Diurnal changes in leaf gas exchange characteristics in the uppermost canopy of a rain forest tree, Dryobalanops aromatica Gaertn. f.

Abstract

Dryobalanops aromatica Gaertn. f. is a major tropical canopy species in lowland tropical rain forests in Peninsular Malaysia. Diurnal changes in net photosynthetic rate (A) and stomatal conductance to water vapor (g(s)) were measured in fully expanded young and old leaves in the uppermost canopy (35 m above ground). Maximum A was 12 and 10 micro mol m(-2) s(-1) in young and old leaves, respectively; however, because of large variation in A among leaves, mean maximum A in young and old leaves was only 6.6 and 5.5 micro mol m(-2) s(-1), respectively. Both g(s) and A declined in young leaves when T(leaf) exceeded 34 degrees C and leaf-to-air vapor pressure deficit (DeltaW) exceeded 0.025, whereas in old leaves, g(s) and A did not start to decline until T(leaf) and DeltaW exceeded 36 degrees C and 0.035, respectively. Under saturating light conditions, A was linearly related to g(s). The coefficient of variation (CV) for the difference between the CO(2) concentrations of ambient air and the leaf intercellular air space (C(a) - C(i)) was smaller than the CV for A or g(s), suggesting that maximum g(s) was mainly controlled by mesophyll assimilation (A/C(i)). Minimum C(i)/C(a) ratios were relatively high (0.72-0.73), indicating a small drought-induced stomatal limitation to A and non-conservative water use in the uppermost canopy leaves.

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Summary
tropical canopy species in lowland tropical rain forests in
Peninsular Malaysia. Diurnal changes in net photosynthetic
rate (A) and stomatal conductance to water vapor (gs) were
measured in fully expanded young and old leaves in the upper-
most canopy (35 m above ground). Maximum A was 12 and 10
µmol m−2 s−1 in young and old leaves, respectively; however,
because of large variation in A among leaves, mean maximum
A in young and old leaves was only 6.6 and 5.5 µmol m−2 s−1,
respectively. Both gs and A declined in young leaves when
Tleaf exceeded 34 °C and leaf-to-air vapor pressure deficit (∆W)
exceeded 0.025, whereas in old leaves, gs and A did not start to
decline until Tleaf and ∆W exceeded 36 °C and 0.035, respec-
tively. Under saturating light conditions, A was linearly related
to gs. The coefficient of variation (CV) for the difference
between the CO2 concentrations of ambient air and the leaf
intercellular air space (Ca − Ci) was smaller than the CV for A
or gs, suggesting that maximum gs was mainly controlled by
mesophyll assimilation (A/Ci). Minimum Ci/Ca ratios were
relatively high (0.72--0.73), indicating a small drought-induced
stomatal limitation to A and non-conservative water use in the
uppermost canopy leaves.
Dryobalanops aromatica Gaertn. f. is a major
Keywords: drought, irradiance, stomatal conductance, photo-
synthesis, tropical rain forest, uppermost canopy, water deficit.
Introduction
Tropical rain forest trees are generally shallow rooted (Doley
1981) and are easily damaged by relatively small water deficits
(Grubb 1977, Buckley et al. 1980). Because the regulation of
stomatal conductance is an important trait in drought avoid-
ance, several studies have examined stomatal responses in rain
forest tree species. In some tropical species, stomatal conduc-
tance remains high throughout the midday period (Grace et. al.
1982, Aylett 1985, Oberbauer et al. 1987), whereas in other
tropical species, stomatal conductance shows a midday depres-
sion when the leaves are exposed to a high leaf-to-air water
vapor deficit (∆W) (Aylett 1985, Roberts et al. 1990, Koch et
al. 1994).
Zotz and Winter (1993) found that maximum net photosyn-
thetic rate (A) was linearly related to diurnal CO2 uptake
among canopy plants in a tropical rain forest in Panama.
However, several published works have reported substantial
variation in A among individual leaves in a canopy. These
variations are caused by either differences in light intensity at
the leaf surface (Doley et al. 1988), or leaf-age-related changes
in intrinsic biochemical capacity and stomatal limitation to
CO2 fixation (Reich and Borchert 1982, Witkowski et al. 1992,
Sobrado 1994). To obtain a more detailed understanding of
how environmental variables limit primary production at the
single leaf level, we examined diurnal changes in microclimate
and leaf gas exchange characteristics in uppermost canopy
leaves of a tropical canopy tree, Dryobalanops aromatica
Gaertn. f. (Dipterocarpaceae) during the rainy season.
Materials and methods
Plant species
Dryobalanops aromatica is one of the canopy tree species in
lowland (elevation 70--400 m) dipterocarp forests in southeast
Asia and sometimes forms mono-specific dominant (mono-
dominant) forests in its natural state (Foxworthy 1932, Vincent
1961, Kachi et al. 1993). Some trees exceed 60 m in height
(Symington 1974).
Study site and access to the uppermost canopy
The study site was a forest stand at the Forest Research Insti-
tute of Malaysia (FRIM) in Kuala Lumpur (3°13′ N,
101°37′ E). Mean annual air temperature is about 26 °C and
annual precipitation is about 2400 mm at Kuala Lumpur (Taka-
hashi and Arakawa 1981). There are two rainy seasons, March
to April and October to December, caused by seasonal mon-
soons. However, because the mean monthly precipitation in
the rainy and other seasons is about 260 and 150 mm, respec-
tively, there is no well-defined dry season.
Diurnal changes in leaf gas exchange characteristics in the uppermost
canopy of a rain forest tree, Dryobalanops aromatica Gaertn. f.
ATSUSHI ISHIDA,1 TAKESHI TOMA,1 YOOSUKE MATSUMOTO,1 SON KHEONG YAP2 and
YUTAKA MARUYAMA3
1 Forest Environment Division, Forestry and Forest Products Research Institute, P.O. Box 16, Tsukuba Norin Danchi, Ibaraki 305, Japan
2 Forest Research Institute of Malaysia, Kepong 52109, Kuala Lumpur, Malaysia
3 Japan International Research Center for Agricultural Science, Tsukuba, Ibaraki 305, Japan
Received October 13, 1995
Tree Physiology 16, 779--785
© 1996 Heron Publishing----Victoria, Canada
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In 1994, the forest stand consisted mainly of 60-year-old
D. aromatica (Appanah and Weinland 1993, Kachi et al. 1993)
and the average height of the uppermost canopy in the stand
was about 35 m. Although leaf litter was thin, there was little
bare soil on the forest floor. Soil was of laterite origin.
To measure micrometeorogical parameters just above the
closed forest canopy, a 40-m-tall scaffolding tower (UP-
RIGHT Co., Republic of Ireland) was installed. Gas exchange
measurements were made on foliage accessible from the tower
at a height of 35 m.
Chlorophyll content
Leaf color, leaf chlorophyll content and leaf order were used
to determine a relative leaf age scale (Sobrado and Medina
1980, Sobrado 1994). Leaf chlorophyll content was measured
at 650 and 940 nm with a chlorophyll meter (Model SPAD-
502, Minolta Co. Ltd., Tokyo, Japan). Twenty-two leaves were
sampled to determine the relationships between leaf chloro-
phyll content and the chlorophyll meter readings (SPAD val-
ues). After a SPAD value was determined for a leaf,
chlorophyll was extracted with 80% (v/v) acetone and the
amounts of chlorophyll a and b were determined by the method
of Arnon (1949).
Diurnal changes in microclimate and leaf gas exchange
characteristics
Measurements were made on October 20 and 21 and Novem-
ber 11, 1994. Photosynthetically active photon flux density
(PPFD, µmol m−2 s−1) at the leaf surface was determined with
a quantum sensor attached to the leaf chamber (PLC-4, Ana-
lytical Development Company (ADC), Hoddeson, UK). An
additional quantum sensor (IKS-25, Koito-Kogyo Co., Tokyo,
Japan), set at the top of the tower, was used on November 11,
1994. Relative humidity of ambient air (RH, %) was deter-
mined from wet and dry bulb temperatures on October 20 and
21, 1994, and with a thin-film capacitance sensor (CHS-APS
XD3, TDK Co., Tokyo, Japan) on November 11, 1994. Ambi-
ent air temperature (Tair, °C) was measured with a platinum
resistance thermometer probe (Pt 100 Ohm, Model KDC-S3,
KONA system Co., Tokyo, Japan). An aluminum roof was
installed above the humidity and air temperature sensors to
prevent exposure to direct solar radiation. Leaf temperature
(Tleaf, °C) was measured with fine-wire copper constantan
thermocouples (diameter 0.1 mm, Hayashi-Denko Co., Tokyo,
Japan) attached to the abaxial leaf surface by adhesive tape.
Gas exchange rates were measured in fully expanded old
leaves on October 20 and 21, 1994, and in just fully expanded
young leaves on November 11, 1994. Net photosynthetic rate
(A, µmol m−2 s−1) and stomatal conductance to water vapor (gs,
mol m−2 s−1) per unit leaf area were measured with a portable
H2O/CO2 analyzer (LCA-4, ADC, Hoddeson, UK) equipped
with a portable leaf chamber (PLC-4 ADC, Hoddeson, UK) in
an open system. There are technical difficulties associated with
measuring concentrations of CO2 based on infrared absorption
properties; however, the error in our estimates of CO2 concen-
tration was small because the differences between ambient air
RH and RH reference readings of the LCA-4 were small. The
error was further minimized by use of the computerized cor-
rection routine supplied with the LCA-4 analyzer.
Leaf gas exchange characteristics were measured three to
four times an hour. Dew on the leaf surface was dried with
paper in the early morning. Six or seven fully expanded,
sun-exposed leaves near the tips of twigs were randomly se-
lected for each measurement. Because the longevity of canopy
leaves was about 1 year, measured leaves were younger than
1 year old. The leaves of D. aromatica are hard and repeated
clipping of a leaf by the leaf chamber often caused the leaf to
split into two laminae from the midrib. To ensure that leaves
did not split during measurement, we used a new leaf for each
measurement. The total number of leaves measured was about
200 for both young and old leaves.
To examine whether variation in A among leaves was a result
of differences in gs or in assimilation capacity, the coefficients
of variation (CVs) for A, gs and (Ca − Ci) (the difference
between ambient air and leaf intercellular space CO2 concen-
trations) were compared.
Transpiration rate per unit leaf area (E, mmol m−2 s−1) was
calculated based on leaf-to-air vapor pressure deficit (∆W),
stomatal conductance to water vapor (gs, mol m−2 s−1) and
boundary layer conductance (gb, mol m−2 s−1), as follows:
E = ∆W(gsgb/(gs + gb))103, (1)
where ∆W was calculated using the Goff-Gratch formulation
for saturated water vapor pressure. Boundary layer conduc-
tance was determined as follows (Jones 1992, Pearcy et al.
1989):
gb = 0.446(0.715(µ/d)0.5)(273/(Tleaf + 273))(P/101.3), (2)
where µ is wind speed (m s−1), d is leaf length (m), and P is air
pressure (kPa), assuming µ = 1 and d = 0.05. The possible error
in E is about 20% between µ values of 0.1 and 1, when gs = 0.2.
Results
Leaf chlorophyll content
The calibrated relationships between chlorophyll content per
unit leaf area and chlorophyll a/b ratio and the SPAD values
are shown in Figure 1a. Chlorophyll content and chlorophyll
a/b ratio differed with leaf order in a single shoot (Figure 1b).
The leaves selected for gas exchange measurements were
grouped as young or old leaves, based on leaf chlorophyll
content and leaf order. The young leaves were light green and
had a low chlorophyll content and a high chlorophyll a/b ratio
(> 2.7). The old leaves were green and had a high chlorophyll
content and a low chlorophyll a/b ratio (< 2.7). The SPAD
values were about 40 and 50 in the young and old leaves,
respectively.
Microenvironments
Sunrise, sunset and the sun’s zenith were at about 0700, 1900
and 1300 h local time, respectively. Because of dry haze in the
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air, maximum PPFDs on October 20 and 21, 1994 were rela-
tively low----about 1000 µmol m−2 s−1 (Figure 2), whereas on
November 11, 1994 when there was little dry haze, maximum
PPFD reached 2000 µmol m−2 s−1. On November 11, diurnal
PPFD and Tleaf fluctuated widely because of frequent cloud
cover. Direct solar radiation caused Tleaf to increase 6--8 °C
above Tair and ∆W to increase to 0.04. Diurnal Tair seldom
exceeded 32 °C and RH decreased to 65--50% during the
daytime.
Diurnal changes in gas exchange
When uppermost canopy leaves were exposed to high solar
radiation, A, gs and E were especially variable among leaves
(Figure 3). In young and old leaves, maximum A was 12 and
10 µmol m−2 s−1, respectively, and the corresponding values of
maximum gs were 0.6 and 0.3 mol m−2 s−1. However, mean
maximum A was only 6.6 and 5.5 µmol m−2 s−1 in young and
old leaves, respectively, and mean maximum gs was 0.25 and
0.14 mol m−2 s−1 in young and old leaves, respectively (Fig-
ure 4). During the day, the CO2 concentration of ambient air
(Ca) at the uppermost canopy level (35 m height) decreased
from about 380 to 310 µmol mol−1, mean CO2 concentration
of the leaf intercellular space (Ci) decreased to 220 µmol
mol−1 and mean Ci/Ca ratio decreased to 0.72--0.73. Mean
water use efficiency (WUE, A/E) decreased to 1.7 and 2.3
µmol mmol−1 in young and old leaves, respectively, during the
day.
In young leaves, gs and A declined when Tleaf exceeded
34 °C and ∆W exceeded 0.025 (Figure 5). In old leaves, gs and
A did not begin to decline until Tleaf and ∆W exceeded 36 °C
and 0.035, respectively.
When PPFD at the leaf surface exceeded 500 µmol m−2 s−1
(under near light-saturating conditions), we observed positive
relationships between A and gs (Figure 6a) in both young and
old leaves with low gs (< 0.3 mol m−2 s−1); however, some
young leaves with high gs (> 0.3 mol m−2 s−1) showed smaller
slopes of A versus gs than other leaves. When PPFD at the leaf
surface exceeded 500 µmol m−2 s−1, A varied considerably for
a given Ci in both young and old leaves (Figure 6b). When
young leaves with high gs (> 0.3 mol m−2 s−1) were excluded
from the analysis, CVs for A, gs and (Ca − Ci) were 0.603, 0.490
and 0.286, respectively. When PPFD at the leaf surface was
less than 500 µmol m−2 s−1 (Figure 6b), A declined with
increasing Ci because low PPFD resulted in reduced assimila-
tion capacity.
Discussion
Diurnal changes in leaf gas exchange characteristics
Stomatal conductance (gs) is sensitive to environmental factors
such as light, CO2 concentration and ∆W (e.g., Körner 1994,
Lee and Bowling 1995). In the morning (before 1100 h), both
gs and A increased and Ci decreased as PPFD increased (Fig-
ure 4). Because ∆W was low (< 0.025), gs and A were mainly
controlled by PPFD in the morning.
In young leaves, both gs and A declined when ∆W exceeded
0.025 and Tleaf exceeded 34 °C, whereas gs and A of old leaves
did not decline until ∆W and Tleaf exceeded 0.035 and 36 °C,
respectively (Figure 5). Similar trends have been observed in
other lowland rain forest trees such as Dialium pachyphyllum
Harms (Caesalpinaceae) in southwest Cameroon, in which gs
decreases in the dry season when ∆W and Tleaf are above 0.025
and 36 °C, respectively (Koch et al. 1994). After 1400 h on
November 11, A was limited by reduced Ci as a result of low
gs (Figure 4). The reduction in gs was the result of a high ∆W
(above 0.025--0.035). Such severe ∆W accounted for only
5--15% of all values recorded during the daytime on the three
measurement days.
Variation in gas exchange characteristics among canopy
leaves
Maximum A in the uppermost canopy leaves reached 10--12
µmol m−2 s−1 (Figure 3). Maximum A in primary tropical
forest tree species ranges from 4 to 15 µmol m−2 s−1, whereas
maximum A in fast-growing secondary tropical trees ranges
from 13 to 16 µmol m−2 s−1 (Koyama 1981, Aylett 1985). Thus,
maximum A in D. aromatica was relatively high compared
Figure 1. (a) Calibrated relationships between chlorophyll a content
(g m−2, ?), chlorophyll b content (g m−2, ?), chlorophyll a/b ratio (?)
and chlorophyll meter readings (SPAD values). The number of sam-
pled leaves was 22. (b) Relationships between chlorophyll a content
(?), chlorophyll b content (?) and chlorophyll a/b ratio (?) and leaf
order within a single shoot.
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Figure 2. Diurnal changes in photo-
synthetic photon flux density
(PPFD), ambient air temperature
(Tair, solid line), relative humidity
(RH, broken line), Tleaf − Tair, and
leaf-to-air vapor pressure deficit
(∆W). Measurements were made
just above the canopy, at 35 m.
Figure 3. Diurnal changes in net photosynthetic rate (A), stomatal conductance to water vapor (gs) and transpiration rate (E) in uppermost canopy
leaves of Dryobalanops aromatica.
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Figure 4. Diurnal changes in mean net photosynthetic rate (A, ?), mean stomatal conductance to water vapor (gs, ?), mean ambient air CO2
concentration (Ca, ?), mean leaf intercellular CO2 concentration (Ci, ?), mean Ci/Ca ratio (?), and water use efficiency (WUE, ?) among the
uppermost canopy leaves of Dryobalanops aromatica. Measurements were made on old leaves on October 20 and 21, 1994 and on young leaves
on November 11, 1994.
Figure 5. Relationships of
mean net photosynthetic rate
(A, ?) and mean stomatal con-
ductance to water vapor (gs, ?)
to photosynthetic photon flux
density (PPFD), leaf-to-air va-
por vapor pressure deficit
(∆W), and leaf temperature
(Tleaf) in the uppermost canopy
leaves of Dryobalanops aro-
matica. Measurements were
made on (a) old leaves on Octo-
ber 20 and 21, 1994 and on (b)
young leaves on November 11,
1994. Vertical bars represent
± 1 SD.
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