Annals of Botany 82 : 665–673, 1998
Article No. bo980729
Biomass Allocation and Light Partitioning Among Dominant and Subordinate
Individuals in Xanthium canadense Stands
N I E L S P. R. A N T E N and T A D A K I H I R O S E*
Biological Institute, Graduate School of Science, Tohoku UniŠersity, Aoba, Sendai 980-8578 Japan
Received : 12 February 1998
Returned for revision : 15 April 1988
Accepted : 20 July 1998
Patterns of above-ground biomass allocation and light capture by plants growing in dense stands or in isolation were
studied in relation to their height. A canopy model was developed to calculate light absorption by individual plants.
This model was combined with data on canopy structure and patterns of biomass allocation for solitary plants and
for plants of different heights in dense mono-specific stands of the dicotyledonous annual Xanthium canadense Mill.
There were four stands, and stand height increased with age and nutrient availability. The allometric relationship
between height and mass differed considerably between plants in stands and those growing in isolation and also
between plants of different heights within stands. The proportion of shoot mass in leaf laminae (LMR) decreased with
increasing plant height, but solitary plants had a higher LMR than competing plants of the same height. Thus, in
contrast to previous assumptions, LMR of competing plants is not strictly determined by biomechanical constraints
but results from a plastic shift in biomass allocation in response to competition. Average leaf area per unit leaf mass
(SLA) decreased with increasing photosynthetic photon flux density (PPFD) independent of nutrient availability.
Consequently, taller, more dominant plants in stands had a lower leaf area ratio (LAR : LAR ¯ LMR¬SLA) than
shorter, more subordinate plants. Dominant plants absorbed more light both per unit leaf area (Φarea) and per unit
mass (Φmass) than subordinate plants. Their greater Φarea more than compensated for a lower LAR (Φmass ¯
Φarea¬LAR). We conclude that the greater Φmass of dominant plants is quantitative evidence that success in
competing for light is disproportionately related to the size of the shoot (i.e. asymmetric competition). We know of
no other study which has demonstrated this with quantitative estimates of light acquisition in relation to plant size.
This result is contrasted with previous studies on multi-specific stands in which Φmass of dominant and subordinate
species is similar, and we discuss why light competition in mono-specific stands is asymmetric whereas it can be sizesymmetric in multi-specific stands, with similar or greater LAI.
# 1998 Annals of Botany Company
Key words : Canopy structure, cockleburr, competition, height growth, leaf mass ratio, light capture, specific leaf area,
Xanthium canadense.
INTRODUCTION
Patterns of biomass allocation are important determinants
of competitive interactions between plants. The relationship
between a plant’s biomass, height and projected leaf area
determines the efficiency with which plants are able to
intercept and absorb light for photosynthesis (Geber, 1989 ;
Weiner and Thomas, 1992). An understanding of the
relationship between biomass allocation, light interception
and competition is therefore essential to explain the
contribution of individual plants to vegetation structure
(Weiner, 1990 ; Weiner and Thomas, 1992 ; Hara and
Yokozawa, 1994).
In dense stands of vegetation, light availability decreases
with increasing depth in the canopy (Monsi and Saeki,
1953). In such stands, taller plants will have an advantage
because they are able to project their leaves in the highest
positions of the canopy where they receive the highest light
intensities (photosynthetic photon flux density ; PPFD)
while shorter plants grow in the shade of their taller
neighbours (Ford, 1975 ; Weiner and Thomas, 1986 ; Schmitt
and Wulff, 1993 ; Anten and Werger, 1996). On the other
hand, there are costs related to increased height : plants have
* For correspondence. Fax ­81 (0)22 2176699, e-mail : hirose!
mail.cc.tohoku.ac.jp
0305-7364}98}110665­09 $30.00}0
to invest a disproportionate amount of biomass in support
tissue (i.e. stems), and leaf (i.e. leaf laminae) mass per unit
of total above-ground mass (LMR) is generally found to
decrease with increasing plant height (Givnish, 1982 ;
Menges, 1987). In addition, leaf area per unit leaf mass
(specific leaf area ; SLA) is often found to decrease with
increasing PPFD (e.g. Dijkstra, 1989 ; Anten and Werger,
1996). Thus, leaf area per unit above-ground mass (leaf area
ratio ; LAR), which is the product of LMR and SLA, can be
expected to decrease with increasing plant height and light
availability per unit leaf area.
Hirose and Werger (1995) compared above-ground
biomass allocation and light absorption in tall, dominant
and short, subordinate species in a herbaceous multi-species
stand on a floating fen. They analysed light absorption per
unit above-ground biomass (Φmass) and argued that if mass
is considered to be the investment to capture light, then
Φmass can be considered to be the efficiency of light
absorption. Moreover, Φmass should be strongly correlated
with photosynthesis and growth per unit mass (RGR),
which are closely related to a plant’s competitive ability
(Poorter and Remkes, 1990). Surprisingly, Hirose and
Werger (1995) found that short, subordinate species had a
slightly higher Φmass than tall dominant species. Φmass is the
product of the average light absorption per unit leaf area
# 1998 Annals of Botany Company
666
Anten and Hirose—Light Partitioning Among IndiŠidual Plants
(Φarea) and the LAR. Although subordinate species obviously had lower Φarea values than dominant ones, they had
a considerably higher LAR which more than compensated
for their low Φarea. Hirose and Werger (1995) suggested that
this similarity in the efficiency with which plants of dominant
and subordinate species used biomass to acquire light could
help explain the co-existence of these species.
The results obtained by Hirose and Werger (1995) apply
to interspecific competition and may not necessarily apply
to intraspecific competition in mono-specific stands. Plants
of different species may differ in their intrinsic structural
layout (e.g. rosettes Šs. stoloniferous clonal plants). In
contrast, intraspecific differences in plant structure depend
largely on phenotypic plastic responses that modify a
common layout. Differences in characteristics such as SLA,
LMR and consequently LAR between taller, more dominant
and shorter, more subordinate plants could therefore be
smaller in mono- than in multi-specific stands. Consequently, we hypothesize that dominant plants in monospecific stands have a higher Φmass than subordinate plants
and that they are therefore disproportionately more successful in acquiring light than are dominant plants in multispecies stands.
Light acquisition by plants is also determined by the leaf
area index (leaf lamina area per unit soil area, LAI) of a
stand since PPFD in the lowest layers of the canopy
decreases with increasing LAI. While subordinate plants
growing in a stand with relatively low LAI may compensate
for a lower light absorbtion per unit leaf area (Φarea) by
having a higher LAR and thus capture equal or higher
amounts of light per unit mass (Φmass) than dominant plants,
this may not be possible when LAI is relatively high.
In this study we address the following questions : (1) how
are relationships between biomass, height and projected leaf
area of plant affected by their competitive status ; (2) what
are the differences in whole plant light capture and light
capture per unit area (Φarea) and mass (Φmass) between
dominant and subordinate plants within mono-specific
stands and between competing and solitary plants ; and (3)
how different is intraspecific competition for light in monospecific stands from interspecific competition in mixed
stands ? For this purpose we develop a model to estimate
light absorption of individual plants as a function of their
canopy structure (leaf area and leaf angle distribution) and
position in the light gradient created by the canopy. This
model is combined with data on patterns of biomass allocation and canopy structure for solitary plants of Xanthium
canadense and for plants of different heights in dense monospecific stands which differ in LAI and height.
MATERIALS AND METHODS
Model calculations
The model developed here is a modification of previously
published models for light distribution in homogeneous
canopies (Goudriaan, 1977, 1988 ; Gates, 1980 ; Spitters,
1986 ; Anten, 1997). Modifications were made to accommodate differences in height between plants i.e. plants are
divided into height classes. Calculation of the total amount
of light absorbed by a plant of height class j (Φ) is given in
the Appendix (note that the subscript j used in the Appendix
has been omitted here). Φ is integrated over the day, from
sunrise to sunset, to obtain daily values of light absorption
(ΦD). In this calculation, the daily courses of diffuse and
direct PPFD above the canopy (Iodr and Iodf) and the solar
inclination angle are calculated as a function of latitude and
date according to eqns 6±27 and 6±31 in Gates (1980).
Daily light absorption per unit leaf area (Φarea) can be
obtained following Hirose and Werger (1995) :
Φarea ¯ ΦD}F
(1)
where F denotes the total leaf lamina area of a plant.
Similarly, daily light absorption per unit above-ground
plant mass (Φmass) can be found as :
Φmass ¯ ΦD}M
(2)
where M is the above-ground biomass of a plant. As noted
in the Introduction, Φmass can be considered to be the
efficiency of using above-ground biomass to absorb light.
The following relationship holds between Φmass and Φarea :
Φmass ¯ Φarea¬LAR
(3)
where LAR (the above-ground area ratio) is the amount of
leaf area per unit of above-ground dry mass (Hirose and
Werger, 1995). LAR can be divided into its two components :
the above-ground leaf mass ratio (LMR), which is the dry
mass of leaf laminae per unit total above-ground dry mass,
and the specific leaf area (SLA) :
LAR ¯ LMR¬SLA
(4)
Study site and plant material
Natural mono-specific stands of Xanthium canadense Mill.
(Asteraceae) growing along the shore of Lake Kamahusa,
20 km west of Sendai, Japan (38° 13« N, 140° 50« E) were
studied. X. canadense is a fast growing annual which is
common in disturbed areas such as flood plains of rivers or
shores of dam lakes, where nutrient and light availability are
high (Shitaka and Hirose, 1993). Between late May and late
July, the water level of Lake Kamahusa gradually drops.
Synchronous germination of large populations of X.
canadense seeds, following the receding water, allows for
pure, dense (generally 300–800 plants m−#) stands of almost
even-aged plants of this species to develop. The gradual
receding of the water also generates a gradient in time of
germination, with seed populations on the most elevated
parts of the shore germinating in late May and those on the
lowest parts in late July.
For this study we selected stands in two areas along the
lake shore. One area was at a relatively high elevation where
seeds had germinated in late May. The other area was lower
on the shore, where germination had occurred during the
middle of June. Within each area we selected two stands
with one stand being taller and having a higher leaf area
index (LAI) than the other. Differences in height and LAI at
a given elevation were probably due to differences in
nutrient availability, as indicated by measurements of leaf
nitrogen content, and, in the case of the low elevation area,
slight differences in plant age (i.e. elevation and germination
time). Within each stand, there were slight differences in the
667
Anten and Hirose—Light Partitioning Among IndiŠidual Plants
T     1. General characteristics of stands and solitary plants of Xanthium canadense inŠestigated in this study
Stand
Stand I
Estimated age (d)
80
Density (plants m−#)
335±0 (10±99)
Stand height (cm)
115
Frequency distribution of plant height
Skewness
®0±06
Kurtosis
®1±40
Above-ground biomass (g m−#)
Total
412±6 (20±71)
Leaves
107±4 (5±08)
Stems
305±3 (15±2)
Leaf area index (LAI, m# m−#)
5±17 (0±253)
Above-ground leaf area ratio (LAR, m# g−")
0±0126 (0±0003)
Above-ground leaf mass ratio (LMR, g g−")
0±260 (0±0011)
Specific leaf area (SLA, m# g−")
0±0482 (0±0012)
Fraction of leaf area in leaf inclination angle classes
F 0–30°
0±59 (0±021)
"
F 30–60°
0±32 (0±014)
#
F 60–90°
0±09 (0±016)
$
Extinction coefficient for diffuse light (Kdf)
0±892 (0±026)
Photosynthetic photon flux density (PPFD)
0±9 (0±084)
below the canopy relative to the PPFD
above the canopy (%)
Stand II
Stand III
Stand IV
Solitary
80
269±0 (16±15)
75
64
382±8 (34±04)
62
58
751±6 (53±05)
32
®1±73
4±04
®0±64
®0±38
®0±87
®0±53
265±9 (11±30)
78±1 (2±56)
187±7 (8±99)
3±40 (0±184)
0±0127 (0±0002)
0±295 (0±0042)
0±0434 (0±0011)
228±2 (2±33)
81±2 (1±96)
147±0 (2±21)
3±48 (0±072)
0±0152 (0±0002)
0±356 (0±0080)
0±0429 (0±0013)
166±4 (7±29)
67±8 (5±05)
98±6 (2±96)
2±66 (0±124)
0±0159 (0±0002)
0±405 (0±0131)
0±0395 (0±0013)
—
—
—
—
—
—
—
0±61 (0±011)
0±34 (0±015)
0±05 (0±012)
0±836 (0±054)
5±3 (0±57)
0±66 (0±022)
0±31 (0±021)
0±03 (0±018)
0±868 (0±051)
4±5 (0±49)
0±60 (0±046)
0±31 (0±030)
0±09 (0±045)
0±853 (0±039)
9±1 (0±23)
0±65 (0±021)
0±27 (0±043)
0±08 (0±029)
—
—
64
1±6
10
055
0±55
Values in parentheses denote s.e. (n ¯ 4, except for F , F and F where n ¯ 10)
"
#
$
time of germination and plants which germinated first
generally grew taller than those which germinated later. At
the low elevation site we also sampled solitary plants (no
neighbours present within 75 cm). These plants were
growing on a slope where high seed densities can not easily
accumulate. Leaf nitrogen contents were similar to those of
the shorter stands from both high and low shore elevations.
Stands will be denoted as Stands I and II for the taller and
shorter stands from the high elevation site and Stands III
and IV for the taller and shorter stands from the low
elevation area, respectively.
Canopy structure and light distribution
Canopy structure and light distribution were determined
on 7 Aug. 1995 shortly before flowering. This was about
80 d after germination (DAG) for Stands I and II, 64 DAG
for Stand III and the solitary plants and about 58 DAG for
Stand IV. A 1¬1 m quadrat was established in each stand.
Photosynthetic photon flux density (PPFD, 400–700 nm)
was measured at height increments of 10 cm in Stands I and
II and 5 cm in Stands III and IV for five replicates, using an
SF80 line sensor (Decagon Devices Ltd., UK) under an
overcast sky. Reference PPFD at the top of the canopy was
measured simultaneously using a point sensor (LI190SA,
LiCor, Lincoln NE, USA) connected to a datalogger
(LI1000, LiCor). Leaf angles were measured with a
protractor.
After measurement of PPFD, the area of each quadrat
was divided into four 0±5¬0±5 m subquadrats. All plants
within each subquadrat were cut at ground level, sealed in
polyethylene bags and taken to the laboratory. Plant height
from the base to the tip of the highest leaf was measured to
the nearest cm and plants were divided into height classes
differing by 10 cm in Stands I and II, and 5 cm in Stands III
and IV. In the remainder of this paper, we assume that
height class means represent characteristics of individual
plants. All plants were clipped every 5 cm from the base and
divided into leaves and stems. Leaf area was measured with
a leaf area meter (LI3100, LiCor). Dry mass was determined
after oven-drying at 70 °C for at least 72 h.
Canopy structure of solitary plants was measured differently to that of plants in stands. A 9¬4 m area was selected
from which all plants (52 in total) were harvested. Plants
were divided into height classes differing by 1 cm but they
were not sub-divided into layers. All other procedures were
as described for plants taken from stands.
Chlorophyll per unit area (chl µmol m−#) was determined
with a chlorophyll meter (SPAD-502, Minolta, Japan).
SPAD values were calibrated with independent measurements of chlorophyll according to Hikosaka (1996). The
regression line was chl ¯ 15±5¬SPAD®124±8 (r# ¯ 0±945).
RESULTS
Stand characteristics : density, height and canopy structure
Plant density was similar in stands from the high shore
elevation (Stands I and II) and the taller stand at the lower
elevation (Stand III, about 300 plants m−#), but much higher
in the shorter stand from the low elevation (Stand IV, about
750 plants m−#, Table 1). This difference was probably the
result of higher seed density at the site where Stand IV was
growing, though greater plant mortality in the other stands
may also have contributed to the difference. The mean
density of solitary plants was about 1±6 plants m−#.
Stand I was tallest followed by Stands II, III and IV,
respectively (Table 1). Stands II, III and IV had typical Jshaped frequency distributions of plant height while the
frequency distribution in Stand I tended to be bimodal.
668
Anten and Hirose—Light Partitioning Among IndiŠidual Plants
5
Stand I
Stand II
160
1
120
80
40
0
0
10 20 30 40 50 60 70 80 90 100 110
Dry mass (g per plant)
Plant frequency (plants m–2)
200
Stand I
0.1
Stand II
Stand III
Stand IV
Solitary
Plant frequency (plants m–2)
300
Stand III
Stand IV
240
0.01
5
10
100
180
F. 2. Allometric relationship between above-ground biomass and
height for plants from crowded stands (I, II, III and IV) and solitary
plants of Xanthium canadense. Characteristics of the different stands
are given in Table 1. Bars indicate s.e. (n ¯ 4 for crowded stands ;
n ¯ 2 to 14 for solitary plants).
120
60
0
0
5 10 15 20 25 30 35 40 45 50 55 60
T     2. Results of analysis of coŠariance (ANCOVA) with
relatiŠe PPFD on plants (relatiŠe PPFD on the highest, most
illuminated leaf of plants), plant height and plant aboŠeground biomass as coŠariates and stands as factors
0.50
Plant frequency (plants m–2)
200
Plant height (cm)
P value
Solitary
0.40
Dependent
variable
Masssol.log
Masslog
LMRlog
0.30
0.20
SLA
0.10
LARlog
0.00
6
7
8
9
10 11 12 13 14 15 16 17
Plant height class (cm)
F. 1. Frequency distribution of plant height for crowded stands and
solitary plants of Xanthium canadense. The characteristics of the stands,
including the skewness and kurtosis values for the height distribution,
are given in Table 1. Bars indicate s.e. (n ¯ 4).
Solitary plants had a slightly L-shaped distribution (Table
1 ; Fig. 1).
Stand I had the greatest amount of above-ground biomass
followed by Stands II, III and IV. Leaf area index (LAI) was
also highest in Stand I and lowest in Stand IV. Stands II and
III had similar LAIs, intermediate between those of the
other two stands (Table 1). All stands had canopies with
predominantly horizontal leaves with about 60 % of the
leaves having an inclination angle of less than 30° (i.e. F
"
was about 0±6, Table 1). There was little difference in leaf
Covariate
Heightlog
Heightlog
Height
Masslog
Relative PPFDlog
Height
Masslog
Relative PPFDlog
Height
Masslog
Relative PPFDlog
Among slopes
Among lines
0±0019
0±0017
! 0±0001
! 0±0001
NC
! 0±0001
! 0±0001
0±201
! 0±0001
0±0192
! 0±0001
NR
NR
NR
NR
NR
NR
0±942
NR
NR
NR
LMR, leaf mass ratio ; SLA, specific leaf area ; LAR, leaf area ratio ;
NR, indicates that the ‘ Among lines ’ analysis is not relevant because
slopes are not the same ; NC, no significant correlation between these
parameters and ANCOVA was not performed ; log, indicates data have
been log-transformed ; sol, shows where solitary plants were included in
the analysis (in other cases only the crowded stands were included).
angle distribution between stands. The extinction coefficient
for diffuse light (Kdf) was similar for each stand and
averaged about 0±86 (Table 1, r# ¯ 0±97–0±99).
Plant characteristics : biomass allocation and leaf
characteristics
Figure 2 shows the allometric relationship between the
height (H ) and above-ground biomass (M ) of individual
669
Anten and Hirose—Light Partitioning Among IndiŠidual Plants
0.80
A
B
C
D
E
F
0.70
LMR (g g–1)
0.60
0.50
0.40
0.30
0.20
0.10
0.10
SLA (m2 g–1)
0.08
0.06
0.04
0.02
0.00
0.05
LAR (m2 g–1)
0.04
0.03
0.02
0.01
0.00
0
20
40
60
80
100
Plant height (cm)
120
140 0.01
0.1
Relative PPFD on plants
1
F. 3. Above-ground leaf mass ratio (LMR ; A and B), specific leaf area (SLA ; C and D) and above-ground leaf area ratio (LAR ; E and F) as
a function of height (A, C and E) and relative PPFD on plants (i.e. the relative PPFD on the top most illuminated leaf of a plant ; B, D and F)
for plants from crowded stands and solitary plants of Xanthium canadense. Since all solitary plants received the same, full, PPFD, average values
were plotted in B, D and F and indicated with arrows for clarity. Characteristics of the stands are given in Table 1 and an explanation of symbols
in Fig. 2. Bars indicate s.e. (n ¯ 4 for crowded stands ; n ¯ 2 to 14 for solitary plants in A, C and E and n ¯ 52 for solitary plants in B,
D and F).
plants (i.e. log H Šs. log M ). In solitary plants, log M
increased linearly with log H. Plants growing in crowded
stands exhibited significantly concave (i.e. a significantly
positive second order polynomial term, P ! 0±05) log
M®log H relationships. Analysis of covariance (ANCOVA)
showed a significant stand effect on the slope of the log
M®log H relationship (P ! 0±0001, Table 2). Solitary plants
were considerably shorter than plants from crowded stands
with the same mass. When comparing crowded stands,
subordinate plants were always taller than dominant plants
with the same mass. For example, compare the most
dominant plants from Stand IV to plants with the same
mass from the other taller stands (Fig. 2). Note that
dominant refers to taller plants and subordinate to shorter
plants within a given stand.
When data for all plants were pooled, the above-ground
leaf mass ratio (LMR) was more strongly related to plant
height (H, r# ¯ 0±692, Fig. 3 A) than to other parameters
such as relative PPFD (no correlation, Fig. 3 B) or plant
mass (r# ¯ 0±214). The LMR of plants in crowded stands, as
670
Anten and Hirose—Light Partitioning Among IndiŠidual Plants
Φarea (mol d–1 m–2)
40.0
10.0
1.0
A
B
C
D
0.5
Φmass (mol d–1 g–1)
1.00
0.10
0.01
0
20
40
60
80
100
Plant height (cm)
120
140 0.01
0.1
Relative PPFD on plants
1
F. 4. Daily light absorption per unit of leaf area (Φarea, A and B) and per unit of above-ground mass (Φmass, C and D) as a function of plant
height (A and C) and relative PPFD on plants (i.e. the relative PPFD on the top most illuminated leaf of a plant ; B and D) for plants from crowded
stands and solitary plants of Xanthium canadense. Characteristics of the stands are given in Table 1, explanation of symbols in Fig. 2 and the
explanation of error bars and arrows in Fig. 3.
well as solitary plants, decreased with increasing height (Fig.
3 A). There was a significant stand effect on the slope of the
LMR–height relationship (P ! 0±0001, Table 2), and the
plants from Stand I had lower LMR values than plants of
the same height from other stands, especially at low plant
height. Note however, that the significant stand effect on the
slope of the LMR Šs. height relationship could, in part, have
been driven by the tallest plants in Stand I which were lying
well outside the height range of plants in the other stands.
Solitary plants had considerably higher LMR values than
any of the plants from the crowded stands including those
with a similar height (Fig. 3 A).
When data for all plants were pooled, average specific leaf
area (SLA) decreased with increasing relative incident
PPFD (i.e. the relative PPFD on the highest, most
illuminated leaf of a plant, r# ¯ 0±858, Fig. 3 D) but was
almost independent of plant height (r# ¯ 0±104, Fig. 3 C).
There was no significant difference in the SLA–PPFD
relationship between stands (Table 2). Solitary plants and
the most dominant plants in crowded stands, both receiving
full PPFD, had very similar SLA values.
The above-ground leaf area ratio was negatively correlated with both plant height (r# ¯ 0±574, Fig. 3 E) and
relative PPFD (r# ¯ 0±504, Fig. 3 F). Consequently, the most
subordinate plants had a three- to four-fold higher LAR
than the most dominant plants in the same stand (Fig. 3 E).
When comparing plants from different stands which received
an equal relative PPFD, shorter plants had significantly
higher LAR values than taller ones (Table 2, Fig. 3 E and
F). Solitary plants had higher LAR values than dominant
plants in stands.
Light absorption per unit leaf area and per unit plant mass
Figure 4 shows the light absorption by plants per unit leaf
area [Φarea, eqn (1)] and per unit above-ground mass [Φmass,
eqn (2)] as a function of relative PPFD on plants and as a
function of plant height. Φarea increased considerably with
increasing relative PPFD (Fig. 4 A and B). Among crowded
stands, there was little difference in the Φarea–PPFD relationship. Solitary plants, however, had higher Φarea values
than the most dominant plants in crowded stands with both
receiving full PPFD (Fig. 4 B). This difference can be
explained by the fact that, in the model, all leaves of solitary
plants were assumed to receive full PPFD (which was
probably a slight overestimation) while the dominant plants
in stands had much of their leaf area shaded by leaves of
their neighbours as well as their own.
Within stands, taller, more dominant plants absorbed
more light per unit mass (Φmass) than shorter, subordinate
ones (Fig. 4 C and D). The difference in Φmass between the
Anten and Hirose—Light Partitioning Among IndiŠidual Plants
most dominant and subordinate plants was greater in Stand
I (ten-fold) than in the other stands (two- to three-fold).
Between stands, however, plants from shorter stands had
higher Φmass values than plants from taller stands that
received the same PPFD. Since Φmass is the product of Φarea
and the above-ground leaf area ratio [eqn (3)], and since the
Φarea–PPFD relationship did not differ between stands (Fig.
4 B), this difference should be attributed to differences in
LAR. As stated above, LMR and therefore LAR [see eqn
(4)] decreased with increasing plant height (Fig. 3 A and E).
Solitary plants had an even higher Φmass than the most
dominant plants in stands. In this case, the difference can be
explained by the higher LAR plus the higher Φarea of solitary
plants (Figs 4 F, 5 A and B).
DISCUSSION
Plant height, biomass allocation and leaf area
Yokozawa and Hara (1992) showed that competitive
interactions between plants are strongly influenced by the
allometric relationship between plant height and aboveground mass. Most models on competition (White, 1981 ;
Weller, 1987 ; Yokozawa and Hara, 1992 ; Hara and
Yokozawa, 1994), however, treat this relationship as
genetically fixed. Results from the present study show this
to be an unrealistic assumption. When comparisons are
made between plants with similar above-ground mass,
plants in crowded stands are considerably (four- to fivetimes) taller than solitary plants, while subordinate plants
(in taller stands) were always taller than dominant plants (in
shorter stands). This indicates that height-mass allometries
may differ considerably between competing and noncompeting plants and between dominant competing and
subordinate competing plants. Similar results were obtained
by Weiner and Thomas (1992) and Nagashima and
Terashima (1995). X. canadense is a fast-growing shadeintolerant plant. Such plants generally respond plastically to
the presence of neighbours by increasing stem extension at
the expense of lateral growth (Corre! , 1983 ; Schmitt and
Wulff, 1993). Apparently, this preferential allocation of
mass to height growth was much stronger in subordinate
than in dominant plants.
McMahon (1973) pointed out that with increasing
height, plants must invest disproportionate amounts of
mass in support tissue to maintain mechanical stability.
From this point of view, Givnish (1982, 1995) hypothesized
that fractional allocation of biomass to leaves (LMR)
should decrease with plant height, that this relationship
should be strictly determined by the degree of mechanical
stress imposed on plants (e.g. windiness), and that it should
thus be independent of the competitive status of plants. In
the present study, LMR decreased with plant height, but
competing plants had considerably lower LMR values than
solitary plants of the same height. This was in spite of the
fact that competing plants shield each other from wind and
are thus subject to less mechanical stress. Competing plants
were growing at considerably lower PPFD levels, and were
thus photosynthesizing at a lower rate than solitary plants
of the same height. Increased stem elongation in response to
671
the presence of neighbours acted as a strong sink and this
probably limited the biomass allocation to leaves. This
would also explain why subordinate plants in Stand I, which
were growing at the lowest light availability, had lower
LMRs than subordinate plants of the same height from the
other stands. It is also interesting to note that LMR values
of subordinate plants were lower than values observed for
shade-tolerant plants of similar height and growing at
similar PPFD (Hirose and Werger, 1995 ; Anten and Hirose,
1998). As noted above, shade-tolerant plants generally
exhibit less stem elongation in response to neighbours than
shade-intolerant plants such as X. canadense (Kohyama,
1987 ; King, 1990). In contrast to Givnish’s hypothesis
(1982, 1995), biomass allocation to leaves of competing
plants in the current study was apparently not determined
by requirements for mechanical stability but rather by
reduced resource availability resulting from a plastic shift in
biomass allocation in response to the presence of neighbours.
The average specific leaf area decreased considerably with
increasing PPFD. Negative correlations between SLA and
PPFD are generally found (e.g. Dijkstra, 1989 ; Anten and
Werger, 1996). On the other hand, SLA is often found to
correlate positively with nitrogen availability (Hirose, 1986 ;
Dijkstra, 1989). In the present study, N availability (data
not shown) differed between the stands, but the SLA was
similar when plants from different stands were compared at
similar PPFD levels.
X. canadense is a fast-growing species common in nutrientrich environments, and the plants analysed here exhibited
high SLA values similar to those of other fast-growing
species (Poorter and Remkes, 1990). SLA values of the most
dominant plants in the stands were significantly higher than
values for dominant species in a multi-species stand on a
nutrient-poor soil (Hirose and Werger, 1995), but SLA
values for the shortest, most subordinate plants were similar
to those of the subordinate species in that stand. Dominant
species from nutrient-poor environments typically have low
SLA values (Poorter and Remkes, 1990) while subordinate
species are usually shade-tolerant and therefore have
relatively high SLAs. In mono-specific stands of X.
canadense, on the other hand, variation in SLA largely
depends on the plastic response of this trait to changing
environmental conditions (Bradshaw, 1965). Apparently,
this plastic response resulted in smaller differences in SLA
between dominant and subordinate plants (three-fold) than
those which resulted from interspecific differences in a
multi-species stand (six-fold ; Hirose and Werger, 1995).
Light absorption per unit leaf area and per unit mass
Dominant plants absorbed more light per unit leaf area
(Φarea) than subordinate plants. Light absorption per unit
above-ground mass (Φmass), the product of the above-ground
leaf area ratio and Φarea [eqn (2)], was also higher for
dominant than for subordinate plants within a stand.
Apparently, the higher light absorbtion per unit leaf area
of dominant plants more than compensated for their lower
allocation to leaves. In contrast to these results, short subordinate species in multi-specific stands studied by Hirose
672
Anten and Hirose—Light Partitioning Among IndiŠidual Plants
and Werger (1995) and Anten and Hirose (1998) had similar
Φmass values to tall dominant species. These results are particularly interesting because they were obtained for stands
which had a higher leaf area index and similar difference in
Φarea between dominant and subordinate plants as Stand IV
in this study, in which Φmass increased with increasing
dominance. This discrepancy can be explained by the fact
that the difference in LAR between dominant and subordinate species found by Hirose and Werger (1995) and
Anten and Hirose (1998) was much greater than the difference found here. As discussed previously, the greater
variation in LAR resulted from : (1) interspecific differences
in specific leaf area ; and (2) subordinate species (which
were shade-tolerant) having greater leaf mass ratios (LMR)
than subordinate plants in the current study (which were
shade-intolerant). Note that LAR ¯ LMR¬SLA. Hirose
and Werger (1995) argued that by virtue of their contrasting
morphologies, dominant and subordinate species utilized
different layers of the canopy to capture light with almost
equal efficiency, and that this was an important mechanism
allowing those species to co-exist. Here we show that in
stands of only one species, with similar shoot morphology,
such an efficient utilization of different layers in the canopy
does not take place.
Competition between plants is generally referred to as
being either asymmetric or symmetric (Ford, 1975). Asymmetric competition, in turn, has enormous implications for
plant populations in that it will lead to increased differences
in relative growth rate between plants and thus to increased
size inequality (e.g. Weiner and Thomas, 1986 ; Weiner,
1990 ; Hara, 1992). Competition is asymmetric when larger
individuals are able to obtain a greater than proportional
share of a given resource, relative to some measure of their
size, and size-symmetric when they obtain a proportional
share (Weiner, 1990). From this point of view, it has been
hypothesized that competition for light is asymmetric, since
larger plants can shade smaller plants but not Šisa Šersa
(Ford, 1975 ; Weiner and Thomas, 1986). Although this
hypothesis is widely accepted, we know of no study which
has tested it with quantitative estimates of light acquisition
in relation to plant size. In this study we developed a
detailed model for light capture by individual plants and
found that larger, dominant plants absorbed more light per
unit mass (Φmass) than smaller, subordinate plants. We
consider this to be quantitative evidence that competition
for light in these stands was asymmetric. In contrast, the
similarity in Φmass between dominant and subordinate species
in multi-species stands as found by Hirose and Werger
(1995) and Anten and Hirose (1998) indicates that light
competition between species is size-symmetric. This similarity in Φmass is attributed to interspecific differences in
plant morphology and allocation patterns. From this we
conclude that competition in multi-specific stands can be
size-symmetric, whereas in mono-specific stands with the
same or even lower LAI (see discussion above) it can be
asymmetric.
A C K N O W L E D G E M E N TS
We thank Kouki Hikosaka, Hisae Nagashima and
Katherine Preston for valuable comments on the manuscript
and Cindy Kranendonk and K. Shibazaki for technical
assistance. This work was supported by grant P-95091 from
the Japan Society for the Promotion of Science (JSPS) and
by a grant-in-aid from the Japan Ministry of Education,
Science and Culture.
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APPENDIX
Light absorbtion by indiŠidual plants
The total amount of light absorbed by a plant of height class
j(Φj) is found by :
Φj ¯
3 Φij
i
(A 1)
nj
where Φij is the light absorbed by the foliage of the plants in
the jth class in the ith canopy layer and nj the number of
plants in height class j. Canopy layers i are counted from the
top to the bottom. Φij is calculated as :
Φij ¯ Φdr,ij­Φdf,ij
(A 2)
where Φdr,ij and Φdf,ij are the amounts of direct and diffuse
light absorbed by foliage class ij. The values of Φdr,ij and
Φdf,ij are :
α!±& f
Φdr,ij ¯ ij ij Φdr,i
(A 3)
±
&
!
α
f
3 ij ij
j
Φdf,ij ¯
α!ij±& fij
3 α!ij & fij
±
Φdf,i
(A 4)
j
where Φdr,i and Φdf,i are the total direct and diffuse light
absorbed by layer i, and fij and αij the leaf area index (LAI)
and leaf absorbance of the foliage class ij, respectively. The
0±5 power in the term αij!±& was used to model the effects of
leaf reflectance and transmittance on the light climate in the
canopy according to Goudriaan (1977) under the simplifying
assumption that these are equal to each other. The light
absorbed per layer i, Φdr,i and Φdf,i, can be quantified
following the approach of Spitters (1986) :
0
0
11
Φdr,i ¯ Iodr,i 1®exp ®Kdr 3 α!ij±& fij
0
j
0
11
Φdf,i ¯ Iodf,i 1®exp ®Kdf 3 fij
j
(A 5)
(A 6)
with Iodr,i and Iodf,i the direct and diffuse photosynthetic
photon flux density (PPFD) at the top of layer i, Kdr the
extinction coefficient of ‘ black ’ non-scattering leaves for
direct light and Kdf the extinction coefficient for diffuse light
of scattering leaves. The values of Iodr,i and Iodf,i can be
found by subtracting the light absorbed by layer i®1 from
the PPFD at the top of that layer :
(A 7)
Iodr,i ¯ Iodr,i− ®Φdr,i−
"
"
Iodf,i ¯ Iodf,i− ®Φdf,i−
(A 8)
"
"
PPFD values above the canopy (Iodr and Iodf), which are
equivalent to PPFD values above the uppermost layer are
multiplied by 1 minus the canopy reflectance (τ) which is
assumed to be 0±05 (Goudriaan, 1977).
The calculation described above applied to plants which
grow in homogenous stands. In the case of solitary plants,
we assume that all the leaf area receives full PPFD. The total
light absorption per plant (Φ) is thus calculated as :
Φ ¯ Fα(Iodr Kdr­Iodf Kdf}α!±&)
(A 9)
where F and α denote the total leaf area of a plant and leaf
absorbtance, and Kdf}α!±& the extinction coefficient of ‘ black ’
non-scattering leaves for diffuse light (Spitters, 1986). Note,
however, that the assumption that all the leaf area receives
full PPFD leads to overestimation of Φ since some selfshading occurs within canopies of solitary plants.
The extinction coefficient for direct light (Kdr) can be
calculated as a function of the solar elevation and the leaf
angle distribution by assuming three leaf inclination angle
classes (0–30, 30–60 and 60–90°) and by assuming all leaves
in a class have an inclination angle equal to the centre angle
of their class (i.e. 15, 45 and 75°, respectively ; Goudriaan,
1988). Here, Kdr was thus calculated using eqns (3)–(5) from
Anten (1997). The extinction coefficient for diffuse PPFD
(Kdf) was estimated from light measurements under an
overcast sky (see Materials and Methods) and by fitting
Beer’s law to the measured data (Monsi and Saeki, 1953).
Leaf absorbance was calculated following Evans (1993) :
α ¯ chl}(chl­76)
(A 10)
where chl is the chlorophyll content per unit leaf area
( µmol m−#).