J. Grieve. 1966. Measurement of sap flow in a
eucalypt by thermo-electric methods. Aust. For. Res. 2:3--27.
Dunn, G.M. and D.J. Connor. 1993. An analysis of sap flow in
mountain ash (Eucalyptus regnans) forests of different age. Tree
Physiol. 13:321--336.
Edwards, W.R.N., P. Becker and J. Èermak. 1997. A unified nomenclature for sap flow measurements. Tree Physiol. 17:65--67.
Granier, A., P. Biron, N. Bréda, J.Y. Pontailler and B. Saugier. 1996.
Transpiration of trees and forest stands: short and long-term monitoring using sapflow methods. Global Change Biol. 2:265--274.
Hatton, T.J. and R.A. Vertessy. 1989. Variability of sapflow in a Pinus
radiata plantation and the robust estimation of transpiration. Australian Institute of Engineers, Hydrology and Water Resources
Symp., National Conference Publication No. 89/19, Canberra,
pp 6--10.
Hatton, T.J. and H.I. Wu. 1995. Scaling theory to extrapolate individual tree water use to stand water use. Hydrol. Processes 9:527--540.
Hatton, T.J., S.J. Moore and P.H. Reece. 1995. Estimating stand
transpiration in Eucalyptus populnea woodland with the heat pulse
method: method errors and sampling strategies. Tree Physiol.
4:219--227.
Jayasuriya, M.D.A., G. Dunn, R. Benyon and P.J. O’Shaughnessy.
1993. Some factors affecting water yield from mountain ash (Eucalyptus regnans) dominated forests in south-east Australia. J. Hydrol.
150:345--367.
Kuczera, G.A. 1987. Prediction of water yield reductions following a
bushfire in ash-mixed species eucalypt forest. J. Hydrol. 94:215--236.
Landsberg, J.J., T.W. Blanchard and B. Warrit. 1976. Studies on the
movement of water through apple trees. J. Exp. Bot. 27:579--596.
Langford, K.J. 1976. Change in yield of water following a bushfire in
a forest of Eucalyptus regnans. J. Hydrol. 29:87--114.
Langford, K.J. and P.J. O’Shaughnessy. 1977. First progress report,
North Maroondah. Melbourne and Metropolitan Board of Works,
Catchment Hydrology Research Report Number MMBW-W-0005,
Melbourne, Australia, 340 p.
Mencuccini, M. and J. Grace. 1996. Hydraulic conductance, light
interception and needle nutrient concentration in Scots pine stands
and their relations with net primary productivity. Tree Physiol.
16:459--468.
Olbrich, B.W. 1991. The verification of the heat pulse velocity technique for measuring sap flow in Eucalyptus grandis. Can. J. For.
Res. 21:836--841.
Roberts, J. 1977. The use of tree-cutting in the study of the water
relations of mature Pinus sylvestris L. J. Exp. Bot. 28:751--767.

TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.com

756

VERTESSY ET AL.

Rosner, B.A. 1990. Fundamentals of biostatistics, 3rd Edn. PWS-Kent
Publishing, Boston, 655 p.
Teskey, R.O. and D.W. Sheriff. 1996. Water use by Pinus radiata trees
in a plantation. Tree Physiol. 16:273--279.
Vertessy, R., R. Benyon and S. Haydon. 1994. Melbourne’s forest
catchments: effect of age on water yield. Water 21:17--20.
Vertessy, R.A., R.G. Benyon, S.K. O’Sullivan and P.R. Gribben. 1995.
Relationship between stem diameter, sapwood area, leaf area and
transpiration in a young mountain ash forest. Tree Physiol. 15:559-568.
Waring, R.H. 1983. Estimating forest growth and efficiency in relation
to canopy leaf area. Adv. Ecol. Res. 13:327--354.
Waring, R.H. and W.B. Silvester. 1994. Variation in foliar δ13C values
within the crowns of Pinus radiata trees. Tree Physiol. 14:1203-1213.

Waring, R.H., H.L. Gholz, C.C. Grier and M.L. Plummer. 1977.
Evaluating stem conducting tissue as an estimator of leaf area in
four woody angiosperms. Can. J. Bot. 55:1474--1477.
Watson, F.G.R. and R.A. Vertessy. 1996. Estimating leaf area index
from stem diameter measurements in mountain ash forest. Cooperative Research Centre for Catchment Hydrology, Technical Report 96/7, Canberra, Australia, 102 p.
Welles, J.M. and J.M. Norman. 1991. Instrument for indirect measurement of canopy architecture. Agron. J. 83:818--825.
Whitehead, D. 1978. The estimation of foliage area from sapwood
basal area in Scots pine. Forestry 51:35--47.
Yoder, B.J., M.G. Ryan, R.H. Waring, A.W. Schoettle and M.R. Kaufmann. 1994. Evidence of reduced photosynthetic rates in old trees.
For. Sci. 40:513--527.

TREE PHYSIOLOGY VOLUME 17, 1997