Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:
Tree Physiology 16, 267--274
© 1996 Heron Publishing----Victoria, Canada
Growth and water relations of Eucalyptus marginata (jarrah) stands in
response to thinning and fertilization
G. L. STONEMAN,1,2 D. S. CROMBIE,1 K. WHITFORD,1 F. J. HINGSTON,3 R. GILES,1
C. C. PORTLOCK,1 J. H. GALBRAITH3 and G. M. DIMMOCK3
1
Department of Conservation and Land Management, Research Centre, Dwellingup, WA 6213, Australia
2
Present address: Department of Conservation and Land Management, Research Centre, Como, WA 6152, Australia
3
CSIRO, Division of Forest Research, Private Bag, PO Wembley, WA 6014, Australia
Received June 2, 1994
Summary We studied the effects of five thinning treatments
(T1 = 5.5, T2 = 11, T3 = 16.5, T4 = 22.5 and T5 = 28.5 m2 ha −1
basal area under bark) × two fertilizer treatments (F0 = unfertilized and F1 = fertilized with 400 kg ha −1 N plus 229 kg
ha −1 P) on growth and water relations of pole-sized Eucalyptus
marginata J. Donn ex Sm. trees growing in southwestern
Australia. Thinning reduced leaf area index (LAI) from 2.1 in
the T4 and T5 treatments to 0.8 in the T1F0 treatment. Fertilizer
had no effect on LAI in the T2, T4 or T5 treatments, but
increased LAI by 45 and 20% in the T1 and T3 treatments,
respectively. Thinning plus fertilizing increased diameter
growth most in the fastest growing trees, from 0.4 cm year −1
for trees in the T5F0 and T5F1 treatments to 0.7 and 1.2 cm
year −1 for trees in the T1F0 and T1F1 treatments, respectively.
In both fertilizer treatments, stand basal area and volume
growth increased with increasing stand density up to 15 m2
ha −1, and thereafter declined with increasing stand density,
such that the growth rate of trees in the T5 treatment was only
half of that at a stand density of 15 m2 ha −1. In response to
fertilizer, growth rates of the slowest and fastest-growing trees
increased from 0.35 and 3.5 m2 ha −1 year −1 (F0) to 0.56 and 5.4
m3 ha −1 year −1 (F1), respectively. Stand growth efficiency
(growth per unit LAI) increased in response to thinning, and
fertilizer increased stand growth efficiency at all stand densities. Throughout the dry season, T5 trees had lower predawn
shoot water potentials (Ψpd) (minimum of −1.5 MPa) than T1
or T2 trees (minimum of −0.7 MPa). Fertilizer decreased Ψpd
in T5 trees (by −0.9 and −1.5 MPa, respectively, in F0 and F1),
but not in T1 or T2 trees. Stand growth rate was closely related
to cumulative midday water stress (CMWS) over the dry season, and volume growth rate declined sharply from 6 m3 ha −1
year −1 at a CMWS of 130 MPa days, to zero at a CMWS of
220 MPa days. Application of fertilizer to thinned stands
increased LAI, stand growth efficiency and stand growth. In
unthinned stands, fertilizer increased stand growth efficiency
and stand growth; however, it also increased tree water stress,
which limited the fertilizer-induced increases in LAI and
growth. We attribute the increase in tree and stand growth in
response to application of fertilizer to increased photosynthetic
rates, increased allocation to stem wood, and in thinned stands
also to higher LAIs.
Keywords: drought, growth efficiency, leaf area index, shoot
water potential.
Introduction
Water and nutrients are two of the most important factors
controlling the growth of forest trees (Nambiar et al. 1990).
The relative importance of each factor in controlling growth
varies with species, soil and climatic conditions. Understanding how these factors control forest growth in a range of
forest conditions (see Landsberg 1986, Landsberg 1989,
Pereira et al. 1989, Gower et al. 1992, Raison and Myers 1992)
is fundamental for the development of physiologically based
forest growth models (Kimmins et al. 1990, McMurtrie and
Landsberg 1992).
We have investigated the effects of thinning and application
of fertilizer on Eucalyptus marginata J. Donn ex Sm. tree and
stand growth. The specific objectives of the study were to
quantify the effects of thinning and fertilization on tree and
stand growth, leaf area index (LAI) and plant water stress, and
to determine the effect of water stress on stand growth.
Methods
Study area
The experiment was located in an E. marginata stand at Inglehope Forest Block (32°45′ S, 116°11′ E), 9 km southeast of
Dwellingup, Western Australia. Soil on the site is 0.5 to 1 m of
yellow sandy loam overlying ferruginous concretionary gravels in a sandy loam matrix up to 4 m thick. Beneath this,
kaolinitic clay extends to granite bedrock at depths between
18 m and 57 m, with groundwater tables at between 20 and
32 m. The soils are of low fertility (Leeper 1970, Hingston
et al. 1981). The climate of the area is typically Mediterranean.
During the study, average annual rainfall at Dwellingup was
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STONEMAN ET AL.
1217 mm, average annual pan evaporation was 1277 mm,
average annual maximum temperature was 21.6 °C and average annual minimum temperature was 9.8 °C (Figure 1). Longterm average annual rainfall at the site is 1100 mm, 200 mm
less than at Dwellingup. Average monthly maximum temperatures varied from about 30 °C in January and February to 14 °C
in July.
Experimental design
Plots (40 m by 40 m with an 8 m buffer on all sides) were first
thinned in 1964 (at age 40 years) to stand densities of about 7
(T1), 11 (T2), 15 (T3), and 18 (T4) m2 ha −1 basal area under
bark, and stand density of the unthinned plots was about 22 m2
ha −1 (T5). In 1986, a fertilizer treatment and a second thinning
were imposed. The experimental design comprised 10 treatments of five thinning treatments (stand basal area under bark
of 5.5 (T1), 10.9 (T2), 16.4 (T3), 22.4 (T4) or 28.5 (T5) m 2 ha −1
(unthinned) in 1986) × two fertilizer treatments (F0 = unfertilized and F1 = fertilized with 400 kg ha −1 N and 229 kg ha −1 P)
× three replicate plots. For the F1 plots, 200 kg ha −1 N and 229
kg ha −1 P were applied in autumn 1987 as 1145 kg ha −1 of
diammonium phosphate, and 200 kg ha −1 N was applied in
autumn 1988 as 588 kg ha −1 of ammonium nitrate. This rate of
fertilizer application was chosen to give an optimum growth
response (Stoneman et al. 1989). In both the first thinning in
1964 and the second thinning in 1986, the smaller, slowergrowing trees were removed (Smith 1962). Table 1 shows the
effect of the treatments on stand structure.
Growth and leaf area measurements
To estimate growth in diameter, basal area and bole volume,
tree diameter over bark and bark thickness were measured on
all trees in all plots in 1987 and 1991, and bole height was
measured in 1985. Volume was calculated for each tree based
on the equation of Pearce et al. (1992). To estimate the seasonal
diameter growth pattern, stem diamter was measured monthly
from dendrometer bands fitted at breast height on six trees per
plot with diameter of about 30 cm. Leaf area index (LAI) of
each plot was estimated based on allometric equations and
Table 1. Average stand basal area under bark (SBA), average stocking,
average diameter at breast height under bark (DBH), and average tree
height for all trees in each of the thinning and fertilizer treatments.
Treatment
SBA
(m2 ha −1)
Stocking
(stems ha −1)
DBH
(cm)
Height
(m)
T1F0
T1F1
T2F0
T2F1
T3F0
T3F1
T4F0
T4F1
T5F0
T5F1
5.3
4.6
10.7
9.7
15.7
16.2
22.3
21.8
29.1
27.1
46
42
150
375
205
290
470
435
975
960
36.5
36.3
27.0
30.4
29.1
25.0
21.3
21.8
16.6
16.4
24.6
24.8
22.1
23.6
24.3
22.3
20.9
21.0
18.2
17.8
hemispherical photographs (Whitford 1991, Whitford unpublished data). The mean LAI for each plot was based on up to
six separate estimates of LAI taken between spring 1987 and
autumn 1990. Stand growth efficiency (Waring 1983) is defined as the basal area growth per plot per unit of LAI (m2 ha −1
year −1/LAI).
Water relations
Predawn shoot water potential (Ψpd) and midday shoot water
potential (Ψmd) were measured with a pressure chamber
(Scholander et al. 1965, Turner 1988) at 2- to 8-week intervals,
from May 1988 to May 1989, and at about 2-week intervals
from November 1989 to May 1990. These measurements were
made in one plot of each of the T1F0, T1F1, T2F0, T2F1, T5F0
and T5F1 treatments. Additionally, shoot water potential (Ψs)
was measured throughout the day on several occasions from
November to April. These measurements were taken in one
plot of each of the T1F0 and T1F1, or T2F0 and T2F1 treatments as well as in one plot of each of theT5F0 and T5F1
treatments. Shoot water potential was measured on two twigs
on each of four dominant or codominant trees in each plot on
each occasion for the predawn, midday and diurnal sets of
measurements. Cumulative water stress (CWS) was calculated
separately from both the Ψpd and Ψmd data for the 1988--1989
and 1989--1990 seasons as described by Ritchie and Hinckley
(1975) and Myers (1988).
Results
Diameter growth
Figure 1. Average monthly values for maximum and minimum air
temperature, rainfall and pan evaporation at Dwellingup over the study
period.
Diameter growth rate increased with decreasing stand density
from 0.02 cm year −1 for trees on T5F0 plots to about 0.75 cm
year −1 for trees on T1F0 plots (Figure 2). Application of
fertilizer increased average diameter growth in all thinning
treatments, with the greatest increase in rate occurring in trees
growing at the lowest stand density (1.2 cm year −1 for trees in
the T1F1 plots).
Analysis of average growth rates biased the data because the
slower growing trees had been removed from the heavily
thinned plots, whereas they remained in the unthinned and
EUCALYPTUS MARGINATA RESPONSE TO THINNING
269
thinning and fertilizing resulting in a larger response to fertilizer at the low stand densities.
Diameter growth rates of trees in all thinning treatments
were not significantly affected by fertilizer application in
1987, the first year after fertilizer was applied (Figure 4);
however, in the second and third years, fertilizer increased
diameter growth rates of trees in all thinning treatments. Diameter growth occurred mainly during the late autumn, winter
and spring periods. The high growth rates recorded when the
trees were water stressed during the February to April period
were caused by bark swelling following rainfall events, and not
to diameter growth (see Figure 7a).
Figure 2. Average diameter growth in relation to stand basal area for
the fertilized (F1) and unfertilized (F0) treatments. Error bars represent one standard error. The fitted equations are F1: y = 2.250 − 0.6545
lnx, r 2 = 0.98, n = 15, F0: y = 1.384 − 0.4034lnx, r 2 = 0.90, n = 15.
lightly thinned plots and depressed the average growth rate (cf.
Figures 2 and 3a). The fastest growing 200 stems ha −1 in the
unthinned and lightly thinned plots exhibited the greatest response to both thinning and fertilizer, whereas the remaining
trees exhibited little response to either treatment. In the unfertilized plots, diameter growth of the fastest growing 50 stems
ha −1 increased from 0.35 cm year−1 for T5 plots to 0.75 cm
year −1 for T1 plots, and the corresponding values for the
fertilized plots were 0.53 and 1.25 cm year −1, respectively.
Fertilizer caused only a small increase in diameter growth at
high stand densities, but there was an interaction between
Stand growth
Trees in the F0 treatment exhibited maximum basal area
growth and volume growth rates at a stand basal area under
bark of about 15 m2 ha −1 (Figures 5). Basal area growth rates
of trees in the T1 and T5 treatments were about 58 and 50%,
respectively, of that of trees growing at a stand density of 15
m2 ha −1. The percentage increase in basal area growth rate in
response to fertilizer application was similar over the range of
stand densities studied, with increases of 60, 70 and 60% at 5,
15 and 30 m2 ha −1, respectively. The largest absolute increases
in basal area growth rate and volume growth rate in response
to fertilizing were at a stand basal area of about 15 m2 ha −1,
where basal area growth increased from 0.35 to 0.56 m2 ha −1
year −1 and volume growth increased from 3.5 to 5.4 m3 ha −1
year −1.
Leaf area index and stand growth efficiency
Thinning reduced LAI from 2.1 in the T4 and T5 treatments to
0.8 in the T1F0 treatment (Figure 6a). Fertilizing did not
increase LAI in the T2, T4 or T5 treatments, but it increased
LAI in the T1 and T3 treatments by 45 and 20%, respectively.
Figure 3. Diameter growth in relation to stand basal area for the
fertilized (F1) and unfertilized (F0) treatments for (a) the fastest
growing 50 stems per hectare, (b) the second fastest growing 50 stems
per hectare, (c) the second fastest growing 100 stems per hectare, and
(d) the third fastest growing 100 stems per hectare. Error bars represent one standard error. The fitted equations are for (a) F1: y = 1.443
− 0.0359x, r 2 = 0.86, F0: y = 0.839 − 0.0171x, r 2 = 0.66, (b) F1: y =
1.00 − 0.0242x, r 2 = 0.70, F0: y = 0.437 − 0.0063x, r 2 = 0.33, (c) F1:
y = 0.569 − 0.0111x, r 2 = 0.35, F0: y = 0.213−0.0012x, r 2 = 0.03, (d)
F1: y = 0.417 − 0.0092x, r 2 = 0.31, F0: y = 0.229 − 0.0045x, r 2 = 0.09.
Figure 4. Mean seasonal diameter growth rates based on six trees with
diameter ~30 cm in each of the three replicate plots for fertilized (F1)
and unfertilized (F0) trees in the (a) unthinned treatment (T5), and (b)
treatment thinned to a basal area under bark of 10.9 m 2 ha −1 (T2).
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STONEMAN ET AL.
in the T5 treatment and from 0.27 to 0.43 m2 ha −1 year −1 in the
T1 treatment.
Water relations
Predawn shoot water potential Trees in thinned plots had
higher Ψpd than trees in unthinned plots, and fertilization
tended to lower the Ψpd of trees in all T treatments (Figure 7a).
In T5 and T2 trees, Ψpd fell below −0.4 MPa in mid-December
1988 and in mid- to late January 1989, respectively, whereas
T1 trees maintained Ψpd above −0.4 MPa until after mid-February 1989 (Figure 7a). The differences between the thinning
treatments increased as the Ψpd of all trees continued to fall
during the remainder of the dry season to reach minimum
values in late April when the Ψpd values of trees in the T5F0,
T2F0 and T1F0 treatments were −0.94, −0.69 and −0.55 MPa,
respectively.
Figure 5. (a) Stand basal area growth for fertilized (F1) and unfertilized (F0) treatments in relation to stand basal area. Error bars represent
one standard error. The fitted equations are Weibull curves of the type
y = a(c/b)((x/b)^(c −1))(exp(−),(x/b)^c)) for F1, where a = 14.88, b =
21.76, c = 1.885, r 2 = 0.59, n = 15, and for F0, where a = 9.028, b =
22.19, c = 1.878, r 2 = 0.33 and n = 15. (b) Stand volume growth for
fertilized (F1) and unfertilized (F0) treatments in relation to stand
basal area. Error bars are for one standard error. The fitted equations
are Weibull curves of the type y = a(c/b)((x/b)^(c −1))(exp(−(x/b)^c))
for F1where a = 137.5, b = 21.09, c = 1.924, r 2 = 0.71, n = 15, and for
F0 where a = 80.73, b = 21.15, c = 2.042, r 2 = 0.46 and n = 15.
Midday shoot water potential The Ψmd values were about
−1.2 MPa during the wet season and they started to decline at
the end of August (Figure 7b). Trees in the T5F0 plot had
significantly lower Ψmd (−2 MPa) during November--December 1988 than trees in the other plots. The lowest Ψmd values
Figure 6. (a) Stand leaf area index (LAI) for fertilized (F1) and
unfertilized (F0) treatments in relation to stand basal area. Error bars
represent one standard error. The fitted equations are F1: y =
2.432e^(−4.119/x), r 2 = 0.83, F0: y = 2.503e^(6.175/x), r 2 = 0.90. (b)
Stand growth efficiency for fertilized (F1) and unfertilized (F0) treatments in relation to stand basal area. Error bars are for one standard
error. The fitted equations are F1: y = 0.488 − 0.0121x, r 2 = 0.43,
n = 15, F0: y = 0.316 − 0.0079x, r 2 = 0.70, n = 15.
Stand growth efficiency increased with decreasing stand
density (Figure 6b). Fertilizing increased stand growth efficiency at all stand densities, from 0.08 to 0.17 m2 ha −1 year −1
Figure 7. (a) Predawn shoot water potential (Ψpd) and rainfall for
1988--1989; (b) midday shoot water potential (Ψmd) and rainfall for
1988--1989. Error bars represent one standard error.
EUCALYPTUS MARGINATA RESPONSE TO THINNING
271
occurred in March 1989, when Ψmd of trees in the T1F0, T1F1
and T5F0 plots reached −2, −2.2 and −2.7 MPa, respectively.
Diurnal course of shoot water potential Early in the dry season (November 14, 1988), there was little difference in Ψs
among the treatments during the day, and the minimum Ψs was
about −1.2 to −1.5 MPa (Figure 8a). By mid-January, the T5
trees had lower Ψs over most of the day, except around midday,
than trees in the other treatments (Figure 8b). The difference
between T5 trees and trees in the other treatments was still
evident late in the dry season (April 19, 1989) (Figure 8d).
Fertilizer decreased Ψs of T5 trees but not of T2 trees.
Effect of water deficits on growth Stand growth rate was inversely related (r 2 = 0.91) to cumulative midday water stress
(CMWS) over the dry season, such that volume growth rate
declined from 6 m3 ha −1 year −1 at a CMWS of 130 MPa days,
to zero at a CMWS of 220 MPa days (Figure 9a). Stand growth
was less closely related (r 2 = 0.59) to cumulative predawn water
stress (CPWS) over the dry season, although the curve indicated a sharp decline in volume growth as CPWS increased
from 10 to 30 MPa days (Figure 9b).
Figure 9. Stand volume growth in relation to (a) cumulative midday
water stress (CMWS), and (b) cumulative predawn water stress
(CPWS). Data from the T1 plots are not included because there are
too few trees on these plots to fully occupy the site. The fitted
equations are: (a) y = 15379x^(−0.012x), r 2 = 0.91, n = 4, and (b) y =
−0.342+65.216/x, r 2 = 0.59, n = 4.
Discussion
Both thinning and fertilizer application increased the growth
rate of E. marginata trees and stands; however, the increase
was limited to the fastest growing 200 stems per hectare.
Thinning reduced LAI, resulting in less water stress and increased stand growth efficiency. Fertilizer increased stand
growth efficiency, and increased LAI of moderately to heavily
thinned stands, but did not significantly increase LAI of lightly
thinned or unthinned stands. Application of fertilizer had no
effect on predawn water stress in heavily thinned stands, but
led to greater water stress in the unthinned stand.
When contrasted with previous studies on this site (Abbott
and Loneragan 1983, Stoneman et al. 1989), it seems that as
the stand has aged the optimum stand density for growth has
increased (Figure 10) (cf. Smith 1962, Shepherd and Forrest
1973). For trees aged 40--60 years, basal area growth was
> 90% of maximum growth over the range of stand basal areas
Figure 8. Diurnal courses of shoot water potential (Ψs) for fertilized
(F1) and unfertilized (F0) trees in the unthinned treatment (T5), the
treatment thinned to 11 m2 ha −1 (T2), and the treatment thinned to
5 m2 ha −1 (T1) during the 1988--1989 dry season. Error bars represent
one standard error.
Figure 10. Stand basal area growth in response to stand basal area for
the unfertilized treatment (F0) for the present growth period (63 to 67
years old), and a previous growth period (40 to 60 years old). Data for
the previous growth period are from Stoneman et al. (1989), the curve
is a Weibull curve with the same form as those in Figures 5 and 6,
where a = 9.476, b = 22.7, c = 1.708 and r 2 = 0.33.
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STONEMAN ET AL.
under bark from 8.5 to 19.5 m2 ha −1, with maximum growth at
13.5 m2 ha −1. In the present study, which covered a growth
period from age 63 to 67 years, basal area growth was > 90%
of maximum growth over the range of stand basal areas of 10
to 20 m2 ha −1, with maximum growth at 15 m2 ha −1.
The increase in tree growth rate with thinning was associated with a reduction in shoot water stress. Based on the
observed relationship between photosynthesis and Ψpd for
E. marginata seedlings (Stoneman et al. 1994), we conclude
that the reduced shoot water stress of trees in the thinned stands
enabled greater rates of photosynthesis than in the unthinned
stand. Additionally, because it usually favors allocation of
photosynthates to shoots in preference to roots (Begg and
Turner 1976, Axelson and Axelson 1986, Pereira and Pallardy
1989, Gower et al. 1992), reduced shoot water stress would
have further increased bole growth rates. Reductions in stand
density also result in increased light availability to remaining
tree crowns and increased soil temperatures (Jenkins and
Chambers 1989, Chen et al. 1993, Kinal 1993, Stoneman and
Dell 1993). However, Stoneman and Dell (1993) found that
these factors had less effect on E. marginata seedling growth
than shoot water stress. Thinning had no effect on leaf nutrient
concentrations (Hingston unpublished data), indicating that
nutrition has little effect on the response of tree growth rates to
thinning.
Stand density had a major influence on the pattern of E. marginata stand growth. At low stand densities, even though tree
growth rates were high, there were too few trees to occupy the
site fully, and stand growth was less than maximal. Thus, there
was a range of stand densities over which growth in an increasing number of trees counteracted reductions in tree growth
rates with increasing stand density. At high stand densities,
stand growth declined in E. marginata stands. Although this
relationship between stand growth and stand density was hypothesized by Langsaeter (1941) (as quoted by Smith 1962 and
Assmann 1970) there has been little published evidence to
support it. We attribute the decline in stand growth at high
stand densities to the low rate of self-thinning of this species.
Thus at high stand densities, a higher proportion of assimilates
was used for respiration and leaf formation by the large number of surviving trees and so a lower proportion of assimilates
was available for stem growth than at lower stand densities.
The application of fertilizer increases leaf nitrogen and
phosphorus concentrations (Hingston unpublished data), and
photosynthetic rates have been found to increase with increasing leaf nitrogen or phosphorus concentrations in E. marginata
(Stoneman unpublished data) and other Eucalyptus seedlings
(Kirschbaum and Tompkins 1990, Sheriff and Nambiar 1991).
Application of fertilizer containing nitrogen or phosphorus
also reduces the root/shoot ratio of E. marginata seedlings
(Stoneman unpublished data), other eucalypt seedlings
(Cromer et al. 1984, Cromer and Jarvis 1990, Kirschbaum
et al. 1992) and other forest tree species (Axelsson and
Axelsson 1986, Gower et al. 1992), which could result in
increased bole growth. Furthermore, the fertilizer treatment
resulted in an increase in LAI of moderately and heavily
thinned stands. Because an increase in LAI will increase radia-
tion interception (Monteith 1977, Legg et al. 1979, Linder
1985), an increase in LAI will generally also lead to an increase in stand growth. Based on this information, we attribute
the increase in tree and stand growth in response to application
of fertilizer to increased photosynthetic rates, increased allocation to stem wood and higher LAIs.
The fertilizer-induced increase in plant water stress in the
unthinned stands may be associated with an increased allocation of assimilates to aboveground tissues relative to root
tissues, which in the unthinned plots, with the highest LAI,
resulted in an imbalance between water use by leaves and
water supply by roots. This response is unusual because fertilizer usually results in less plant water stress (Hillerdal-Hagstromer et al. 1982, Brix and Mitchell 1986, Myers and Talsma
1992), as was observed in the thinned plots (Figure 10).
Increased stand growth efficiency in response to application
of fertilizer was a reflection of the increased rates of photosynthesis per unit leaf area and increased allocation to stem wood.
Increases in growth efficiency in response to fertilization have
also been observed for Pseudotsuga menziesii (Mirb.) Franco
(Binkley and Reid 1984), Pinus sylvestris L. (Waring and
Schlesinger 1985), Pinus taeda L. and Pinus elliottii var.
elliottii Engelm. (Colbert et al. 1990). However, Vose and
Allen (1988) found no increase in growth efficiency of Pinus
taeda following fertilization. It seems likely that application of
fertilizer will increase growth efficiency to a greater extent on
nutrient-deficient soils and for species that require a high
nutrient status.
Increased stand growth efficiency in response to thinning
was partly a reflection of the improved water status of trees in
thinned stands, which resulted in increased photosynthetic
rates, increased allocation to stem wood and decreased allocation to respiration and leaf formation. The increased stand
growth efficiency in response to thinning was also partly a
result of the selective removal of the slower growing trees
(Stoneman unpublished data). Increases in growth efficiency
in response to thinning have also been observed for Pseudotsuga menziesii (Waring et al. 1981, Binkley and Reid 1984,
Velazquez-Martinez et al. 1992) and Pinus resinosa Ait. (Law
et al. 1992). However, thinning did not increase the growth
efficiency of Pinus contorta var. latifolia Engelm ex S. Wats.
(Amman et al. 1988) or Pseudotsuga menziesii in the experiment of O’Hara (1989). Thinning may increase growth efficiency to a greater extent for species and sites where thinning
causes large reductions in plant water stress, or in unthinned
stands with a high LAI where thinning increases light availability to leaves that would otherwise have been in deep shade.
Acknowledgments
We thank S. Bellgard, I. Freeman and M. Reynolds for technical
assistance and I. Abbott, F.J. Bradshaw, P. Kimber, J. McGrath and
M. Rayner for reviewing the manuscript.
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© 1996 Heron Publishing----Victoria, Canada
Growth and water relations of Eucalyptus marginata (jarrah) stands in
response to thinning and fertilization
G. L. STONEMAN,1,2 D. S. CROMBIE,1 K. WHITFORD,1 F. J. HINGSTON,3 R. GILES,1
C. C. PORTLOCK,1 J. H. GALBRAITH3 and G. M. DIMMOCK3
1
Department of Conservation and Land Management, Research Centre, Dwellingup, WA 6213, Australia
2
Present address: Department of Conservation and Land Management, Research Centre, Como, WA 6152, Australia
3
CSIRO, Division of Forest Research, Private Bag, PO Wembley, WA 6014, Australia
Received June 2, 1994
Summary We studied the effects of five thinning treatments
(T1 = 5.5, T2 = 11, T3 = 16.5, T4 = 22.5 and T5 = 28.5 m2 ha −1
basal area under bark) × two fertilizer treatments (F0 = unfertilized and F1 = fertilized with 400 kg ha −1 N plus 229 kg
ha −1 P) on growth and water relations of pole-sized Eucalyptus
marginata J. Donn ex Sm. trees growing in southwestern
Australia. Thinning reduced leaf area index (LAI) from 2.1 in
the T4 and T5 treatments to 0.8 in the T1F0 treatment. Fertilizer
had no effect on LAI in the T2, T4 or T5 treatments, but
increased LAI by 45 and 20% in the T1 and T3 treatments,
respectively. Thinning plus fertilizing increased diameter
growth most in the fastest growing trees, from 0.4 cm year −1
for trees in the T5F0 and T5F1 treatments to 0.7 and 1.2 cm
year −1 for trees in the T1F0 and T1F1 treatments, respectively.
In both fertilizer treatments, stand basal area and volume
growth increased with increasing stand density up to 15 m2
ha −1, and thereafter declined with increasing stand density,
such that the growth rate of trees in the T5 treatment was only
half of that at a stand density of 15 m2 ha −1. In response to
fertilizer, growth rates of the slowest and fastest-growing trees
increased from 0.35 and 3.5 m2 ha −1 year −1 (F0) to 0.56 and 5.4
m3 ha −1 year −1 (F1), respectively. Stand growth efficiency
(growth per unit LAI) increased in response to thinning, and
fertilizer increased stand growth efficiency at all stand densities. Throughout the dry season, T5 trees had lower predawn
shoot water potentials (Ψpd) (minimum of −1.5 MPa) than T1
or T2 trees (minimum of −0.7 MPa). Fertilizer decreased Ψpd
in T5 trees (by −0.9 and −1.5 MPa, respectively, in F0 and F1),
but not in T1 or T2 trees. Stand growth rate was closely related
to cumulative midday water stress (CMWS) over the dry season, and volume growth rate declined sharply from 6 m3 ha −1
year −1 at a CMWS of 130 MPa days, to zero at a CMWS of
220 MPa days. Application of fertilizer to thinned stands
increased LAI, stand growth efficiency and stand growth. In
unthinned stands, fertilizer increased stand growth efficiency
and stand growth; however, it also increased tree water stress,
which limited the fertilizer-induced increases in LAI and
growth. We attribute the increase in tree and stand growth in
response to application of fertilizer to increased photosynthetic
rates, increased allocation to stem wood, and in thinned stands
also to higher LAIs.
Keywords: drought, growth efficiency, leaf area index, shoot
water potential.
Introduction
Water and nutrients are two of the most important factors
controlling the growth of forest trees (Nambiar et al. 1990).
The relative importance of each factor in controlling growth
varies with species, soil and climatic conditions. Understanding how these factors control forest growth in a range of
forest conditions (see Landsberg 1986, Landsberg 1989,
Pereira et al. 1989, Gower et al. 1992, Raison and Myers 1992)
is fundamental for the development of physiologically based
forest growth models (Kimmins et al. 1990, McMurtrie and
Landsberg 1992).
We have investigated the effects of thinning and application
of fertilizer on Eucalyptus marginata J. Donn ex Sm. tree and
stand growth. The specific objectives of the study were to
quantify the effects of thinning and fertilization on tree and
stand growth, leaf area index (LAI) and plant water stress, and
to determine the effect of water stress on stand growth.
Methods
Study area
The experiment was located in an E. marginata stand at Inglehope Forest Block (32°45′ S, 116°11′ E), 9 km southeast of
Dwellingup, Western Australia. Soil on the site is 0.5 to 1 m of
yellow sandy loam overlying ferruginous concretionary gravels in a sandy loam matrix up to 4 m thick. Beneath this,
kaolinitic clay extends to granite bedrock at depths between
18 m and 57 m, with groundwater tables at between 20 and
32 m. The soils are of low fertility (Leeper 1970, Hingston
et al. 1981). The climate of the area is typically Mediterranean.
During the study, average annual rainfall at Dwellingup was
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STONEMAN ET AL.
1217 mm, average annual pan evaporation was 1277 mm,
average annual maximum temperature was 21.6 °C and average annual minimum temperature was 9.8 °C (Figure 1). Longterm average annual rainfall at the site is 1100 mm, 200 mm
less than at Dwellingup. Average monthly maximum temperatures varied from about 30 °C in January and February to 14 °C
in July.
Experimental design
Plots (40 m by 40 m with an 8 m buffer on all sides) were first
thinned in 1964 (at age 40 years) to stand densities of about 7
(T1), 11 (T2), 15 (T3), and 18 (T4) m2 ha −1 basal area under
bark, and stand density of the unthinned plots was about 22 m2
ha −1 (T5). In 1986, a fertilizer treatment and a second thinning
were imposed. The experimental design comprised 10 treatments of five thinning treatments (stand basal area under bark
of 5.5 (T1), 10.9 (T2), 16.4 (T3), 22.4 (T4) or 28.5 (T5) m 2 ha −1
(unthinned) in 1986) × two fertilizer treatments (F0 = unfertilized and F1 = fertilized with 400 kg ha −1 N and 229 kg ha −1 P)
× three replicate plots. For the F1 plots, 200 kg ha −1 N and 229
kg ha −1 P were applied in autumn 1987 as 1145 kg ha −1 of
diammonium phosphate, and 200 kg ha −1 N was applied in
autumn 1988 as 588 kg ha −1 of ammonium nitrate. This rate of
fertilizer application was chosen to give an optimum growth
response (Stoneman et al. 1989). In both the first thinning in
1964 and the second thinning in 1986, the smaller, slowergrowing trees were removed (Smith 1962). Table 1 shows the
effect of the treatments on stand structure.
Growth and leaf area measurements
To estimate growth in diameter, basal area and bole volume,
tree diameter over bark and bark thickness were measured on
all trees in all plots in 1987 and 1991, and bole height was
measured in 1985. Volume was calculated for each tree based
on the equation of Pearce et al. (1992). To estimate the seasonal
diameter growth pattern, stem diamter was measured monthly
from dendrometer bands fitted at breast height on six trees per
plot with diameter of about 30 cm. Leaf area index (LAI) of
each plot was estimated based on allometric equations and
Table 1. Average stand basal area under bark (SBA), average stocking,
average diameter at breast height under bark (DBH), and average tree
height for all trees in each of the thinning and fertilizer treatments.
Treatment
SBA
(m2 ha −1)
Stocking
(stems ha −1)
DBH
(cm)
Height
(m)
T1F0
T1F1
T2F0
T2F1
T3F0
T3F1
T4F0
T4F1
T5F0
T5F1
5.3
4.6
10.7
9.7
15.7
16.2
22.3
21.8
29.1
27.1
46
42
150
375
205
290
470
435
975
960
36.5
36.3
27.0
30.4
29.1
25.0
21.3
21.8
16.6
16.4
24.6
24.8
22.1
23.6
24.3
22.3
20.9
21.0
18.2
17.8
hemispherical photographs (Whitford 1991, Whitford unpublished data). The mean LAI for each plot was based on up to
six separate estimates of LAI taken between spring 1987 and
autumn 1990. Stand growth efficiency (Waring 1983) is defined as the basal area growth per plot per unit of LAI (m2 ha −1
year −1/LAI).
Water relations
Predawn shoot water potential (Ψpd) and midday shoot water
potential (Ψmd) were measured with a pressure chamber
(Scholander et al. 1965, Turner 1988) at 2- to 8-week intervals,
from May 1988 to May 1989, and at about 2-week intervals
from November 1989 to May 1990. These measurements were
made in one plot of each of the T1F0, T1F1, T2F0, T2F1, T5F0
and T5F1 treatments. Additionally, shoot water potential (Ψs)
was measured throughout the day on several occasions from
November to April. These measurements were taken in one
plot of each of the T1F0 and T1F1, or T2F0 and T2F1 treatments as well as in one plot of each of theT5F0 and T5F1
treatments. Shoot water potential was measured on two twigs
on each of four dominant or codominant trees in each plot on
each occasion for the predawn, midday and diurnal sets of
measurements. Cumulative water stress (CWS) was calculated
separately from both the Ψpd and Ψmd data for the 1988--1989
and 1989--1990 seasons as described by Ritchie and Hinckley
(1975) and Myers (1988).
Results
Diameter growth
Figure 1. Average monthly values for maximum and minimum air
temperature, rainfall and pan evaporation at Dwellingup over the study
period.
Diameter growth rate increased with decreasing stand density
from 0.02 cm year −1 for trees on T5F0 plots to about 0.75 cm
year −1 for trees on T1F0 plots (Figure 2). Application of
fertilizer increased average diameter growth in all thinning
treatments, with the greatest increase in rate occurring in trees
growing at the lowest stand density (1.2 cm year −1 for trees in
the T1F1 plots).
Analysis of average growth rates biased the data because the
slower growing trees had been removed from the heavily
thinned plots, whereas they remained in the unthinned and
EUCALYPTUS MARGINATA RESPONSE TO THINNING
269
thinning and fertilizing resulting in a larger response to fertilizer at the low stand densities.
Diameter growth rates of trees in all thinning treatments
were not significantly affected by fertilizer application in
1987, the first year after fertilizer was applied (Figure 4);
however, in the second and third years, fertilizer increased
diameter growth rates of trees in all thinning treatments. Diameter growth occurred mainly during the late autumn, winter
and spring periods. The high growth rates recorded when the
trees were water stressed during the February to April period
were caused by bark swelling following rainfall events, and not
to diameter growth (see Figure 7a).
Figure 2. Average diameter growth in relation to stand basal area for
the fertilized (F1) and unfertilized (F0) treatments. Error bars represent one standard error. The fitted equations are F1: y = 2.250 − 0.6545
lnx, r 2 = 0.98, n = 15, F0: y = 1.384 − 0.4034lnx, r 2 = 0.90, n = 15.
lightly thinned plots and depressed the average growth rate (cf.
Figures 2 and 3a). The fastest growing 200 stems ha −1 in the
unthinned and lightly thinned plots exhibited the greatest response to both thinning and fertilizer, whereas the remaining
trees exhibited little response to either treatment. In the unfertilized plots, diameter growth of the fastest growing 50 stems
ha −1 increased from 0.35 cm year−1 for T5 plots to 0.75 cm
year −1 for T1 plots, and the corresponding values for the
fertilized plots were 0.53 and 1.25 cm year −1, respectively.
Fertilizer caused only a small increase in diameter growth at
high stand densities, but there was an interaction between
Stand growth
Trees in the F0 treatment exhibited maximum basal area
growth and volume growth rates at a stand basal area under
bark of about 15 m2 ha −1 (Figures 5). Basal area growth rates
of trees in the T1 and T5 treatments were about 58 and 50%,
respectively, of that of trees growing at a stand density of 15
m2 ha −1. The percentage increase in basal area growth rate in
response to fertilizer application was similar over the range of
stand densities studied, with increases of 60, 70 and 60% at 5,
15 and 30 m2 ha −1, respectively. The largest absolute increases
in basal area growth rate and volume growth rate in response
to fertilizing were at a stand basal area of about 15 m2 ha −1,
where basal area growth increased from 0.35 to 0.56 m2 ha −1
year −1 and volume growth increased from 3.5 to 5.4 m3 ha −1
year −1.
Leaf area index and stand growth efficiency
Thinning reduced LAI from 2.1 in the T4 and T5 treatments to
0.8 in the T1F0 treatment (Figure 6a). Fertilizing did not
increase LAI in the T2, T4 or T5 treatments, but it increased
LAI in the T1 and T3 treatments by 45 and 20%, respectively.
Figure 3. Diameter growth in relation to stand basal area for the
fertilized (F1) and unfertilized (F0) treatments for (a) the fastest
growing 50 stems per hectare, (b) the second fastest growing 50 stems
per hectare, (c) the second fastest growing 100 stems per hectare, and
(d) the third fastest growing 100 stems per hectare. Error bars represent one standard error. The fitted equations are for (a) F1: y = 1.443
− 0.0359x, r 2 = 0.86, F0: y = 0.839 − 0.0171x, r 2 = 0.66, (b) F1: y =
1.00 − 0.0242x, r 2 = 0.70, F0: y = 0.437 − 0.0063x, r 2 = 0.33, (c) F1:
y = 0.569 − 0.0111x, r 2 = 0.35, F0: y = 0.213−0.0012x, r 2 = 0.03, (d)
F1: y = 0.417 − 0.0092x, r 2 = 0.31, F0: y = 0.229 − 0.0045x, r 2 = 0.09.
Figure 4. Mean seasonal diameter growth rates based on six trees with
diameter ~30 cm in each of the three replicate plots for fertilized (F1)
and unfertilized (F0) trees in the (a) unthinned treatment (T5), and (b)
treatment thinned to a basal area under bark of 10.9 m 2 ha −1 (T2).
270
STONEMAN ET AL.
in the T5 treatment and from 0.27 to 0.43 m2 ha −1 year −1 in the
T1 treatment.
Water relations
Predawn shoot water potential Trees in thinned plots had
higher Ψpd than trees in unthinned plots, and fertilization
tended to lower the Ψpd of trees in all T treatments (Figure 7a).
In T5 and T2 trees, Ψpd fell below −0.4 MPa in mid-December
1988 and in mid- to late January 1989, respectively, whereas
T1 trees maintained Ψpd above −0.4 MPa until after mid-February 1989 (Figure 7a). The differences between the thinning
treatments increased as the Ψpd of all trees continued to fall
during the remainder of the dry season to reach minimum
values in late April when the Ψpd values of trees in the T5F0,
T2F0 and T1F0 treatments were −0.94, −0.69 and −0.55 MPa,
respectively.
Figure 5. (a) Stand basal area growth for fertilized (F1) and unfertilized (F0) treatments in relation to stand basal area. Error bars represent
one standard error. The fitted equations are Weibull curves of the type
y = a(c/b)((x/b)^(c −1))(exp(−),(x/b)^c)) for F1, where a = 14.88, b =
21.76, c = 1.885, r 2 = 0.59, n = 15, and for F0, where a = 9.028, b =
22.19, c = 1.878, r 2 = 0.33 and n = 15. (b) Stand volume growth for
fertilized (F1) and unfertilized (F0) treatments in relation to stand
basal area. Error bars are for one standard error. The fitted equations
are Weibull curves of the type y = a(c/b)((x/b)^(c −1))(exp(−(x/b)^c))
for F1where a = 137.5, b = 21.09, c = 1.924, r 2 = 0.71, n = 15, and for
F0 where a = 80.73, b = 21.15, c = 2.042, r 2 = 0.46 and n = 15.
Midday shoot water potential The Ψmd values were about
−1.2 MPa during the wet season and they started to decline at
the end of August (Figure 7b). Trees in the T5F0 plot had
significantly lower Ψmd (−2 MPa) during November--December 1988 than trees in the other plots. The lowest Ψmd values
Figure 6. (a) Stand leaf area index (LAI) for fertilized (F1) and
unfertilized (F0) treatments in relation to stand basal area. Error bars
represent one standard error. The fitted equations are F1: y =
2.432e^(−4.119/x), r 2 = 0.83, F0: y = 2.503e^(6.175/x), r 2 = 0.90. (b)
Stand growth efficiency for fertilized (F1) and unfertilized (F0) treatments in relation to stand basal area. Error bars are for one standard
error. The fitted equations are F1: y = 0.488 − 0.0121x, r 2 = 0.43,
n = 15, F0: y = 0.316 − 0.0079x, r 2 = 0.70, n = 15.
Stand growth efficiency increased with decreasing stand
density (Figure 6b). Fertilizing increased stand growth efficiency at all stand densities, from 0.08 to 0.17 m2 ha −1 year −1
Figure 7. (a) Predawn shoot water potential (Ψpd) and rainfall for
1988--1989; (b) midday shoot water potential (Ψmd) and rainfall for
1988--1989. Error bars represent one standard error.
EUCALYPTUS MARGINATA RESPONSE TO THINNING
271
occurred in March 1989, when Ψmd of trees in the T1F0, T1F1
and T5F0 plots reached −2, −2.2 and −2.7 MPa, respectively.
Diurnal course of shoot water potential Early in the dry season (November 14, 1988), there was little difference in Ψs
among the treatments during the day, and the minimum Ψs was
about −1.2 to −1.5 MPa (Figure 8a). By mid-January, the T5
trees had lower Ψs over most of the day, except around midday,
than trees in the other treatments (Figure 8b). The difference
between T5 trees and trees in the other treatments was still
evident late in the dry season (April 19, 1989) (Figure 8d).
Fertilizer decreased Ψs of T5 trees but not of T2 trees.
Effect of water deficits on growth Stand growth rate was inversely related (r 2 = 0.91) to cumulative midday water stress
(CMWS) over the dry season, such that volume growth rate
declined from 6 m3 ha −1 year −1 at a CMWS of 130 MPa days,
to zero at a CMWS of 220 MPa days (Figure 9a). Stand growth
was less closely related (r 2 = 0.59) to cumulative predawn water
stress (CPWS) over the dry season, although the curve indicated a sharp decline in volume growth as CPWS increased
from 10 to 30 MPa days (Figure 9b).
Figure 9. Stand volume growth in relation to (a) cumulative midday
water stress (CMWS), and (b) cumulative predawn water stress
(CPWS). Data from the T1 plots are not included because there are
too few trees on these plots to fully occupy the site. The fitted
equations are: (a) y = 15379x^(−0.012x), r 2 = 0.91, n = 4, and (b) y =
−0.342+65.216/x, r 2 = 0.59, n = 4.
Discussion
Both thinning and fertilizer application increased the growth
rate of E. marginata trees and stands; however, the increase
was limited to the fastest growing 200 stems per hectare.
Thinning reduced LAI, resulting in less water stress and increased stand growth efficiency. Fertilizer increased stand
growth efficiency, and increased LAI of moderately to heavily
thinned stands, but did not significantly increase LAI of lightly
thinned or unthinned stands. Application of fertilizer had no
effect on predawn water stress in heavily thinned stands, but
led to greater water stress in the unthinned stand.
When contrasted with previous studies on this site (Abbott
and Loneragan 1983, Stoneman et al. 1989), it seems that as
the stand has aged the optimum stand density for growth has
increased (Figure 10) (cf. Smith 1962, Shepherd and Forrest
1973). For trees aged 40--60 years, basal area growth was
> 90% of maximum growth over the range of stand basal areas
Figure 8. Diurnal courses of shoot water potential (Ψs) for fertilized
(F1) and unfertilized (F0) trees in the unthinned treatment (T5), the
treatment thinned to 11 m2 ha −1 (T2), and the treatment thinned to
5 m2 ha −1 (T1) during the 1988--1989 dry season. Error bars represent
one standard error.
Figure 10. Stand basal area growth in response to stand basal area for
the unfertilized treatment (F0) for the present growth period (63 to 67
years old), and a previous growth period (40 to 60 years old). Data for
the previous growth period are from Stoneman et al. (1989), the curve
is a Weibull curve with the same form as those in Figures 5 and 6,
where a = 9.476, b = 22.7, c = 1.708 and r 2 = 0.33.
272
STONEMAN ET AL.
under bark from 8.5 to 19.5 m2 ha −1, with maximum growth at
13.5 m2 ha −1. In the present study, which covered a growth
period from age 63 to 67 years, basal area growth was > 90%
of maximum growth over the range of stand basal areas of 10
to 20 m2 ha −1, with maximum growth at 15 m2 ha −1.
The increase in tree growth rate with thinning was associated with a reduction in shoot water stress. Based on the
observed relationship between photosynthesis and Ψpd for
E. marginata seedlings (Stoneman et al. 1994), we conclude
that the reduced shoot water stress of trees in the thinned stands
enabled greater rates of photosynthesis than in the unthinned
stand. Additionally, because it usually favors allocation of
photosynthates to shoots in preference to roots (Begg and
Turner 1976, Axelson and Axelson 1986, Pereira and Pallardy
1989, Gower et al. 1992), reduced shoot water stress would
have further increased bole growth rates. Reductions in stand
density also result in increased light availability to remaining
tree crowns and increased soil temperatures (Jenkins and
Chambers 1989, Chen et al. 1993, Kinal 1993, Stoneman and
Dell 1993). However, Stoneman and Dell (1993) found that
these factors had less effect on E. marginata seedling growth
than shoot water stress. Thinning had no effect on leaf nutrient
concentrations (Hingston unpublished data), indicating that
nutrition has little effect on the response of tree growth rates to
thinning.
Stand density had a major influence on the pattern of E. marginata stand growth. At low stand densities, even though tree
growth rates were high, there were too few trees to occupy the
site fully, and stand growth was less than maximal. Thus, there
was a range of stand densities over which growth in an increasing number of trees counteracted reductions in tree growth
rates with increasing stand density. At high stand densities,
stand growth declined in E. marginata stands. Although this
relationship between stand growth and stand density was hypothesized by Langsaeter (1941) (as quoted by Smith 1962 and
Assmann 1970) there has been little published evidence to
support it. We attribute the decline in stand growth at high
stand densities to the low rate of self-thinning of this species.
Thus at high stand densities, a higher proportion of assimilates
was used for respiration and leaf formation by the large number of surviving trees and so a lower proportion of assimilates
was available for stem growth than at lower stand densities.
The application of fertilizer increases leaf nitrogen and
phosphorus concentrations (Hingston unpublished data), and
photosynthetic rates have been found to increase with increasing leaf nitrogen or phosphorus concentrations in E. marginata
(Stoneman unpublished data) and other Eucalyptus seedlings
(Kirschbaum and Tompkins 1990, Sheriff and Nambiar 1991).
Application of fertilizer containing nitrogen or phosphorus
also reduces the root/shoot ratio of E. marginata seedlings
(Stoneman unpublished data), other eucalypt seedlings
(Cromer et al. 1984, Cromer and Jarvis 1990, Kirschbaum
et al. 1992) and other forest tree species (Axelsson and
Axelsson 1986, Gower et al. 1992), which could result in
increased bole growth. Furthermore, the fertilizer treatment
resulted in an increase in LAI of moderately and heavily
thinned stands. Because an increase in LAI will increase radia-
tion interception (Monteith 1977, Legg et al. 1979, Linder
1985), an increase in LAI will generally also lead to an increase in stand growth. Based on this information, we attribute
the increase in tree and stand growth in response to application
of fertilizer to increased photosynthetic rates, increased allocation to stem wood and higher LAIs.
The fertilizer-induced increase in plant water stress in the
unthinned stands may be associated with an increased allocation of assimilates to aboveground tissues relative to root
tissues, which in the unthinned plots, with the highest LAI,
resulted in an imbalance between water use by leaves and
water supply by roots. This response is unusual because fertilizer usually results in less plant water stress (Hillerdal-Hagstromer et al. 1982, Brix and Mitchell 1986, Myers and Talsma
1992), as was observed in the thinned plots (Figure 10).
Increased stand growth efficiency in response to application
of fertilizer was a reflection of the increased rates of photosynthesis per unit leaf area and increased allocation to stem wood.
Increases in growth efficiency in response to fertilization have
also been observed for Pseudotsuga menziesii (Mirb.) Franco
(Binkley and Reid 1984), Pinus sylvestris L. (Waring and
Schlesinger 1985), Pinus taeda L. and Pinus elliottii var.
elliottii Engelm. (Colbert et al. 1990). However, Vose and
Allen (1988) found no increase in growth efficiency of Pinus
taeda following fertilization. It seems likely that application of
fertilizer will increase growth efficiency to a greater extent on
nutrient-deficient soils and for species that require a high
nutrient status.
Increased stand growth efficiency in response to thinning
was partly a reflection of the improved water status of trees in
thinned stands, which resulted in increased photosynthetic
rates, increased allocation to stem wood and decreased allocation to respiration and leaf formation. The increased stand
growth efficiency in response to thinning was also partly a
result of the selective removal of the slower growing trees
(Stoneman unpublished data). Increases in growth efficiency
in response to thinning have also been observed for Pseudotsuga menziesii (Waring et al. 1981, Binkley and Reid 1984,
Velazquez-Martinez et al. 1992) and Pinus resinosa Ait. (Law
et al. 1992). However, thinning did not increase the growth
efficiency of Pinus contorta var. latifolia Engelm ex S. Wats.
(Amman et al. 1988) or Pseudotsuga menziesii in the experiment of O’Hara (1989). Thinning may increase growth efficiency to a greater extent for species and sites where thinning
causes large reductions in plant water stress, or in unthinned
stands with a high LAI where thinning increases light availability to leaves that would otherwise have been in deep shade.
Acknowledgments
We thank S. Bellgard, I. Freeman and M. Reynolds for technical
assistance and I. Abbott, F.J. Bradshaw, P. Kimber, J. McGrath and
M. Rayner for reviewing the manuscript.
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