Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol15.1995:
Tree Physiology 15, 559--567
© 1995 Heron Publishing----Victoria, Canada
Relationships between stem diameter, sapwood area, leaf area and
transpiration in a young mountain ash forest
R. A. VERTESSY,1,4 R. G. BENYON,2,4 S. K. O’SULLIVAN3,4 and P. R. GRIBBEN3,4
1
CSIRO Division of Water Resources, GPO Box 1666, Canberra, ACT 2601, Australia
2
Melbourne Water, Box 4342, Melbourne, Victoria 3001, Australia
3
Monash University, Wellington Road, Clayton, Victoria 3168, Australia
4
Cooperative Research Center for Catchment Hydrology, Wellington Road, Clayton, Victoria 3168, Austr
alia
Received October 20, 1994
Summary We examined relationships between stem diameter, sapwood area, leaf area and transpiration in a 15-year-old
mountain ash (Eucalyptus regnans F. Muell.) forest containing
silver wattle (Acacia dealbata Link.) as a suppressed overstory
species and mountain hickory (Acacia frigescens J.H. Willis)
as an understory species. Stem diameter explained 93% of the
variation in leaf area, 96% of the variation in sapwood area and
88% of the variation in mean daily spring transpiration in 19
mountain ash trees. In seven silver wattle trees, stem diameter
explained 87% of the variation in sapwood area but was a poor
predictor of the other variables. When transpiration measurements from individual trees were scaled up to a plot basis, using
stem diameter values for 164 mountain ash trees and 124 silver
wattle trees, mean daily spring transpiration rates of the two
species were 2.3 and 0.6 mm day −1, respectively. The leaf area
index of the plot was estimated directly by destructive sampling, and indirectly with an LAI-2000 plant canopy analyzer
and by hemispherical canopy photography. All three methods
gave similar results.
Keywords: Acacia dealbata, Acacia frigescens, Eucalyptus
regnans, heat pulse method, leaf area index, mountain hickory,
silver wattle.
Introduction
Water for the city of Melbourne is obtained from 155,000 ha
of forested catchments in the central highlands of Victoria,
Australia. Mountain ash (Eucalyptus regnans F. Muell.) forests
cover just under half this area, but yield 80% of the streamflow
because they grow on the higher rainfall sites.
In 1939, about 80% of the extensive old-growth mountain
ash forest in Melbourne’s water supply catchments was killed
in bushfires. It regenerated naturally with dense re-growth
mountain ash forest. The widespread destruction and regeneration of the forests was followed by a prolonged decline in
streamflows. In the three decades following the fires, mean
annual streamflows from five catchments were 25% less than
the pre-1939 mean (Langford 1976). The reduction in water
yield after the fire in each catchment was proportional to the
percentage of mountain ash forest burnt (Langford 1976,
Kuczera 1987). The maximum decline in mean annual streamflow occurred 27 years after the fires and was equivalent to
6 mm for each 1% of catchment converted from old-growth to
re-growth (Kuczera 1987).
There have been many studies of the Melbourne water
supply area. Field experiments have examined stand-age-dependent rates of fog drip (O’Connell and O’Shaughnessy
1975), rainfall interception (Langford and O’Shaughnessy
1978) and evapotranspiration (Legge 1980, Dunn and Connor
1993, Jayasuriya et al. 1993). Fog drip was shown to be less
than 1% of gross precipitation and did not vary significantly
between re-growth and old-growth stands. Although Langford
and O’Shaughnessy (1978) found that interception in 34-yearold re-growth mountain ash forest was lower than in a 160year-old mature mountain ash forest, a recent, more
comprehensive study has determined that interception peaks at
26% of gross rainfall at age 30 years and steadily declines to
17% by age 240 years (S. Haydon, Melbourne Water, personal
communication). However, changes in interception explained
only about 25% of the changes in streamflow resulting from
conversion of old-growth forests to a re-growth state.
Although the causal mechanisms of the water yield response
in the Melbourne water supply area have not been fully elucidated, it is now assumed that differences in evapotranspiration
are the prime cause of the stand age--water yield relationship
in mountain ash forests. Recent studies by Dunn and Connor
(1993) and Jayasuriya et al. (1993) emphasize the role of
changing leaf area and sapwood area in determining changes
in forest evapotranspiration. However, no studies have provided quantitative data linking leaf area, sapwood area and
transpiration in mountain ash forests.
As a first step in elucidating the factors underlying the stand
age--water yield relationship in mountain ash forests, we have
examined relationships between stem diameter, sapwood area,
leaf area and transpiration in a 15-year-old mountain ash forest
560
VERTESSY, BENYON, O’SULLIVAN AND GRIBBEN
and obtained evidence that similar relationships could be derived for older stands. We determined leaf area by destructively
sampling individual trees. Because such sampling is impractical in older forests with large trees, we also evaluated two
methods for indirectly measuring leaf area index and show that
these can be used in older stands of mountain ash.
Materials and methods
Site description
The study site was located in the North Maroondah experimental area (Figure 1), at an elevation of 890 m, with a mean annual
rainfall of 1713 mm. Soils in the area are deep krasnozems of
high permeability and high water-holding capacity (Langford
and O’Shaughnessy 1977). The even-aged stand of mountain
ash was regenerated from direct seeding after clear-cutting in
1978. As a result of natural thinning, the stocking density of
mountain ash trees decreased from 8500 stems ha −1 in 1983 to
656 stems ha −1 at the time of the study in the spring of 1993.
Silver wattle (Acacia dealbata Link.) is a subdominant canopy
species, present at a stocking density of 496 stems ha −1. The
understory is mostly mountain hickory (Acacia frigescens J.H.
Willis), rising to a height of 3 m, and assorted ferns reaching
about 1 m in height.
A 50 × 50 m plot was established, containing 164 mountain
ash and 124 silver wattle trees, with mean heights of 28 and
24 m, respectively. The stem diameter at breast height over
bark (DBH) of each tree was measured. Mean DBH values for
the mountain ash and silver wattle trees were 20.4 and 15.6 cm,
respectively. Minimum DBH values were 9.6 and 10.0 cm, and
maximum DBH values were 39.2 and 27.1 cm, respectively.
Nineteen mountain ash and seven silver wattle trees, spanning
the range of DBH values in the plot, were selected for detailed
examination. For each of these trees, we determined the mean
daily sapflow velocity, mean daily transpiration rate, DBH,
height, total leaf weight, total leaf area and sapwood cross-sectional area (Table 1).
Transpiration measurements
Mean daily minimum and maximum temperatures for the
period of record were 4.4 and 12.8 °C, respectively, slightly
lower than the long-term October--November averages of 7.3
and 13.4 °C (Langford and O’Shaughnessy 1977). Total solar
radiation varied between 3.1 and 25.1 MJ m −2 day −1, with a
mean value of 13.9 MJ m −2 day −1. This mean daily value is
similar to the local October--November average of 13.2 MJ
m −2 day −1 (Langford and O’Shaughnessy 1977).
Over a 39-day period between September and November
1993, measurements of half-hourly transpiration were made
on the sample trees by the heat pulse technique (Cohen et al.
1985, Green and Clothier 1988, Hatton and Vertessy 1990,
Olbrich 1991, Dunn and Connor 1993). Half-hourly measurements of sapflow velocities were multiplied by sapwood crosssectional area to compute transpiration flux in individual trees.
Dunn and Connor (1993) validated this technique in young
mountain ash trees and showed that no significant wound
effect takes place after implanting sensor probes.
Six sapflow sensor units (Greenspan Technology, Warwick,
Queensland) were deployed for a period of 39 days using a
roaming sensor technique. One sensor unit was allocated to a
reference mountain ash tree (MA-REF) and another to a reference silver wattle tree (SW-REF) for the entire 39-day measurement period. The other four sensor units were moved from
tree to tree every 4 to 7 days (Table 1). At all times, three
roaming sensors were deployed in mountain ash trees and one
in a silver wattle tree. Measurements of total daily transpiration obtained from the roaming sensors were regressed against
those obtained with the reference sensors. The resulting regression equations were used to predict total daily transpiration in
the roaming sensor trees outside the period of measurement in
Figure 1. Location of the North Maroondah experimental area.
TREE PARAMETERS AND TRANSPIRATION IN E. REGNANS
561
Table 1. Parameter values for all sample trees. The value of r2 is relative to mean daily transpiration in reference trees MA-REF and SW-REF.
Tree
DBH
(cm)
Leaf weight
(kg)
Leaf area
(m2)
Sapwood area
(cm2)
Sap flux record
(day)
r2
Mean sap flux
(l day −1 )
Mean sap velocity
(cm h −1)
MA-REF
MA1
MA2
MA3
MA4
MA5
MA6
MA7
MA8
MA9
MA10
MA11
MA12
MA13
MA14
MA15
MA16
MA17
MA18
SW-REF
SW1
SW2
SW3
SW4
SW5
SW6
22.4
18.3
27.9
18.2
30.6
21.2
15.6
23.8
31.5
22.5
16.4
28.4
20.5
27.0
37.1
39.2
25.8
34.7
36.6
27.1
25.0
24.9
23.7
15.4
20.7
20.7
15.06
9.05
26.96
6.69
27.98
10.83
3.68
26.13
27.50
13.50
3.69
26.16
17.09
25.40
60.85
63.12
27.53
55.70
63.49
11.95
---0.46
---
36.33
21.83
65.03
16.13
67.48
26.13
8.89
63.02
66.32
32.57
8.89
63.08
41.21
61.27
146.78
152.26
66.41
134.34
153.13
23.34
---0.90
---
121.3
96.1
215.0
66.0
233.0
93.5
50.5
212.6
261.0
140.0
62.5
234.7
116.1
236.4
451.8
401.1
198.0
352.9
406.4
236.2
278.5
204.5
208.3
72.1
178.3
113.7
39
5
7
4
4
5
5
5
7
5
7
4
7
7
5
5
5
7
5
39
5
5
5
7
7
4
-0.42
0.92
0.81
0.94
0.96
0.46
0.88
0.92
0.96
0.85
0.92
0.77
0.71
0.88
0.96
0.96
1.00
0.96
-0.94
0.98
0.96
0.06
0.31
0.04
25.4
22.4
56.7
23.3
87.4
26.2
6.6
74.2
67.9
38.2
15.2
75.6
35.8
69.1
150.6
135.1
64.4
115.2
87.8
19.8
58.9
45.0
20.0
----
8.7
9.7
11.0
14.7
15.6
11.7
5.5
14.5
10.8
11.4
10.1
13.4
12.8
12.2
13.9
14.0
13.6
13.6
9.0
3.5
8.8
9.2
4.0
----
those trees. This enabled us to estimate total daily transpiration
for each sample tree over the entire 39-day measurement
period. No transpiration measurements were made in the understory.
Fractions of wood, water and air in the sapwood were
determined gravimetrically on one 5-mm diameter increment
core from each tree at the end of the transpiration experiment.
In the mountain ash trees, the wood fraction varied between
0.26 and 0.34 (mean = 0.29), whereas the water fraction varied
between 0.55 and 0.67 (mean = 0.60). In the silver wattle trees,
the wood fraction varied between 0.31 and 0.37 (mean = 0.32),
and the water fraction varied between 0.28 and 0.49 (mean =
0.36). Hence, the silver wattle trees had a much larger air
fraction in the sapwood than the mountain ash trees.
Nondestructive measurement of leaf area index
An LAI-2000 plant canopy analyzer (PCA) (Li-Cor Inc., Lincoln, NE) and hemispherical canopy photographs were used to
estimate the leaf area index (LAI) of the 50 × 50 m plot. Both
methods combine the leaf area of the overstory and understory
species to give a total LAI value for the plot.
Leaf area index was computed from the PCA measurements
using a canopy gap fraction model (Welles and Norman 1991).
We deployed two PCAs; one device was left in a clearing
(1.5 km from the site) to record light conditions automatically
every 15 s, whereas the other PCA was used manually beneath
the canopy in the experimental plot. All PCA measurements
were made over a 360° field of view, under cloudy (diffuse
radiation dominated) conditions or at dawn or dusk when there
was no direct sunlight on the canopy. We only used readings
from the four inner rings of the sensor, thereby confining the
PCA’s field of view to a radius of 41 m.
Three sampling protocols (random, transect and fixed position) were used in the application of the PCA to the experimental plot. In the random sampling protocol, the PCA operator
moved around the plot making readings at random locations.
These measurements were repeated on four different occasions
to include a variety of sky conditions. In the transect sampling
protocol, the PCA operator made measurements at roughly
1-m intervals along two 50-m transects laid out across the plot
and two 70-m transects running along the diagonals of the plot.
One of the 50-m transects was measured twice and the other
50-m transect was measured three times. In the fixed position
sampling protocol, the PCA operator made measurements at
six fixed points in the plot on six occasions.
Multiple hemispherical photographs were taken at the same
six points within the plot. We used Ilford pan F (50 ASA) film
in a NIKON FG camera, fitted with a Nikkor 8 mm fisheye
lens. All photographs were taken under cloudy conditions to
ensure even back-lighting. Each negative was digitized to
provide a 512 × 512 8-bit image. The images were analyzed to
determine the percent canopy gap as a function of zenith and
azimuth angles. The PISCES program (Jupp et al. 1980) was
used to compute the generalized foliage area index (GFAI),
which is analogous to LAI, from the canopy gap fraction data.
Although the scanned images extended to a zenith angle of
562
VERTESSY, BENYON, O’SULLIVAN AND GRIBBEN
84.3°, the foliage was so dense that measurements beyond 65°
had little influence on the analysis; hence, the hemispherical
photographic and PCA methods had similar fields of view.
Destructive measurement of leaf area index
After completion of the transpiration and PCA measurements,
nineteen mountain ash trees and two silver wattle trees were
felled. The total leaf mass from each tree was harvested and
weighed. The height of each tree was measured and stem discs
were cut at breast height (1.3 m above ground level) for
measurements of sapwood cross-sectional area.
Five circular subplots, each with a radius of 5 m, were
established within the 50 × 50 m plot, covering a total area of
392.75 m2, or 15.7% of the experimental plot. Leaves were
stripped and weighed from all mountain hickory trees located
within each subplot.
Leaf subsamples from each of the three tree species were
weighed, then immediately fed through a planimeter to obtain
a leaf area to weight ratio for each species. It was difficult to
obtain reliable leaf area estimates for the silver wattle trees
because the fragile pinnate leaves were easily dislodged when
the trees hit the ground and were awkward to feed through the
planimeter. Leaf area estimates were therefore obtained for
only two of the seven sample silver wattle trees, including the
largest and one of the smallest trees in the plot.
Sapwood area measurements
Sapwood thickness and sapwood cross-sectional area of each
sample tree were determined from measurements on several
5-mm diameter increment cores, taken at breast height in each
of the sample trees before felling. The sapwood width measurements were used to determine appropriate depths for implanting heat pulse sensor probes. After felling, more accurate
measurements of sapwood area were made on wood discs, cut
at breast height. Measurements of bark thickness, sapwood
thickness and heartwood diameter at up to 15 points around
each wood disc were used to compute sapwood area. These
determinations were used in the transpiration calculations.
In the silver wattle trees, the heartwood and sapwood were
clearly distinguishable on the basis of color. To identify sapwood in the mountain ash increment cores, it was necessary to
hold each core up against a bright light. The open vessels of
the sapwood were clearly visible as pin-points of light,
whereas the blocked vessels of the heartwood did not transmit
light. The heartwood in the mountain ash discs could be easily
distinguished after a day or two of exposure to air because the
heartwood became brown, whereas the sapwood remained
straw colored. In mountain ash trees, the no-flow boundary
detected with the sapflow sensor corresponded to the independent estimates of the sapwood--heartwood transition.
by the measured leaf area/leaf wet weight ratio of 2.412 m2
kg −1 (Table 1). To scale up individual tree leaf areas to a plot
LAI value, a regression equation was developed that related
leaf area to stem diameter (Figure 2). Because a linear fit gave
an undesirable y-intercept value, we applied a power function
that passed through the origin and hence reflected the growth
process. The r2 value of 0.93 (n = 19) indicated a strong
association between stem diameter and leaf area. This regression equation was evaluated for all 164 mountain ash trees in
the plot, resulting in a total mountain ash leaf area of 5342 m2,
or an LAI of 2.1.
Leaf wet weights were determined for two silver wattle trees
(Table 1), and these were multiplied by the measured leaf
area/leaf wet weight ratio of 1.953 m2 kg −1 to determine leaf
area for both trees. Silver wattle leaf area was regressed against
stem diameter, based on the regression equation used for the
mountain ash trees. Although the power function was fitted to
only two data points, we believe the fitting was valid because
the function passed through the origin, and data points for the
largest tree and one of the smallest trees in the plot were
included. The small diameter silver wattle trees supported little
leaf area and were beginning to die out as a result of over-topping by the mountain ash canopy. The large silver wattle trees
occurred in canopy gaps and were therefore less affected by
competition with mountain ash trees. The regression equation
relating silver wattle leaf area to stem diameter was used to
estimate total leaf area for all 124 silver wattle trees in the 50
× 50 m plot. This yielded an estimate of 290.23 m2 of leaf area,
equivalent to an LAI of 0.1.
The leaf area/leaf wet weight ratio for the mountain hickory
was 3.646 m2 kg −1. The total leaf mass harvested from five
circular plots was 194.38 kg, equivalent to a total leaf area of
708.72 m2, or an LAI of 1.8.
Indirect leaf area index estimates
Table 2 lists means of all of the PCA-based LAI estimates
made for the 50 × 50 m plot using the random, transect and
fixed position sampling protocols. For each sampling protocol,
multiple determinations were made under varying irradiances
Results
Direct leaf area index estimates
Leaf area estimates for 19 mountain ash trees were based on
the wet weight of leaves collected from each tree, multiplied
Figure 2. Stem diameter (DBH) versus leaf area for the sampled
mountain ash and silver wattle trees.
TREE PARAMETERS AND TRANSPIRATION IN E. REGNANS
Table 2. Various LAI estimates for the 50 × 50 m plot obtained using
the PCA; n = number of readings per sample.
Sampling protocol
Random
Random
Random
Random
n
Run identifier
LAI
1
2
3
4
228
200
60
200
3.6
4.5
4.5
3.8
Transect (east)
Transect (east)
Transect (north)
Transect (north)
Transect (north)
Transect (both diagonals)
5
6
7
8
9
10
50
50
55
55
55
130
4.4
4.2
4.4
5.0
4.5
3.9
Fixed position
Fixed position
Fixed position
Fixed position
Fixed position
Fixed position
11
12
13
14
15
16
6
6
6
6
6
6
3.8
4.3
4.1
4.4
4.0
3.8
Total/mean
1--16
1119
4.2
to obtain an average value of LAI. Mean LAI values for the
random, transect and fixed position sampling protocols were
4.1, 4.4 and 4.1, respectively, and the mean of all PCA readings
obtained from all sampling protocols was 4.2. None of the
sampling protocols was more accurate or reproducible than the
others.
Fixed-position LAI estimates based on the PCA and photographic methods are compared in Table 3. For the PCA
method, we list the mean and range of LAI values measured
for each plot. For the photographic method, we list the LAI
determination and error margin for a single photograph from
each plot. With the exception of Plot 3, the individual fixed-position LAI estimates obtained by the PCA and photographic
methods differed by 0.5 or less. The mean photograph-based
LAI estimate for the plot was 4.4, which is almost the same as
the mean fixed-position estimate of 4.1 obtained with the PCA.
Although we applied the PCA under ideal light conditions,
LAI determinations for some plots varied by 1.0 or more
between samplings. Error bounds for the photograph-based
563
LAI estimates were large for individual photographs (up to 2.1
units) but small for the lumped plot value (only 0.3).
Sapwood area estimates
Sapwood areas in the mountain ash trees ranged between 50.5
and 451.8 cm2 and averaged 207.8 cm2, whereas silver wattle
sapwood areas ranged between 72.1 and 278.5 cm2 and averaged 184.5 cm2 (Table 1). Sapwood areas were reliably predicted from stem diameter measurements in both species,
yielding r2 values of 0.96 and 0.87, respectively (Figure 3).
Regression equations for the two species were similar, though
statistically different.
The sapwood areas for all mountain ash and silver wattle
trees in the plot were estimated from regression equations for
each species. Total sapwood area for the 164 mountain ash
trees in the plot was 8.04 m2 ha −1. Total sapwood area for the
124 silver wattle trees in the plot was 4.16 m2 ha −1, or 34.1%
of the combined overstory sapwood area. There was a strong
relationship between sapwood area and leaf area in the mountain ash trees (r2 = 0.96, n = 19) (Figure 4). There were
insufficient leaf area measurements for the silver wattle trees
to make a similar comparison.
Figure 3. Stem diameter (DBH) versus sapwood area for the sampled
mountain ash and silver wattle trees.
Table 3. Fixed position LAI estimates for the six circular plots obtained using the PCA and hemispherical canopy photography.
Plot
no.
LAI
(PCA)
Range
(PCA)
LAI
(photo)
Error
(photo)
1
2
3
4
5
6
All
3.2
4.1
4.5
4.2
4.3
4.1
4.1
2.9--3.5
3.6--4.7
4.1--4.7
3.8--4.8
4.0--4.6
3.2--4.6
2.9--4.8
3.7
4.0
6.1
3.7
4.2
4.4
4.4
± 0.8
± 0.6
± 1.3
± 1.4
± 1.0
± 2.1
± 0.3
Figure 4. Sapwood area versus leaf area for the sampled mountain ash
trees.
564
VERTESSY, BENYON, O’SULLIVAN AND GRIBBEN
Figure 5. Daily transpiration in the two
reference trees (MA-REF and SW-REF)
over the 39-day sampling period.
Transpiration estimates in the sample trees
Over the 39-day measurement period, transpiration in tree
MA-REF varied between 9 and 43 l day −1 , and the mean daily
value was 25 l day−1 (Figure 5). In tree SW-REF, transpiration
varied between 1 and 40 l day −1 , with a mean daily value of
20 l day −1 (Figure 5). Tree SW-REF had a larger diameter than
tree MA-REF (27.1 versus 22.4 cm) and greater sapwood area
(236.2 versus 121.3 cm2), but a lower leaf area (23.34 versus
36.33 m2). Despite these differences, daily transpiration rates
in the two trees were similar, except that when transpiration in
tree MA-REF dropped below 20 l day −1, transpiration in tree
SW-REF declined sharply (Figure 6). On these days, radiation
was usually below average, and wind speeds were significantly
lower than normal. The silver wattle canopy was on average
about 4 m below that of the mountain ash and would have been
poorly ventilated on days of low wind speed, so the results
suggest uncoupling of the silver wattle canopy from the atmospheric boundary layer during these times.
For periods of concurrent record, daily transpiration values
for the other 18 mountain ash trees were regressed against
daily transpiration values for tree MA-REF. Because the statistical association between daily transpiration totals in the different mountain ash trees was strong, with r2 values exceeding
0.9 in over half the cases and exceeding 0.7 in all but two cases
Figure 6. Daily transpiration in tree MA-REF versus daily transpiration in tree SW-REF. Solid lines indicate best fit linear regressions for
conditions where transpiration of tree MA-REF is greater than (s) and
less than (+) 20 l day −1.
(Table 1), we were able to estimate daily transpiration in the
other 18 mountain ash trees for the entire 39-day period. The
mean daily transpiration for these trees varied between 6.6
(MA6) and 150.6 l day −1 (MA14), with a mean of 62 l day −1
(Table 1).
Daily transpiration values for tree SW-REF were regressed
against daily transpiration values for the other six silver wattle
trees sampled during periods of concurrent record. Although
trees SW1, SW2 and SW4 displayed daily transpiration behavior similar to tree SW-REF (r2 = 0.9), trees SW3, SW5 and
SW6 did not. This difference may be attributable to the rapid
thinning of the silver wattle trees in the plot as the mountain
ash canopy began to over-top the silver wattle trees. With the
exception of tree SW4, all of the positive results came from the
larger silver wattle trees. Consequently, robust transpiration
totals for the 39-day sampling period were derived for only
four of the seven silver wattle trees sampled. For these four
trees, mean daily transpiration varied between 19.8 (SW-REF)
and 58.9 l day −1 (SW1), with a mean of 35.9 l day −1 (Table 1).
There was considerable variation in mean daily sapflow
velocity among the 19 mountain ash trees sampled, with values
ranging between 5.5 (MA6) and 15.6 cm h −1 (MA4), and
averaging 11.9 cm h −1 (Table 1). There was no systematic
relationship between stem diameter and mean daily sapflow
velocity in the mountain ash trees. Mean daily sapflow velocities for the four silver wattle trees varied between 3.5 (SWREF) and 9.2 cm h −1 (SW2), and averaged 6.4 cm h −1
(Table 1). This mean daily value was just over half that observed for the mountain ash trees, but almost twice that observed for mature A. frigescens (Dunn and Connor 1993).
Mean daily transpiration in the mountain ash trees was
strongly related to both DBH (r2 = 0.88, n = 19) and leaf area
(r2 = 0.91, n = 19) (Figures 7 and 8). The relationship between
leaf area and transpiration was almost linear (cf. Running and
Coughlan 1988, Hatton et al. 1992, Band et al. 1993, Vertessy
et al. 1993). However, the larger trees appeared to have a
slightly lower leaf area efficiency than the smaller trees, even
though they experienced higher radiation and wind speeds, and
lower humidity than the smaller trees. This finding is consistent with observations by Yoder et al. (1994) that large trees
tend to experience plumbing problems and consequent stomatal limitations more than small trees.
TREE PARAMETERS AND TRANSPIRATION IN E. REGNANS
565
largest mountain ash trees in the plot (occupying 22% of the
basal area) accounted for 28% of the mountain ash transpiration, whereas the 90 smallest mountain ash trees in the plot
(also occupying 22% of the basal area) were responsible for
only 22% of the mountain ash transpiration (Figure 9). This
finding emphasizes the need to sample a spectrum of tree sizes
when attempting to estimate plot water use by the heat pulse
method.
Discussion
Figure 7. Stem diameter (DBH) versus mean daily transpiration for the
sampled mountain ash trees.
Figure 8. Leaf area versus mean daily transpiration for the sampled
mountain ash trees.
Plot transpiration
The mean daily transpiration for all 164 mountain ash trees in
the experimental plot was calculated to be 5740 l day −1, or 2.3
mm day −1 when expressed on an area basis. Although we could
not obtain a statistically significant relationship between mean
daily transpiration and DBH in the silver wattle trees, by
multiplying the total sapwood area (1.04 m2) by the mean daily
sapflow velocity (6.4 cm h −1), we obtained a mean daily
transpiration estimate of 1597 l day −1, or 0.6 mm day −1. The
estimate of plot transpiration for the silver wattle trees is less
robust than the estimate for mountain ash, because the estimate
of mean sapflow velocity was based on only four trees and
these were amongst the largest silver wattle trees in the plot.
However, we observed no systematic relationship between
DBH and sapflow velocity in the mountain ash trees. For the
entire 39-day measurement period, the mean daily transpiration rate for the mountain ash and silver wattle trees combined
was 2.9 mm day −1 .
Transpiration among the mountain ash trees in the experimental plot was highly skewed. When the relative mean daily
transpiration (expressed as a percentage of the plot total) of
mountain ash trees was ranked on the basis of DBH, the 14
In the mountain ash, we found strong statistical associations
between stem diameters and all other stand parameters analyzed. Stem diameter measurements explained 93% of the
variation in leaf area, 96% of the variation in sapwood area and
88% of the variation in mean daily transpiration. Because stem
diameters are easy to measure, it is possible to make reliable
assessments of related stand parameters in large plots. We did
not test whether the same statistical associations hold for older
mountain ash stands.
A strong association between sapwood area and leaf area has
been noted in other eucalypts (Brack et al. 1985, Moore 1993,
Hatton et al. 1995). Although Hatton and Wu (1995) cautioned
that leaf area can decline quickly in response to drought,
whereas sapwood area remains stable, this effect is probably
not significant in mountain ash trees because they are rarely
drought stressed. However, it is not known whether leaf area
varies temporally in these forests in response to seasonal
changes in irradiance or temperature, or both.
When estimating tree transpiration using the heat pulse
technique, accurate measurement of sapwood area is essential.
Even in young mountain ash trees that appear symmetrical, we
found considerable variation in the width of the sapwood at
breast height within individual trees (Table 4). Thus, a single
sample from MA15 may have produced an estimated sapwood
width anywhere between 25 and 48 mm, whereas the true
mean was 38 mm. In most of the 19 mountain ash trees
analyzed, at least two or three core samples would be needed
to give an estimate of the mean sapwood width within 5 mm
Figure 9. Relative transpiration by mountain ash trees of different
sizes in the 50 × 50 m plot. Class 151-164 contains the 14 largest trees
in the plot.
566
VERTESSY, BENYON, O’SULLIVAN AND GRIBBEN
Table 4. Variability of sapwood widths within individual mountain ash
trees; n = number of samples, SD = standard deviation, P95 = number
of samples needed to estimate mean sapwood thickness within 95%
confidence limits of 5 mm.
Tree
n
Range
(mm)
Mean
(mm)
SD
(mm)
P95
MA-REF
MA1
MA2
MA3
MA4
MA5
MA6
MA7
MA8
MA9
MA10
MA11
MA12
MA13
MA14
MA15
MA16
MA17
MA18
8
8
8
8
12
8
8
7
8
8
8
12
8
8
10
13
8
8
14
15--23
18--22
28--32
10--18
21--33
14--20
10--13
33--40
26--36
15--31
12--17
22--42
18--26
29--37
41--53
25--48
27--33
29--38
32--52
20
20
29
14
28
16
12
36
31
24
14
31
21
33
47
38
30
39
42
3
1
2
3
4
2
1
2
3
5
2
6
3
3
3
8
3
3
6
2
1
2
2
3
2
1
2
2
4
2
6
2
2
4
10
2
2
6
of the true mean with 95% confidence (Table 4). Our stem disc
analyses indicated that sapwood width was more variable in
the large-diameter trees than in the small-diameter trees. Because of slight buttressing, wider sapwood occurred at the
buttresses, whereas the narrower sapwood occurred between
the buttresses.
Dunn and Connor (1993) reported sapwood area values of
6.74, 6.09, 4.23 and 4.04 m2 ha −1 for 50-, 90-, 150- and
230-year-old mountain ash stands, respectively. Our plotbased estimate of mountain ash sapwood area of 8.04 m2 ha −1
therefore lends support to Dunn and Connor’s (1993) hypothesis that sapwood area declines with age in mountain ash forests. Our data also support Dunn and Connor’s (1993)
contention that mean daily sapflow velocity in mountain ash
trees appears to be independent of stand age. Over a spring-summer--autumn sampling period, they estimated mean daily
sapflow velocities of 11.5, 11.4, 9.9 and 11.8 cm h −1 for 50-,
90-, 150- and 230-year-old mountain ash stands, respectively.
In our 15-year-old stand, we estimated a mean daily sapflow
velocity of 11.9 cm h −1. This implies that spring transpiration
in our stand was almost twice that of a 230-year-old stand.
The strong relationship between stem diameter and transpiration in mountain ash permits calculations to be made on the
implications of forest thinning for catchment water balance.
We estimate that a 50% basal area reduction would reduce
mean daily (spring) transpiration by 58% if the biggest trees
were felled, or by 42% if the smaller trees were felled. Leaf
area (and hence rainfall interception) would be reduced by 61
or 39%, depending on whether the biggest or smallest mountain ash trees were removed (Figure 2). These data imply that
naturally occurring water yield deficits in re-growth mountain
ash forests could be significantly off-set by selectively felling
the largest trees every few years after re-growth commences.
However, recent forest thinning experiments in mountain ash
forests suggest that instantaneous reductions in transpiration
and interception would not be converted entirely into streamflow gains (Jayasuriya et al. 1993).
We note that our plot transpiration estimates are deficient in
two key respects if they are to be used in water balance
calculations. First, they are only valid for spring conditions. To
estimate annual transpiration from stem diameter measurements, it would be necessary to repeat sapflow measurements
in the trees at different times of the year. Second, we did not
measure the transpiration rate of the mountain hickory understory, though we have reason to believe that this is small,
despite its significant contribution to plot leaf area. In a mature
(230-year-old) mountain ash forest with a well developed
mountain hickory understory and low density mountain ash
overstory (72 stems ha −1), Dunn and Connor (1993) estimated
that the understory was responsible for about 27% of the total
forest transpiration during summer. However, in a soil water
balance study carried out in denser 38-year-old re-growth
forest, Langford and O’Shaughnessy (1979) concluded that
the woody understory contributed little to soil water depletion
over a summer period. Although 45% of the total plot leaf area
was associated with the understory at our site, we suspect that
transpiration from this layer would be low because of intense
shading from the overstory trees and minimal turbulent transfer between the understory and overstory layers.
Based on destructive sampling, the leaf area indices of the
mountain ash, silver wattle and mountain hickory vegetation
were 2.1, 0.1 and 1.8, respectively, yielding a total plot LAI
value of 4.0. The PCA and photographic methods yielded
values of 4.2 and 4.4, respectively, thus overestimating LAI by
5 to 10%. This difference is small and could be attributable to
errors in any of the estimation techniques. However, the degree
and direction of difference are consistent with other studies
that have compared direct and indirect estimates of LAI in
forests; these studies have concluded that slight overestimation
arises in indirect methods because tree stems and branches
account for between 4 and 12% of light interception in forests
(Neumann et al. 1989, Chason et al. 1991, Lang et al. 1991).
Leaf area is difficult to determine directly in older mountain
ash trees because of their great size (50--80 m high), thus
indirect LAI estimation techniques, such as the PCA, are
required. The PCA is simple and rapid to apply and does not
require knowledge of the light extinction coefficient for the
stand, as is the case with quantum sensors (Pierce and Running
1988, Rich et al. 1993). However, the estimates of LAI made
with the PCA varied with sampling protocol and irradiance at
the time of measurement (Tables 2 and 3). Also, we do not
know how well the PCA performs in older stands where
canopy structure differs from that in young stands.
Acknowledgments
This study was funded by the Cooperative Research Center for Catchment Hydrology. Melbourne Water and the Victorian Department of
Conservation and Natural Resources assisted with several aspects of
TREE PARAMETERS AND TRANSPIRATION IN E. REGNANS
the field experiment. We thank Kim Whitford of the West Australian
Department of Conservation and Land Management for loaning us
photographic equipment and Dr. Barry Harper of Wollongong University for analyzing the canopy photographs.
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© 1995 Heron Publishing----Victoria, Canada
Relationships between stem diameter, sapwood area, leaf area and
transpiration in a young mountain ash forest
R. A. VERTESSY,1,4 R. G. BENYON,2,4 S. K. O’SULLIVAN3,4 and P. R. GRIBBEN3,4
1
CSIRO Division of Water Resources, GPO Box 1666, Canberra, ACT 2601, Australia
2
Melbourne Water, Box 4342, Melbourne, Victoria 3001, Australia
3
Monash University, Wellington Road, Clayton, Victoria 3168, Australia
4
Cooperative Research Center for Catchment Hydrology, Wellington Road, Clayton, Victoria 3168, Austr
alia
Received October 20, 1994
Summary We examined relationships between stem diameter, sapwood area, leaf area and transpiration in a 15-year-old
mountain ash (Eucalyptus regnans F. Muell.) forest containing
silver wattle (Acacia dealbata Link.) as a suppressed overstory
species and mountain hickory (Acacia frigescens J.H. Willis)
as an understory species. Stem diameter explained 93% of the
variation in leaf area, 96% of the variation in sapwood area and
88% of the variation in mean daily spring transpiration in 19
mountain ash trees. In seven silver wattle trees, stem diameter
explained 87% of the variation in sapwood area but was a poor
predictor of the other variables. When transpiration measurements from individual trees were scaled up to a plot basis, using
stem diameter values for 164 mountain ash trees and 124 silver
wattle trees, mean daily spring transpiration rates of the two
species were 2.3 and 0.6 mm day −1, respectively. The leaf area
index of the plot was estimated directly by destructive sampling, and indirectly with an LAI-2000 plant canopy analyzer
and by hemispherical canopy photography. All three methods
gave similar results.
Keywords: Acacia dealbata, Acacia frigescens, Eucalyptus
regnans, heat pulse method, leaf area index, mountain hickory,
silver wattle.
Introduction
Water for the city of Melbourne is obtained from 155,000 ha
of forested catchments in the central highlands of Victoria,
Australia. Mountain ash (Eucalyptus regnans F. Muell.) forests
cover just under half this area, but yield 80% of the streamflow
because they grow on the higher rainfall sites.
In 1939, about 80% of the extensive old-growth mountain
ash forest in Melbourne’s water supply catchments was killed
in bushfires. It regenerated naturally with dense re-growth
mountain ash forest. The widespread destruction and regeneration of the forests was followed by a prolonged decline in
streamflows. In the three decades following the fires, mean
annual streamflows from five catchments were 25% less than
the pre-1939 mean (Langford 1976). The reduction in water
yield after the fire in each catchment was proportional to the
percentage of mountain ash forest burnt (Langford 1976,
Kuczera 1987). The maximum decline in mean annual streamflow occurred 27 years after the fires and was equivalent to
6 mm for each 1% of catchment converted from old-growth to
re-growth (Kuczera 1987).
There have been many studies of the Melbourne water
supply area. Field experiments have examined stand-age-dependent rates of fog drip (O’Connell and O’Shaughnessy
1975), rainfall interception (Langford and O’Shaughnessy
1978) and evapotranspiration (Legge 1980, Dunn and Connor
1993, Jayasuriya et al. 1993). Fog drip was shown to be less
than 1% of gross precipitation and did not vary significantly
between re-growth and old-growth stands. Although Langford
and O’Shaughnessy (1978) found that interception in 34-yearold re-growth mountain ash forest was lower than in a 160year-old mature mountain ash forest, a recent, more
comprehensive study has determined that interception peaks at
26% of gross rainfall at age 30 years and steadily declines to
17% by age 240 years (S. Haydon, Melbourne Water, personal
communication). However, changes in interception explained
only about 25% of the changes in streamflow resulting from
conversion of old-growth forests to a re-growth state.
Although the causal mechanisms of the water yield response
in the Melbourne water supply area have not been fully elucidated, it is now assumed that differences in evapotranspiration
are the prime cause of the stand age--water yield relationship
in mountain ash forests. Recent studies by Dunn and Connor
(1993) and Jayasuriya et al. (1993) emphasize the role of
changing leaf area and sapwood area in determining changes
in forest evapotranspiration. However, no studies have provided quantitative data linking leaf area, sapwood area and
transpiration in mountain ash forests.
As a first step in elucidating the factors underlying the stand
age--water yield relationship in mountain ash forests, we have
examined relationships between stem diameter, sapwood area,
leaf area and transpiration in a 15-year-old mountain ash forest
560
VERTESSY, BENYON, O’SULLIVAN AND GRIBBEN
and obtained evidence that similar relationships could be derived for older stands. We determined leaf area by destructively
sampling individual trees. Because such sampling is impractical in older forests with large trees, we also evaluated two
methods for indirectly measuring leaf area index and show that
these can be used in older stands of mountain ash.
Materials and methods
Site description
The study site was located in the North Maroondah experimental area (Figure 1), at an elevation of 890 m, with a mean annual
rainfall of 1713 mm. Soils in the area are deep krasnozems of
high permeability and high water-holding capacity (Langford
and O’Shaughnessy 1977). The even-aged stand of mountain
ash was regenerated from direct seeding after clear-cutting in
1978. As a result of natural thinning, the stocking density of
mountain ash trees decreased from 8500 stems ha −1 in 1983 to
656 stems ha −1 at the time of the study in the spring of 1993.
Silver wattle (Acacia dealbata Link.) is a subdominant canopy
species, present at a stocking density of 496 stems ha −1. The
understory is mostly mountain hickory (Acacia frigescens J.H.
Willis), rising to a height of 3 m, and assorted ferns reaching
about 1 m in height.
A 50 × 50 m plot was established, containing 164 mountain
ash and 124 silver wattle trees, with mean heights of 28 and
24 m, respectively. The stem diameter at breast height over
bark (DBH) of each tree was measured. Mean DBH values for
the mountain ash and silver wattle trees were 20.4 and 15.6 cm,
respectively. Minimum DBH values were 9.6 and 10.0 cm, and
maximum DBH values were 39.2 and 27.1 cm, respectively.
Nineteen mountain ash and seven silver wattle trees, spanning
the range of DBH values in the plot, were selected for detailed
examination. For each of these trees, we determined the mean
daily sapflow velocity, mean daily transpiration rate, DBH,
height, total leaf weight, total leaf area and sapwood cross-sectional area (Table 1).
Transpiration measurements
Mean daily minimum and maximum temperatures for the
period of record were 4.4 and 12.8 °C, respectively, slightly
lower than the long-term October--November averages of 7.3
and 13.4 °C (Langford and O’Shaughnessy 1977). Total solar
radiation varied between 3.1 and 25.1 MJ m −2 day −1, with a
mean value of 13.9 MJ m −2 day −1. This mean daily value is
similar to the local October--November average of 13.2 MJ
m −2 day −1 (Langford and O’Shaughnessy 1977).
Over a 39-day period between September and November
1993, measurements of half-hourly transpiration were made
on the sample trees by the heat pulse technique (Cohen et al.
1985, Green and Clothier 1988, Hatton and Vertessy 1990,
Olbrich 1991, Dunn and Connor 1993). Half-hourly measurements of sapflow velocities were multiplied by sapwood crosssectional area to compute transpiration flux in individual trees.
Dunn and Connor (1993) validated this technique in young
mountain ash trees and showed that no significant wound
effect takes place after implanting sensor probes.
Six sapflow sensor units (Greenspan Technology, Warwick,
Queensland) were deployed for a period of 39 days using a
roaming sensor technique. One sensor unit was allocated to a
reference mountain ash tree (MA-REF) and another to a reference silver wattle tree (SW-REF) for the entire 39-day measurement period. The other four sensor units were moved from
tree to tree every 4 to 7 days (Table 1). At all times, three
roaming sensors were deployed in mountain ash trees and one
in a silver wattle tree. Measurements of total daily transpiration obtained from the roaming sensors were regressed against
those obtained with the reference sensors. The resulting regression equations were used to predict total daily transpiration in
the roaming sensor trees outside the period of measurement in
Figure 1. Location of the North Maroondah experimental area.
TREE PARAMETERS AND TRANSPIRATION IN E. REGNANS
561
Table 1. Parameter values for all sample trees. The value of r2 is relative to mean daily transpiration in reference trees MA-REF and SW-REF.
Tree
DBH
(cm)
Leaf weight
(kg)
Leaf area
(m2)
Sapwood area
(cm2)
Sap flux record
(day)
r2
Mean sap flux
(l day −1 )
Mean sap velocity
(cm h −1)
MA-REF
MA1
MA2
MA3
MA4
MA5
MA6
MA7
MA8
MA9
MA10
MA11
MA12
MA13
MA14
MA15
MA16
MA17
MA18
SW-REF
SW1
SW2
SW3
SW4
SW5
SW6
22.4
18.3
27.9
18.2
30.6
21.2
15.6
23.8
31.5
22.5
16.4
28.4
20.5
27.0
37.1
39.2
25.8
34.7
36.6
27.1
25.0
24.9
23.7
15.4
20.7
20.7
15.06
9.05
26.96
6.69
27.98
10.83
3.68
26.13
27.50
13.50
3.69
26.16
17.09
25.40
60.85
63.12
27.53
55.70
63.49
11.95
---0.46
---
36.33
21.83
65.03
16.13
67.48
26.13
8.89
63.02
66.32
32.57
8.89
63.08
41.21
61.27
146.78
152.26
66.41
134.34
153.13
23.34
---0.90
---
121.3
96.1
215.0
66.0
233.0
93.5
50.5
212.6
261.0
140.0
62.5
234.7
116.1
236.4
451.8
401.1
198.0
352.9
406.4
236.2
278.5
204.5
208.3
72.1
178.3
113.7
39
5
7
4
4
5
5
5
7
5
7
4
7
7
5
5
5
7
5
39
5
5
5
7
7
4
-0.42
0.92
0.81
0.94
0.96
0.46
0.88
0.92
0.96
0.85
0.92
0.77
0.71
0.88
0.96
0.96
1.00
0.96
-0.94
0.98
0.96
0.06
0.31
0.04
25.4
22.4
56.7
23.3
87.4
26.2
6.6
74.2
67.9
38.2
15.2
75.6
35.8
69.1
150.6
135.1
64.4
115.2
87.8
19.8
58.9
45.0
20.0
----
8.7
9.7
11.0
14.7
15.6
11.7
5.5
14.5
10.8
11.4
10.1
13.4
12.8
12.2
13.9
14.0
13.6
13.6
9.0
3.5
8.8
9.2
4.0
----
those trees. This enabled us to estimate total daily transpiration
for each sample tree over the entire 39-day measurement
period. No transpiration measurements were made in the understory.
Fractions of wood, water and air in the sapwood were
determined gravimetrically on one 5-mm diameter increment
core from each tree at the end of the transpiration experiment.
In the mountain ash trees, the wood fraction varied between
0.26 and 0.34 (mean = 0.29), whereas the water fraction varied
between 0.55 and 0.67 (mean = 0.60). In the silver wattle trees,
the wood fraction varied between 0.31 and 0.37 (mean = 0.32),
and the water fraction varied between 0.28 and 0.49 (mean =
0.36). Hence, the silver wattle trees had a much larger air
fraction in the sapwood than the mountain ash trees.
Nondestructive measurement of leaf area index
An LAI-2000 plant canopy analyzer (PCA) (Li-Cor Inc., Lincoln, NE) and hemispherical canopy photographs were used to
estimate the leaf area index (LAI) of the 50 × 50 m plot. Both
methods combine the leaf area of the overstory and understory
species to give a total LAI value for the plot.
Leaf area index was computed from the PCA measurements
using a canopy gap fraction model (Welles and Norman 1991).
We deployed two PCAs; one device was left in a clearing
(1.5 km from the site) to record light conditions automatically
every 15 s, whereas the other PCA was used manually beneath
the canopy in the experimental plot. All PCA measurements
were made over a 360° field of view, under cloudy (diffuse
radiation dominated) conditions or at dawn or dusk when there
was no direct sunlight on the canopy. We only used readings
from the four inner rings of the sensor, thereby confining the
PCA’s field of view to a radius of 41 m.
Three sampling protocols (random, transect and fixed position) were used in the application of the PCA to the experimental plot. In the random sampling protocol, the PCA operator
moved around the plot making readings at random locations.
These measurements were repeated on four different occasions
to include a variety of sky conditions. In the transect sampling
protocol, the PCA operator made measurements at roughly
1-m intervals along two 50-m transects laid out across the plot
and two 70-m transects running along the diagonals of the plot.
One of the 50-m transects was measured twice and the other
50-m transect was measured three times. In the fixed position
sampling protocol, the PCA operator made measurements at
six fixed points in the plot on six occasions.
Multiple hemispherical photographs were taken at the same
six points within the plot. We used Ilford pan F (50 ASA) film
in a NIKON FG camera, fitted with a Nikkor 8 mm fisheye
lens. All photographs were taken under cloudy conditions to
ensure even back-lighting. Each negative was digitized to
provide a 512 × 512 8-bit image. The images were analyzed to
determine the percent canopy gap as a function of zenith and
azimuth angles. The PISCES program (Jupp et al. 1980) was
used to compute the generalized foliage area index (GFAI),
which is analogous to LAI, from the canopy gap fraction data.
Although the scanned images extended to a zenith angle of
562
VERTESSY, BENYON, O’SULLIVAN AND GRIBBEN
84.3°, the foliage was so dense that measurements beyond 65°
had little influence on the analysis; hence, the hemispherical
photographic and PCA methods had similar fields of view.
Destructive measurement of leaf area index
After completion of the transpiration and PCA measurements,
nineteen mountain ash trees and two silver wattle trees were
felled. The total leaf mass from each tree was harvested and
weighed. The height of each tree was measured and stem discs
were cut at breast height (1.3 m above ground level) for
measurements of sapwood cross-sectional area.
Five circular subplots, each with a radius of 5 m, were
established within the 50 × 50 m plot, covering a total area of
392.75 m2, or 15.7% of the experimental plot. Leaves were
stripped and weighed from all mountain hickory trees located
within each subplot.
Leaf subsamples from each of the three tree species were
weighed, then immediately fed through a planimeter to obtain
a leaf area to weight ratio for each species. It was difficult to
obtain reliable leaf area estimates for the silver wattle trees
because the fragile pinnate leaves were easily dislodged when
the trees hit the ground and were awkward to feed through the
planimeter. Leaf area estimates were therefore obtained for
only two of the seven sample silver wattle trees, including the
largest and one of the smallest trees in the plot.
Sapwood area measurements
Sapwood thickness and sapwood cross-sectional area of each
sample tree were determined from measurements on several
5-mm diameter increment cores, taken at breast height in each
of the sample trees before felling. The sapwood width measurements were used to determine appropriate depths for implanting heat pulse sensor probes. After felling, more accurate
measurements of sapwood area were made on wood discs, cut
at breast height. Measurements of bark thickness, sapwood
thickness and heartwood diameter at up to 15 points around
each wood disc were used to compute sapwood area. These
determinations were used in the transpiration calculations.
In the silver wattle trees, the heartwood and sapwood were
clearly distinguishable on the basis of color. To identify sapwood in the mountain ash increment cores, it was necessary to
hold each core up against a bright light. The open vessels of
the sapwood were clearly visible as pin-points of light,
whereas the blocked vessels of the heartwood did not transmit
light. The heartwood in the mountain ash discs could be easily
distinguished after a day or two of exposure to air because the
heartwood became brown, whereas the sapwood remained
straw colored. In mountain ash trees, the no-flow boundary
detected with the sapflow sensor corresponded to the independent estimates of the sapwood--heartwood transition.
by the measured leaf area/leaf wet weight ratio of 2.412 m2
kg −1 (Table 1). To scale up individual tree leaf areas to a plot
LAI value, a regression equation was developed that related
leaf area to stem diameter (Figure 2). Because a linear fit gave
an undesirable y-intercept value, we applied a power function
that passed through the origin and hence reflected the growth
process. The r2 value of 0.93 (n = 19) indicated a strong
association between stem diameter and leaf area. This regression equation was evaluated for all 164 mountain ash trees in
the plot, resulting in a total mountain ash leaf area of 5342 m2,
or an LAI of 2.1.
Leaf wet weights were determined for two silver wattle trees
(Table 1), and these were multiplied by the measured leaf
area/leaf wet weight ratio of 1.953 m2 kg −1 to determine leaf
area for both trees. Silver wattle leaf area was regressed against
stem diameter, based on the regression equation used for the
mountain ash trees. Although the power function was fitted to
only two data points, we believe the fitting was valid because
the function passed through the origin, and data points for the
largest tree and one of the smallest trees in the plot were
included. The small diameter silver wattle trees supported little
leaf area and were beginning to die out as a result of over-topping by the mountain ash canopy. The large silver wattle trees
occurred in canopy gaps and were therefore less affected by
competition with mountain ash trees. The regression equation
relating silver wattle leaf area to stem diameter was used to
estimate total leaf area for all 124 silver wattle trees in the 50
× 50 m plot. This yielded an estimate of 290.23 m2 of leaf area,
equivalent to an LAI of 0.1.
The leaf area/leaf wet weight ratio for the mountain hickory
was 3.646 m2 kg −1. The total leaf mass harvested from five
circular plots was 194.38 kg, equivalent to a total leaf area of
708.72 m2, or an LAI of 1.8.
Indirect leaf area index estimates
Table 2 lists means of all of the PCA-based LAI estimates
made for the 50 × 50 m plot using the random, transect and
fixed position sampling protocols. For each sampling protocol,
multiple determinations were made under varying irradiances
Results
Direct leaf area index estimates
Leaf area estimates for 19 mountain ash trees were based on
the wet weight of leaves collected from each tree, multiplied
Figure 2. Stem diameter (DBH) versus leaf area for the sampled
mountain ash and silver wattle trees.
TREE PARAMETERS AND TRANSPIRATION IN E. REGNANS
Table 2. Various LAI estimates for the 50 × 50 m plot obtained using
the PCA; n = number of readings per sample.
Sampling protocol
Random
Random
Random
Random
n
Run identifier
LAI
1
2
3
4
228
200
60
200
3.6
4.5
4.5
3.8
Transect (east)
Transect (east)
Transect (north)
Transect (north)
Transect (north)
Transect (both diagonals)
5
6
7
8
9
10
50
50
55
55
55
130
4.4
4.2
4.4
5.0
4.5
3.9
Fixed position
Fixed position
Fixed position
Fixed position
Fixed position
Fixed position
11
12
13
14
15
16
6
6
6
6
6
6
3.8
4.3
4.1
4.4
4.0
3.8
Total/mean
1--16
1119
4.2
to obtain an average value of LAI. Mean LAI values for the
random, transect and fixed position sampling protocols were
4.1, 4.4 and 4.1, respectively, and the mean of all PCA readings
obtained from all sampling protocols was 4.2. None of the
sampling protocols was more accurate or reproducible than the
others.
Fixed-position LAI estimates based on the PCA and photographic methods are compared in Table 3. For the PCA
method, we list the mean and range of LAI values measured
for each plot. For the photographic method, we list the LAI
determination and error margin for a single photograph from
each plot. With the exception of Plot 3, the individual fixed-position LAI estimates obtained by the PCA and photographic
methods differed by 0.5 or less. The mean photograph-based
LAI estimate for the plot was 4.4, which is almost the same as
the mean fixed-position estimate of 4.1 obtained with the PCA.
Although we applied the PCA under ideal light conditions,
LAI determinations for some plots varied by 1.0 or more
between samplings. Error bounds for the photograph-based
563
LAI estimates were large for individual photographs (up to 2.1
units) but small for the lumped plot value (only 0.3).
Sapwood area estimates
Sapwood areas in the mountain ash trees ranged between 50.5
and 451.8 cm2 and averaged 207.8 cm2, whereas silver wattle
sapwood areas ranged between 72.1 and 278.5 cm2 and averaged 184.5 cm2 (Table 1). Sapwood areas were reliably predicted from stem diameter measurements in both species,
yielding r2 values of 0.96 and 0.87, respectively (Figure 3).
Regression equations for the two species were similar, though
statistically different.
The sapwood areas for all mountain ash and silver wattle
trees in the plot were estimated from regression equations for
each species. Total sapwood area for the 164 mountain ash
trees in the plot was 8.04 m2 ha −1. Total sapwood area for the
124 silver wattle trees in the plot was 4.16 m2 ha −1, or 34.1%
of the combined overstory sapwood area. There was a strong
relationship between sapwood area and leaf area in the mountain ash trees (r2 = 0.96, n = 19) (Figure 4). There were
insufficient leaf area measurements for the silver wattle trees
to make a similar comparison.
Figure 3. Stem diameter (DBH) versus sapwood area for the sampled
mountain ash and silver wattle trees.
Table 3. Fixed position LAI estimates for the six circular plots obtained using the PCA and hemispherical canopy photography.
Plot
no.
LAI
(PCA)
Range
(PCA)
LAI
(photo)
Error
(photo)
1
2
3
4
5
6
All
3.2
4.1
4.5
4.2
4.3
4.1
4.1
2.9--3.5
3.6--4.7
4.1--4.7
3.8--4.8
4.0--4.6
3.2--4.6
2.9--4.8
3.7
4.0
6.1
3.7
4.2
4.4
4.4
± 0.8
± 0.6
± 1.3
± 1.4
± 1.0
± 2.1
± 0.3
Figure 4. Sapwood area versus leaf area for the sampled mountain ash
trees.
564
VERTESSY, BENYON, O’SULLIVAN AND GRIBBEN
Figure 5. Daily transpiration in the two
reference trees (MA-REF and SW-REF)
over the 39-day sampling period.
Transpiration estimates in the sample trees
Over the 39-day measurement period, transpiration in tree
MA-REF varied between 9 and 43 l day −1 , and the mean daily
value was 25 l day−1 (Figure 5). In tree SW-REF, transpiration
varied between 1 and 40 l day −1 , with a mean daily value of
20 l day −1 (Figure 5). Tree SW-REF had a larger diameter than
tree MA-REF (27.1 versus 22.4 cm) and greater sapwood area
(236.2 versus 121.3 cm2), but a lower leaf area (23.34 versus
36.33 m2). Despite these differences, daily transpiration rates
in the two trees were similar, except that when transpiration in
tree MA-REF dropped below 20 l day −1, transpiration in tree
SW-REF declined sharply (Figure 6). On these days, radiation
was usually below average, and wind speeds were significantly
lower than normal. The silver wattle canopy was on average
about 4 m below that of the mountain ash and would have been
poorly ventilated on days of low wind speed, so the results
suggest uncoupling of the silver wattle canopy from the atmospheric boundary layer during these times.
For periods of concurrent record, daily transpiration values
for the other 18 mountain ash trees were regressed against
daily transpiration values for tree MA-REF. Because the statistical association between daily transpiration totals in the different mountain ash trees was strong, with r2 values exceeding
0.9 in over half the cases and exceeding 0.7 in all but two cases
Figure 6. Daily transpiration in tree MA-REF versus daily transpiration in tree SW-REF. Solid lines indicate best fit linear regressions for
conditions where transpiration of tree MA-REF is greater than (s) and
less than (+) 20 l day −1.
(Table 1), we were able to estimate daily transpiration in the
other 18 mountain ash trees for the entire 39-day period. The
mean daily transpiration for these trees varied between 6.6
(MA6) and 150.6 l day −1 (MA14), with a mean of 62 l day −1
(Table 1).
Daily transpiration values for tree SW-REF were regressed
against daily transpiration values for the other six silver wattle
trees sampled during periods of concurrent record. Although
trees SW1, SW2 and SW4 displayed daily transpiration behavior similar to tree SW-REF (r2 = 0.9), trees SW3, SW5 and
SW6 did not. This difference may be attributable to the rapid
thinning of the silver wattle trees in the plot as the mountain
ash canopy began to over-top the silver wattle trees. With the
exception of tree SW4, all of the positive results came from the
larger silver wattle trees. Consequently, robust transpiration
totals for the 39-day sampling period were derived for only
four of the seven silver wattle trees sampled. For these four
trees, mean daily transpiration varied between 19.8 (SW-REF)
and 58.9 l day −1 (SW1), with a mean of 35.9 l day −1 (Table 1).
There was considerable variation in mean daily sapflow
velocity among the 19 mountain ash trees sampled, with values
ranging between 5.5 (MA6) and 15.6 cm h −1 (MA4), and
averaging 11.9 cm h −1 (Table 1). There was no systematic
relationship between stem diameter and mean daily sapflow
velocity in the mountain ash trees. Mean daily sapflow velocities for the four silver wattle trees varied between 3.5 (SWREF) and 9.2 cm h −1 (SW2), and averaged 6.4 cm h −1
(Table 1). This mean daily value was just over half that observed for the mountain ash trees, but almost twice that observed for mature A. frigescens (Dunn and Connor 1993).
Mean daily transpiration in the mountain ash trees was
strongly related to both DBH (r2 = 0.88, n = 19) and leaf area
(r2 = 0.91, n = 19) (Figures 7 and 8). The relationship between
leaf area and transpiration was almost linear (cf. Running and
Coughlan 1988, Hatton et al. 1992, Band et al. 1993, Vertessy
et al. 1993). However, the larger trees appeared to have a
slightly lower leaf area efficiency than the smaller trees, even
though they experienced higher radiation and wind speeds, and
lower humidity than the smaller trees. This finding is consistent with observations by Yoder et al. (1994) that large trees
tend to experience plumbing problems and consequent stomatal limitations more than small trees.
TREE PARAMETERS AND TRANSPIRATION IN E. REGNANS
565
largest mountain ash trees in the plot (occupying 22% of the
basal area) accounted for 28% of the mountain ash transpiration, whereas the 90 smallest mountain ash trees in the plot
(also occupying 22% of the basal area) were responsible for
only 22% of the mountain ash transpiration (Figure 9). This
finding emphasizes the need to sample a spectrum of tree sizes
when attempting to estimate plot water use by the heat pulse
method.
Discussion
Figure 7. Stem diameter (DBH) versus mean daily transpiration for the
sampled mountain ash trees.
Figure 8. Leaf area versus mean daily transpiration for the sampled
mountain ash trees.
Plot transpiration
The mean daily transpiration for all 164 mountain ash trees in
the experimental plot was calculated to be 5740 l day −1, or 2.3
mm day −1 when expressed on an area basis. Although we could
not obtain a statistically significant relationship between mean
daily transpiration and DBH in the silver wattle trees, by
multiplying the total sapwood area (1.04 m2) by the mean daily
sapflow velocity (6.4 cm h −1), we obtained a mean daily
transpiration estimate of 1597 l day −1, or 0.6 mm day −1. The
estimate of plot transpiration for the silver wattle trees is less
robust than the estimate for mountain ash, because the estimate
of mean sapflow velocity was based on only four trees and
these were amongst the largest silver wattle trees in the plot.
However, we observed no systematic relationship between
DBH and sapflow velocity in the mountain ash trees. For the
entire 39-day measurement period, the mean daily transpiration rate for the mountain ash and silver wattle trees combined
was 2.9 mm day −1 .
Transpiration among the mountain ash trees in the experimental plot was highly skewed. When the relative mean daily
transpiration (expressed as a percentage of the plot total) of
mountain ash trees was ranked on the basis of DBH, the 14
In the mountain ash, we found strong statistical associations
between stem diameters and all other stand parameters analyzed. Stem diameter measurements explained 93% of the
variation in leaf area, 96% of the variation in sapwood area and
88% of the variation in mean daily transpiration. Because stem
diameters are easy to measure, it is possible to make reliable
assessments of related stand parameters in large plots. We did
not test whether the same statistical associations hold for older
mountain ash stands.
A strong association between sapwood area and leaf area has
been noted in other eucalypts (Brack et al. 1985, Moore 1993,
Hatton et al. 1995). Although Hatton and Wu (1995) cautioned
that leaf area can decline quickly in response to drought,
whereas sapwood area remains stable, this effect is probably
not significant in mountain ash trees because they are rarely
drought stressed. However, it is not known whether leaf area
varies temporally in these forests in response to seasonal
changes in irradiance or temperature, or both.
When estimating tree transpiration using the heat pulse
technique, accurate measurement of sapwood area is essential.
Even in young mountain ash trees that appear symmetrical, we
found considerable variation in the width of the sapwood at
breast height within individual trees (Table 4). Thus, a single
sample from MA15 may have produced an estimated sapwood
width anywhere between 25 and 48 mm, whereas the true
mean was 38 mm. In most of the 19 mountain ash trees
analyzed, at least two or three core samples would be needed
to give an estimate of the mean sapwood width within 5 mm
Figure 9. Relative transpiration by mountain ash trees of different
sizes in the 50 × 50 m plot. Class 151-164 contains the 14 largest trees
in the plot.
566
VERTESSY, BENYON, O’SULLIVAN AND GRIBBEN
Table 4. Variability of sapwood widths within individual mountain ash
trees; n = number of samples, SD = standard deviation, P95 = number
of samples needed to estimate mean sapwood thickness within 95%
confidence limits of 5 mm.
Tree
n
Range
(mm)
Mean
(mm)
SD
(mm)
P95
MA-REF
MA1
MA2
MA3
MA4
MA5
MA6
MA7
MA8
MA9
MA10
MA11
MA12
MA13
MA14
MA15
MA16
MA17
MA18
8
8
8
8
12
8
8
7
8
8
8
12
8
8
10
13
8
8
14
15--23
18--22
28--32
10--18
21--33
14--20
10--13
33--40
26--36
15--31
12--17
22--42
18--26
29--37
41--53
25--48
27--33
29--38
32--52
20
20
29
14
28
16
12
36
31
24
14
31
21
33
47
38
30
39
42
3
1
2
3
4
2
1
2
3
5
2
6
3
3
3
8
3
3
6
2
1
2
2
3
2
1
2
2
4
2
6
2
2
4
10
2
2
6
of the true mean with 95% confidence (Table 4). Our stem disc
analyses indicated that sapwood width was more variable in
the large-diameter trees than in the small-diameter trees. Because of slight buttressing, wider sapwood occurred at the
buttresses, whereas the narrower sapwood occurred between
the buttresses.
Dunn and Connor (1993) reported sapwood area values of
6.74, 6.09, 4.23 and 4.04 m2 ha −1 for 50-, 90-, 150- and
230-year-old mountain ash stands, respectively. Our plotbased estimate of mountain ash sapwood area of 8.04 m2 ha −1
therefore lends support to Dunn and Connor’s (1993) hypothesis that sapwood area declines with age in mountain ash forests. Our data also support Dunn and Connor’s (1993)
contention that mean daily sapflow velocity in mountain ash
trees appears to be independent of stand age. Over a spring-summer--autumn sampling period, they estimated mean daily
sapflow velocities of 11.5, 11.4, 9.9 and 11.8 cm h −1 for 50-,
90-, 150- and 230-year-old mountain ash stands, respectively.
In our 15-year-old stand, we estimated a mean daily sapflow
velocity of 11.9 cm h −1. This implies that spring transpiration
in our stand was almost twice that of a 230-year-old stand.
The strong relationship between stem diameter and transpiration in mountain ash permits calculations to be made on the
implications of forest thinning for catchment water balance.
We estimate that a 50% basal area reduction would reduce
mean daily (spring) transpiration by 58% if the biggest trees
were felled, or by 42% if the smaller trees were felled. Leaf
area (and hence rainfall interception) would be reduced by 61
or 39%, depending on whether the biggest or smallest mountain ash trees were removed (Figure 2). These data imply that
naturally occurring water yield deficits in re-growth mountain
ash forests could be significantly off-set by selectively felling
the largest trees every few years after re-growth commences.
However, recent forest thinning experiments in mountain ash
forests suggest that instantaneous reductions in transpiration
and interception would not be converted entirely into streamflow gains (Jayasuriya et al. 1993).
We note that our plot transpiration estimates are deficient in
two key respects if they are to be used in water balance
calculations. First, they are only valid for spring conditions. To
estimate annual transpiration from stem diameter measurements, it would be necessary to repeat sapflow measurements
in the trees at different times of the year. Second, we did not
measure the transpiration rate of the mountain hickory understory, though we have reason to believe that this is small,
despite its significant contribution to plot leaf area. In a mature
(230-year-old) mountain ash forest with a well developed
mountain hickory understory and low density mountain ash
overstory (72 stems ha −1), Dunn and Connor (1993) estimated
that the understory was responsible for about 27% of the total
forest transpiration during summer. However, in a soil water
balance study carried out in denser 38-year-old re-growth
forest, Langford and O’Shaughnessy (1979) concluded that
the woody understory contributed little to soil water depletion
over a summer period. Although 45% of the total plot leaf area
was associated with the understory at our site, we suspect that
transpiration from this layer would be low because of intense
shading from the overstory trees and minimal turbulent transfer between the understory and overstory layers.
Based on destructive sampling, the leaf area indices of the
mountain ash, silver wattle and mountain hickory vegetation
were 2.1, 0.1 and 1.8, respectively, yielding a total plot LAI
value of 4.0. The PCA and photographic methods yielded
values of 4.2 and 4.4, respectively, thus overestimating LAI by
5 to 10%. This difference is small and could be attributable to
errors in any of the estimation techniques. However, the degree
and direction of difference are consistent with other studies
that have compared direct and indirect estimates of LAI in
forests; these studies have concluded that slight overestimation
arises in indirect methods because tree stems and branches
account for between 4 and 12% of light interception in forests
(Neumann et al. 1989, Chason et al. 1991, Lang et al. 1991).
Leaf area is difficult to determine directly in older mountain
ash trees because of their great size (50--80 m high), thus
indirect LAI estimation techniques, such as the PCA, are
required. The PCA is simple and rapid to apply and does not
require knowledge of the light extinction coefficient for the
stand, as is the case with quantum sensors (Pierce and Running
1988, Rich et al. 1993). However, the estimates of LAI made
with the PCA varied with sampling protocol and irradiance at
the time of measurement (Tables 2 and 3). Also, we do not
know how well the PCA performs in older stands where
canopy structure differs from that in young stands.
Acknowledgments
This study was funded by the Cooperative Research Center for Catchment Hydrology. Melbourne Water and the Victorian Department of
Conservation and Natural Resources assisted with several aspects of
TREE PARAMETERS AND TRANSPIRATION IN E. REGNANS
the field experiment. We thank Kim Whitford of the West Australian
Department of Conservation and Land Management for loaning us
photographic equipment and Dr. Barry Harper of Wollongong University for analyzing the canopy photographs.
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