Annals of Botany 91: 55±63, 2003
doi:10.1093/aob/mcg008, available online at www.aob.oupjournals.org
Branch Architecture, Light Interception and Crown Development in Saplings of
a Plagiotropically Branching Tropical Tree, Polyalthia jenkinsii (Annonaceae)
N O R I Y U K I O S A D A * and H I R O S H I TA K E D A
Laboratory of Forest Ecology, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
Received: 10 May 2002 Returned for revision: 18 June 2002 Accepted: 3 October 2002 Published electronically: 13 November 2002
To investigate crown development patterns, branch architecture, branch-level light interception, and leaf and
branch dynamics were studied in saplings of a plagiotropically branching tree species, Polyalthia jenkinsii Hk. f.
& Thoms. (Annonaceae) in a Malaysian rain forest. Lengths of branches and parts of the branches lacking leaves
(`bare' branches) were smaller in upper branches than in lower branches within crowns, whereas lengths of
`leafy' parts and the number of leaves per branch were larger in intermediate than in upper and lower branches.
Maximum diffuse light absorption (DLA) of individual leaves was not related to sapling height or branch position within crowns, whereas minimum DLA was lower in tall saplings. Accordingly, branch-level light interception was higher in intermediate than in upper and lower branches. The leaf production rate was higher and leaf
loss rate was smaller in upper than in intermediate and lower branches. Moreover, the branch production rate of
new ®rst-order branches was larger in the upper crowns. Thus, leaf and branch dynamics do not correspond to
branch-level light interception in the different canopy zones. As a result of architectural constraints, branches at
different vertical positions experience predictable light microenvironments in plagiotropic species. Accordingly,
this pattern of carbon allocation among branches might be particularly important for growth and crown development in plagiotropic species.
ã 2003 Annals of Botany Company
Key words: Annonaceae, branch-level light interception, crown development, leaf dynamics, Malaysia, Pasoh Forest
Reserve, plagiotropic species, Polyalthia jenkinsii.
INTRODUCTION
A tree crown develops through the repetitive production of
shoots, i.e. branches and leaves (Maillette, 1982; Room
et al., 1994). Crown architecture directly determines the
pattern of leaf arrangement and thus affects light capture
ef®ciency in the crown (e.g. Pearcy and Yang, 1996); it also
restricts the direction of future growth (Room et al., 1994).
Crown development patterns can therefore be described
through the dynamics of the shoot population (Maillette,
1982). Recent studies of crown development have focused
on relationships between branch light level and branch
development within crowns (Stoll and Schmid, 1998;
Takenaka, 2000; Henriksson, 2001). They have revealed
the importance of correlative inhibition, i.e. export of
photosynthate from low-light branches to high-light
branches within individuals, on crown development of
various trees. However, the light microenvironment of each
leaf on the branches was not investigated; this information is
essential to determine the carbon gain of the shoots. On the
other hand, by computing the light microenvironment of
each leaf in the crown, other studies have investigated the
importance of crown architecture and leaf display on
ef®cient light capture at the crown level (Ackerly and
Bazzaz, 1995; Pearcy and Yang, 1996, 1998; Muraoka et al.,
1998; Yamada et al., 2000; Takenaka et al., 2001).
However, only static aspects of crown architecture were
* For correspondence at: Nikko Botanical Garden, Graduate School of
Science, University of Tokyo, 1842 Hanaishi, Nikko, Tochigi 321-1435,
Japan. Fax +81 288 543178, e-mail [email protected]
measured in these studies and they were not related to the
dynamics of crown development.
Tropical forest understoreys are characterized by light
resource limitations, and seedlings and saplings of shadetolerant tree species require physiological and morphological mechanisms to survive and grow effectively in such
light-limited environments. Plagiotropic branches, in which
leaves grow two-dimensionally along horizontal stems
(Halle et al., 1978), may enhance light capture ef®ciency,
and are considered preferable to orthotropic branches in
shaded environments (Givnish, 1984; King and
Maindonald, 1999). In plagiotropically branching species
(referred to as `plagiotropic species' hereafter), however,
there is a con¯ict between vertical and horizontal growth,
because vertical growth is critical to successful regeneration, whereas horizontal leaf dispersion is necessary for
ef®cient light capture (Zipperlen and Press, 1996). King
et al. (1997) found that branch spacing increased with
increased growth rates and light levels in various plagiotropic species, but these authors did not consider the effects
of branch architecture on the growth patterns of these
species. Zipperlen and Press (1996) found that saplings of
Shorea leprosula maximized height growth, whereas the
more shade-tolerant Dryobalanops lanceolata tended to
produce long branches that increased light interception.
From a dynamic point of view, however, lower
plagiotropic branches are inevitably shaded by newly
produced upper branches. Accordingly, even if plagiotropic
branches are expanded in small saplings, these branches will
be shed during the course of growth and will not contribute
ã 2003 Annals of Botany Company
56
Osada and Takeda Ð Crown Development in Plagiotropic Tree Saplings
to the architecture of later stages of the tree. Thus, there may
be an optimal strategy for the growth of plagiotropic
branches in different vertical positions within saplings;
assimilated carbon may be exported from lower branches to
upper branches to expand new leaves, as well as to the main
stem and roots. Since tall saplings should produce a larger
leaf area than smaller saplings in order to cope with the
larger proportion of non-photosynthetic organs (e.g. Shukla
and Ramakrishnan, 1984; Ardhana et al., 1988), the optimal
strategy for the growth of plagiotropic branches and leaf
display may also change with increasing sapling height. Tall
saplings may produce longer branches and a larger leaf area
per branch than smaller saplings by changing the number of
leaves and/or the individual leaf area.
In this study, branch architecture, branch-level light
interception, and leaf and branch dynamics were analysed in
saplings of a shade-tolerant tree species, Polyalthia jenkinsii
Hk. f. & Thoms., in a Malaysian rain forest. This species
produces plagiotropic branches along a vertical main stem
during the sapling stage. The following questions were
addressed. (1) Do branch architecture and leaf and branch
dynamics differ among branches at different vertical
positions within saplings and among saplings of different
heights? (2) How are these factors related to the light
interception capacity of the leaves on these branches?
oldest leaves (length of `bare' branches); length from the
oldest leaves to the branch tip (length of `leafy' branches);
average angle of a ®rst-order branch from the horizontal;
number of leaves on all ®rst-order branches; and lengths of
leaf blades and internodes. Furthermore, geometric measurements of architectural characters were carried out for four
of the nine saplings of various heights (32, 70, 132 and
210 cm). For each node, lengths, angles and azimuths of the
petiole, leaf surface and internode extending from this node,
and the azimuth of the midrib were recorded using a ruler,
level and compass. Angles and azimuths were almost
constant for all internodes within most plagiotropic branches
and were therefore recorded once for each of these branches.
For branches that had an obvious bend, length, angle and
azimuth measurements were taken for the portions above
and below the bend. For each leaf, the azimuth of the normal
to the surface, the angle of the surface from the horizontal,
and the azimuth of the midrib and the leaf blade length from
the petiole attachment point to the tip were also measured.
Coordinates for leaf-blade shape were obtained by tracing
representative leaves on to graph paper and then recording
the x- and y-coordinates for points on the leaf margins,
starting with x = y = 0 at the point of attachment of the
petiole, with the midline of the leaf as the y-axis.
Data analysis
MATERIALS AND METHODS
Study site and species
The study was conducted at the Pasoh Forest Reserve,
Peninsular Malaysia (2°59¢N, 102°18¢E). The Pasoh Forest
Reserve is a lowland dipterocarp forest of the Red Meranti±
Keruing type, which is dominated by Shorea spp. (Red
Meranti group) and Dipterocarpus spp. (Keruing;
Manokaran et al., 1992). The emergent layer averages
46 m and the height of the main canopy ranges from 20 to
30 m (Manokaran and Swaine, 1994). Branch architecture,
leaf and branch dynamics, and light interception of saplings
of a shade-tolerant tree species, Polyalthia jenkinsii Hk. f. &
Thoms. (Annonaceae), were studied. During the sapling
phase, this species grows vertical main stems with expanding plagiotropic branches and is, therefore, considered to be
a typical plagiotropic species.
Data collection
In April 1997, saplings of P. jenkinsii (n = 11) in a shaded
understorey were selected to investigate leaf and branch
dynamics. Saplings of various heights (30±240 cm) were
chosen to investigate the effect of sapling height. First-order
branches and leaves were tagged, and tree architecture and
leaf positions were sketched to allow for the identi®cation of
each leaf. The number and position of missing and newly
emerged leaves and branches were recorded monthly from
May 1997 to May 1999.
In addition, nine individuals of P. jenkinsii (30±210 cm
tall) were chosen in the shaded understorey in September
2000 on which the following variables were measured:
length of ®rst-order branches; length from branch base to the
Leaf population dynamics were expressed as changes in
leaf number, which is a result of the birth and death of leaves
on a branch. Changes in leaf number were expressed as the
net leaf gain rate (number per year), and this rate was
divided into two components: the leaf production rate
(number per year) and the leaf loss rate (number per year;
Bongers and Popma, 1990; Osada et al., 2002). All indices
were calculated on a per ®rst-order branch basis, and only
branches that existed from the beginning of the census were
used in the analyses. In addition, the rate of branch increase
(number per year) was calculated for each ®rst-order
branch; this accounted for the death of that branch and the
emergence of any second-order branches, i.e. the branch
increase rate was zero when the ®rst-order branch died,
whereas it was >1 when new second-order branches
emerged within the ®rst-order branch system. The rate of
increase in the number of ®rst-order branches (number per
year) was also calculated.
First-order branches of each individual sapling were
categorized into classes based on their vertical position at
the beginning of the census, i.e. upper, intermediate and
lower parts of the crown. The indices of branch architecture
and leaf and branch dynamics were averaged for the
branches in each position class of each sapling. As we were
interested in changes in the patterns of crown development
with increasing sapling height, analysis of covariance
(ANCOVA) was used to investigate relationships between
sapling height and indices of branch architecture, and leaf
and branch dynamics among branches in different position
classes. In the ANCOVA model, relative branch position
(upper, intermediate and lower) was a factor, and sapling
height a covariate. The interaction between branch position
and sapling height was used to test for differences in slopes
Osada and Takeda Ð Crown Development in Plagiotropic Tree Saplings
among branches in different positions. If height yielded
consistently parallel gradients of the indices, then there
would be a signi®cant covariate effect in the ANCOVAs. A
signi®cant interaction term, on the other hand, would
indicate that the slopes of the relationships varied among
branches in different positions. If there was no signi®cance
in the covariate analysis then the signi®cance of differences
among branches at different positions was evaluated using
ANOVA.
The three-dimensional computer model `Y-plant'
(Pearcy and Yang, 1996; Valladares and Pearcy, 1999)
was used to simulate light interception of each leaf of
four saplings on which geometric measurements had
been taken. Branch and petiole diameters were assumed
to be 1 mm, and leaf absorptance and transmittance of
the adaxial surfaces were assumed to be 0´85 and 0´10,
respectively. Fractional diffuse light absorption (DLA)
was estimated for each leaf. Simulations of DLA were
based on vectors for 160 different sky sectors (eight
azimuth and 20 angle classes). To investigate the
generality in the patterns of light interception for the
leaves of branches in different vertical positions, simulations were conducted for the open canopy (no
hemispherical photographs) and for the average understorey light environment. The average understorey light
environment was calculated from 25 hemispherical
photographs taken under closed forest canopy in the
same forest (CoolPix 910 with FC-E8 ®sheye converter;
Nikon, Tokyo, Japan), which took into account the small
contribution of light from a low angular altitude relative
to the total light. Images of hemispherical photographs
were analysed using Hemiview ver. 2´1 (Delta-T
Devices, Cambridge, UK). Both simulations showed
similar results, so only the results obtained from the
open canopy (no hemispherical photographs) are presented here. Fractional DLA was multiplied by the
relative leaf area of each leaf within each individual
sapling, and was then summed for all leaves on a
branch to investigate the relative light interception of
each branch. Relative light interception at the branch
level was also averaged for branches in each vertical
position class (upper, intermediate and lower parts of the
crown) for each sapling. Moreover, to investigate the
effects of branch number on light interception, the
relative light interception of total branches within each
positional class was also calculated.
Theoretically, plagiotropic branches should expand horizontally to capture light ef®ciently. A hypothetical architecture was therefore constructed on a computer by
changing the angles of ®rst-order branches to be horizontal,
and relative light interception was then calculated for the
simulated saplings. Values of relative light interception
were compared for real and simulated saplings.
R E SU L T S
Branch architecture
Figure 1 shows the differences in branch architecture in
relation to sapling height for branches at different vertical
57
positions. An interaction between height and branch
position was detected for branch length and length of bare
branches (Table 1). Branch length and bare branch length
were larger in tall saplings than in small saplings, particularly in the lower branches, whereas these lengths remained
almost constant in the upper branches, irrespective of
sapling height (Fig. 1). In contrast, leafy branch length was
larger in intermediate branches than in upper and lower
branches, and larger in tall saplings. The branch angle
(measured from the horizontal) was larger in upper than in
lower branches, but was not related to sapling height
(Table 2). Leaf number per ®rst-order branch was larger in
intermediate than in upper and lower branches, and was not
related to height (Table 2). Leaf and internode lengths were
not related to branch position, but were larger in tall
saplings, although the relationship between leaf length and
height was only marginally signi®cant [Table 1;
Li = 0´0814H + 18´28, r2 = 0´221; Ll = 0´225H + 118,
r2 = 0´137, n = 27, where Li is internode length (mm), H is
height (cm), and Ll is leaf length (mm)]. The number of ®rstorder
branches
also
increased
with
height
(B = 0´0598H + 3´68, r2 = 0´687, n = 11, where B is the
number of ®rst-order branches). Thus, as saplings grew
taller, the lower branches increased in length while the
length of leafy parts decreased, and they had fewer leaves
than intermediate branches. In contrast, the upper branches
were short and had few leaves, irrespective of the height of
the sapling.
Light interception
Diffuse light absorption of individual leaves changed
with sapling height but not with branch position (Fig. 2;
Table 1). Maximum DLA was not related to height or
branch position (Table 1); in contrast, minimum DLA was
related to height, being reduced for tall saplings.
Consequently, the range of DLA was also related to height,
and was greater in tall saplings (Fig. 2; Table 1).
Relative light interception at the branch level was larger
in intermediate than in upper and lower branches, and was
smaller in tall saplings (Fig. 3; Table 1). This indicates that
relative light interception at the branch level was in¯uenced
more by the difference in standing leaf number on the
branch than by DLA of individual leaves within the branch.
Relative light interception of total branches within each
branch position class (upper, intermediate and lower
branches) was larger in intermediate than in upper and
lower classes, but was not related to height because the
number of ®rst-order branches was larger in tall saplings
(Fig. 3; Table 1). The relative contribution of light
interception was, thus, similarly larger in intermediate
branches than in upper and lower branches within a crown
irrespective of sapling height.
According to the branch-angle simulation experiments,
the relative light interception of the four saplings was
slightly but consistently higher in the simulated horizontally
®xed branches than in actual branches at all branch positions
(Fig. 3). This indicates that variations in the branch angles
of real saplings do not increase light capture ef®ciency.
58
Osada and Takeda Ð Crown Development in Plagiotropic Tree Saplings
F I G . 1. Indices of branch architecture in relation to sapling height for upper, intermediate and lower branches. Solid, broken and dotted lines indicate
the regression lines of the upper, intermediate and lower branches, respectively, and only signi®cant relationships are shown (see Table 1). If the
interaction term was not signi®cant but the effects of height and branch position were signi®cant in the analysis of ANCOVA, a common regression
slope was calculated for the three branch positions. Branch angle was measured from the horizontal.
Leaf and branch dynamics
The rate of leaf production was higher and that of leaf loss
was smaller in upper than in lower branches (Fig. 4;
Table 1). Moreover, leaf production and loss rates were
higher in tall saplings (Fig. 4; Table 1). Consequently, net
leaf gain rate was not related to sapling height, and was
larger for upper than for lower branches (Table 2).
The rate of increase in the number of branches was not
related to sapling height. However, it was larger in upper
than in lower branches, and it was less than one branch per
year for most of the intermediate and lower branches
because of the death of branches (Fig. 5; Table 2). Almost
all ®rst-order branches produced during the census period
were in the upper crown, and the production rate of the new
®rst-order branches was higher in tall saplings (Fig. 5).
DISCUSSION
Branch architecture was clearly related to the relative
vertical positions of branches and to sapling height. Branch
length indicates the trajectory of past growth, and was larger
in lower than in upper branches of tall saplings, whereas it
was almost the same in small saplings. The length of bare
branches, an index of the number of leaves that have
dropped, tended to be larger in lower than in upper branches
of tall saplings. The length of leafy branches was larger and
Osada and Takeda Ð Crown Development in Plagiotropic Tree Saplings
59
TA B L E 1. Results of ANCOVA with branch position as factor and sapling height as covariable
F-value
Index
Branch architecture
Branch length
Length of bare branches
Length of leafy branches
Branch angle
Leaf No.
Leaf length
Internode length
Diffuse light absorption of individual leaves
Maximum DLA
Minimum DLA
Range of DLA
Relative light interception
Per ®rst-order branch
Per total branches within each branch position
Leaf and branch dynamics
Leaf production rate
Leaf loss rate
Net leaf gain rate
Branch increase rate
New ®rst order branch production rate
R2
Interaction
Height
Branch position
0´699
0´808
0´469
0´543
0´339
0´162
0´234
3´75*
11´1***
1´45
0´226
0´241
0´963
0´817
±
±
8´59**
1´16
0´160
3´76+
6´65*
±
±
5´88**
13´1***
5´81**
0´334
0´187
0´493
0´720
0´738
0´782
0´0176
0´415
0´822
14´2**
19´9**
3´48
3´17
1´30
0´833
0´775
0´288
1´11
28´3***
3´53²
5´85*
12´0**
0´629
0´386
0´614
0´368
0´783
3´03+
0´840
0´744
0´238
8´80**
8´05**
5´85*
0´240
0´474
±
20´6***
6´20**
23´0***
8´19**
±
If the interaction between branch position and height was not signi®cant, results are presented of an ANCOVA without the interaction term.
*** P < 0´001; ** P < 0´01; * P < 0´05; ² P < 0´10.
TA B L E 2. F-values of one-way ANOVA for indices of branch architecture, light interception, and leaf and branch
dynamics, and mean values of these indices for branches in different positions
Index
Branch architecture
Branch angle (degree)
Leaf no.
Leaf and branch dynamics
Net leaf gain rate (no. per year)
Branch increase rate (no. per year)
F-value of
ANOVA
Upper branch
(mean 6 s.d.)
Intermediate branch
(mean 6 s.d.)
Lower branch
(mean 6 s.d.)
13´0***
6´02**
24´6 6 11´4a
4´39 6 1´10b
14´7 6 6´30a
8´08 6 3´45a
0´65 6 11´4b
5´34 6 1´83ab
23´6***
8´34**
1´98 6 1´51a
0´97 6 0´10a
±0´60 6 0´91b
0´82 6 0´24a
±3´31 6 2´59c
0´53 6 0´36b
*** P < 0´001; ** P < 0´01.
n = 9 and 11 for the branch architecture and leaf and branch dynamics, respectively, for each branch position.
Values with different superscript letters are signi®cantly different for each row (Tukey multiple comparison test, P < 0´05). Analyses were
conducted for the indices that were signi®cant only in factor (branch position) in ANCOVA (Table 1).
there were more leaves on intermediate branches than on
lower branches, irrespective of sapling height, suggesting
that growth declines in the lower branches because of
shading by highly developed intermediate branches,
whereas the upper branches are still in an early stage of
development. Because leaf and internode lengths were not
related to branch position, differences in leaf size did not
affect the patterns of branch architecture within crowns. In
contrast, an increase in leaf size may be important for
increasing the total leaf area at the branch level in tall
saplings, since leaf size increased as saplings grew taller,
whereas the number of leaves per branch was not related to
height. Accordingly, the length of intermediate and lower
branches increased with increasing sapling height, indicating that crowns became wider as sapling grew taller. This
may be important for increasing light interception at the
sapling level as saplings grow taller.
Although the maximum DLA of individual leaves was
not related to height or branch position, the minimum DLA
was smaller and the range of DLA was larger for tall
saplings. This suggests that the leaves of tall saplings suffer
severe mutual shading compared with those of small
saplings. It is not clear why small saplings were unable to
maintain leaves with lower DLA. Although all the saplings
studied were in a shaded understorey, subtle differences in
light environment might affect this pattern. Relative light
interception at the branch level was higher in intermediate
than in upper and lower branches. This indicates that carbon
gain at the branch level may be larger in intermediate than in
upper and lower branches within saplings, not as a result of
differences in DLA, but instead because of differences in the
standing number of leaves on branches. Owing to the
increase in branch number, relative light interception at the
branch level was smaller in tall saplings than in small
60
Osada and Takeda Ð Crown Development in Plagiotropic Tree Saplings
saplings, but relative light interception per total number of
branches within each branch position class was not related
to height. Thus, the relative contribution of light interception of branches of different vertical position classes was
similar for saplings of different heights.
Branch angles were steeper in upper branches and were
nearly horizontal in lower branches, irrespective of sapling
height. Yamada et al. (2000) showed that unbranched
saplings of Macaranga gigantea, a tropical pioneer species,
produce new leaves near their orthotropic trunks, whereas
they deploy old leaves further from the trunks by increasing
petiole length and increasing the de¯ection angle of the
petiole relative to the trunk in order to capture light
ef®ciently. The branch architecture of P. jenkinsii can be
F I G . 2. Diffuse light absorption (DLA) of individual leaves in three
branch positions (upper, intermediate and lower) of four saplings of
differing heights. The upper and lower borders of the box are the 75th
and 25th percentiles, respectively, and the box is divided at the median.
A vertical line is drawn from the top of the box to the largest observation
within 1´5 interquartile ranges of the top, and from the bottom to the
smallest observation within 1´5 interquartile ranges of the bottom. Upper
and lower dots represent maximum and minimum values of DLA.
F I G . 3. Relative light interception (RLI) per ®rst-order branch (A) and
per total branches at each branch position (B). In B, shaded and open
bars represent values for real saplings and for simulated saplings in
which branch angles were ®xed to be horizontal, respectively, and circles
with lines represent the RLI ratio of simulated to real saplings.
Osada and Takeda Ð Crown Development in Plagiotropic Tree Saplings
61
compared with the leaf morphology of M. gigantea: the
length of the bare part of ®rst-order branches in P. jenkinsii
is analogous to petiole length, and the length of the leafy
part is analogous to leaf blade length in M. gigantea. The
crown projection area of the horizontally ®xed petiole angle
was slightly larger than that of actual plants in M. gigantea
(Yamada et al., 2000). Similarly, the actual branch angle did
not increase light capture ef®ciency in P. jenkinsii compared
with hypothetical branches that were ®xed horizontally.
Yamada et al. (2000) attributed this to the increasing height
growth rate in actual plants by elongating petioles vertically
in young leaves. However, this explanation does not apply
to P. jenkinsii, since the angles of upper branches are only
about 30° from the horizontal and their effect on increasing
height is almost negligible. Changes in branch angle with
decreasing vertical branch position within crowns may thus
be a developmentally constrained process in P. jenkinsii. It
would be dif®cult and costly to keep branch angles strictly
horizontal, and this would only bring minor bene®ts.
Since the rates of leaf production and loss were higher in
tall saplings but net leaf gain rate was not related to height,
leaf turnover was faster in tall saplings. As internode length
was also larger in tall saplings, crown development was
more rapid in tall saplings. Moreover, the rate of leaf
production was higher and that of leaf loss was lower in the
upper branches than in the lower branches. Leaf dynamics
did not, therefore, correspond to branch-level light inter-
F I G . 4. Indices of leaf dynamics in relation to sapling height for upper
(triangles), intermediate (circles) and lower (squares) branches. Solid,
broken and dotted lines indicate the regression lines of the upper,
intermediate and lower branches, respectively, and only signi®cant
relationships are shown (see Table 1).
F I G . 5. Branch increase rate in relation to sapling height for upper
(triangles), intermediate (circles) and lower (squares) branches, and new
®rst-order branch production rate for upper (triangles), intermediate
(circles) and lower (square) crown parts. Solid line is the regression line
of the upper branches; no other regressions were signi®cant (see Table 1).
62
Osada and Takeda Ð Crown Development in Plagiotropic Tree Saplings
ception, and upper branches grew more vigorously than
intermediate and lower branches. Senescence of lower
branches was also evidenced by the low leaf production rate
and by the very low branch increase rate (a branch increase
rate <1 represents a decrease in the number of branches).
Furthermore, new ®rst-order branch production was restricted to the upper crowns, indicating crown expansion
upwards. Thus, the ratio of carbon used within the branch
(including imported carbon) to carbon ®xed by leaves on the
branch, which is an estimate of carbon allocation, was
higher in upper branches than in intermediate and lower
branches. This pattern of carbon allocation parallels that of
temperate seedlings that have orthotropic stems with a spiral
leaf arrangement: photosynthate from recently matured
upper leaves is primarily translocated to developing leaves,
but that from lower leaves is mainly translocated to lower
stems and roots (Isebrands and Nelson, 1983; Dickson,
1986). Correlative inhibition, i.e. photosynthate export from
low-light branches to high-light branches within individuals
was recently found in various orthotropic species (Stoll and
Schmid, 1998; Takenaka, 2000; Henriksson, 2001). Light
environments were measured at the branch level in these
studies, but the authors did not take into account the branchlevel light interception, i.e. the product of the light
environment and the leaf area of the branches. Thus, the
results for P. jenkinsii do not contradict the results of these
studies. In addition, the present results are the ®rst to
describe the relationship between light interception at the
leaf level and leaf and branch dynamics in the crown. This
approach is important in understanding the patterns of
crown development in relation to crown architecture,
because the highly diverse architectures found in tropical
trees (Halle et al., 1978) may be related to differences in
regeneration strategies.
Owing to architectural constraints, branches at different
vertical positions experience predictable light microenvironments in plagiotropic species. If branches were strictly
autonomous in carbon allocation in saplings of P. jenkinsii,
then lower branches would consistently grow and produce
more leaves than upper branches, and the crown width
would be larger and sapling height smaller than measured
values. The cost of carbon allocation would be reduced in
such saplings compared with that in actual saplings.
However, this strategy is not adaptive because (1) the
light environment generally increases with height (Yoda,
1978; Parker, 1995), (2) tree saplings should grow taller to
mature and reproduce, and (3) branches cannot extend
inde®nitely because of mechanical constraints and the high
density of small saplings in a forest understorey.
Accordingly, the pattern of carbon allocation among
branches at different vertical positions found in P. jenkinsii
may be particularly important for growth and crown
development in plagiotropic species.
ACKNOWLEDGEMENTS
We thank R. W. Pearcy for critical comments on the
manuscript, and A. Furukawa, M. Awang, T. Okuda, M.
Yasuda, N. Osawa, Makmom and Shamsuddin and members of the Laboratory of Forest Ecology, Kyoto University,
for valuable suggestions. The present study is a part of a
Joint Research Project between the Forest Research Institute
Malaysia, Universiti Putra Malaysia and the National
Institute for Environmental Studies of Japan (Global
Environment Research Program granted by Japan
Environment Agency, Grant No. E-1). This study was
partly supported by a JSPS Research Fellowship for Young
Scientists to N.O.
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