Sunlight Foraging in Two Tropical Southeast Asian Pioneer Tree Species: Macaranga denticulata and Mallotus paniculatus

12 Pages·2001·0.3 MB·English

by  Kleier, Catherine

NAT. HIST. BULL. SIAM Soc. 49: 231-241，2 001
SUNLIGHT FORAGING IN TWOT ROPICAL SOUTHEAST
ASIAN PIONEER TREE SPECIES: MACARANGA DENTICULATA
ANDM ALLOTUS PANICULATUS
Catherine Kleie ，J，Lisa Ainsworth2，a nd Philip Rundef
ABSTRACT
Photos戸E曲目icresponses to light intensity plus leaf and petiole architectures were com-
pared to determine light ga白巴ringstra旬giesin two pioneer tropica1 tree species，M acaranga
denticulata and Mallotus paniculatus [Euphorbiaceae]，in am ixed deciduous fo陀stne訂 Chiang
Mai，百lai1and.Despite growth in superficially simi1紅 habitats，these two species showed
marked morphologica1 and physiological di白，rer悶 s，supp口氏ing出.ehypothesis that pioneer
species utilize ag radient of adaptive characteristics. These釘eesshare the classica1 pioneer
mo叩hologyof large leaves and long petioles，bu t their petiole arr釦 gementsdiffer. Macaranga
showed a1 泊earcor問lationbetween petiol巴lengthand leaf area，b ut Mallotus showed no
relation between leaf area and petiole length. This result suggests that Mallotus may a110cate
relatively more carbon for leaf grow出泊fullsu凶ightand a1so suggests白紙thisspecies may
have ah igher light lr叫uirementth釦 Macaranga.Because th巴maximumphotosynthetic ra旬
was hi酔erfor Mallotus (8.5μmol m-2.s-l)由加forMacaranga (7.1μmol mm--2.s一1)，petiole
growth in Mallotus is an adaptation to acquire sunlight more efficiently than if petiole grow由
we問 linearlyrelated to leaf area as in Macaranga.
Key wor由:Macaranga denticulata，Ma llotus paniculatus，ph otosynthesis，pi oneer紅ees，peti・
ole length
INτRODUCTION
The ecological meaningfulness of the pioneer concept in tropical forest succession has
recently been questioned (CLARK & CLARK，1 998). Tree species traditionally considered
as干ioneers"may actually represent ag radient of physiological and mo中hologicals回.tegies
of adaptation. In白issωdy，w e investigate pioneer strategies of two related Southeast
Asian pioneer trees，M acaranga denticulata and Mallotus paniculatus [Euphorbiaceae].
Photosynthetic response to solar irradiance and display of photosynthetic tissues represent
physiological and morphological components of light foraging s釘ategies.Differences in
these traits，if present，mi ght correlate with growth paUerns that allow niche segregation
between these two species.
Pioneer tropical forest佐'eeshave many characteristics血atallow adaptation to rapidly
chang泊genvironments. Pioneer species have been defined as species that germinate and
grow in open釘'easwhere light is not limiting (SWAlNE & WHITMORE，1 988; RUNOEL &
IOepartment of Biology，A dams State College，2 05 Edgemont Blvd.，A lamosa，C O8 1101
20ep釘tmentof Crop Sciences，Un iversity of lllinois，37 9 Edward R. Madigan La加ratory，1201 West Greagory
Orive Urbana lllinois，6 1801
30epar簡略ntof Organismic Biology，Ec ology and Evolution，Un iversity of Ca1ifomia，L osA ngeles，62 1 Charles
E. Young Orive，B ox 951606，L osA ngeles，C A 900951606
司
Received 5J uly 2000; accepted 3S eptember 2001.
231
232 CATHERINE KLEIER，LI SA AINSWORTH，A ND PHILlPR UNDEL
GIBSON，1 996). Pioneers typical1yh ave an enh cedcapacity to use high photosynthetic
加
photon flux densities (PPFDs) within ag ap or ac le紅均 (BAZZAZ& PIC阻 π，1980).
Pioneers have higher rates of photosynthesis，r espiration，t ranspiration，s tomatal and
mesophyll conductance，an d quantum efficiency than non-pioneer species，w hose seedlings
can persist in the shade of af orest (BAZZAZ & CARLSON，1 982; RmDocH ETA L.，1 991).
Studies have also shown that pioneers (Cecropia，O chroma dTrema species) have lower
組
shade tolerances，sa turate at higher PPFDs，an d have higher nitrogen efficiency th tropical
佃
climax trees (OBERAUER & STRAIN，1 984; STRAuss-DEBENEDETTI & BAZZAZ，1 991;
RAA町AMAKERSETA L.，19 95; TRAW &A c阻 RLY，1995). Pioneers also play叩 ecologically
important role by stabilizing soil after disturbances and providing initial shading for primary
forest seeds (P町MACK& LEE，1 991).
官官earlysuccessional environment of pioneer species is both variable and unpredictable，
thus providing for evolution of av ariety of adaptive traits and aw ide r geof responses
釦
to light environments (BAZZAZ &町CKET，1980; RUNDEL & GIBSON，1 996; ELLSWORTH
& REICH，1 996). Research has supported the hypothesis that pioneer species have more
flexible physiologies血anclimax species of more stable environments. For example，pi oneers
have high degrees of photosynthetic plasticity，a nd they are thus able to change pattems
of photosynthesis in rponseto varying light intensity levels (BAZZAZ & CARLSON，1 982;
，白
STRAUSS・.DEBENDETTI& BAZZAZ，1 991). Anatomically-based or cel1-based acclimation
allows pioneers to maintain ap ositive carbon balance that supports rapid growth and
survival (CHAZDON & KAUFMANN，19 93). This flexibility泊growthpattem is essential for
pioneers to flourish in ac hanging environment.
Changing environments also present ac hal1enge for org ismsto effectively exploit
加
available resour intheir environment. Foraging is one way organisms increase their
，回
resource acquisition (HUTCHINGS & DE KROON，19 94). Because plants are sessile organisms，
those plants that have greater phenotypic plasticity and c exploitnearby patches with
叩
greater resources will be more success釦1foragers (G則ME，1979). However，u nless the
plant is able to白l1yutilize available resources，th e ability to capture those resources is not
of selective value. In this s旬dy，we investigate how two住opicalpioneer trees forage for
light and th出 relativeabilities to utilize high light intensities.
Phenotypic plasticity凶 lightforaging c組 involvechanges in leaf shape，b ranching
pattems，in temode length，an d pattems of biomass allocation to various plant organs (EvA NS，
1992). Another form of plasticity involves petiole leng血 (ST回目R& HUBER，1 998).
Given that newer leaves occur closer to the apex above older leaves，on togeny predicts that
petiole lengths would be shorter for smal1er，y ounger leaves and longer for larger，o lder
leaves. However，if petiole lengths do not correlate with leaf eas，then there is af actor
紅
other than leaf age or size determining petiole length. That factor may be light; if so，
petioles of old and young leaves should grow until leaf surfaces receive high irradiances，
at which point petiole growth should stop. Such growth govemed by irradiance levels is
an ex倒npleof sunlight foraging. In order for light forag泊gto be佃 adaptivestrategy，
leaves must be able to use fully the additional sunlight g幻nedvia foraging or else carbon
would be expended without providing advantagefor the plant. To determine if differential
加
abilities to use high light intensities exist，w e performed light curves on two species of
plOneer trees.
SUNLIGHT FORAGING IN TWO PIONEER TREE SPECIES 233
MATERIALS AND METHODS
Study Site
Wec onducted 0町 studyat the Queen Sirikit Botanic Garden，n e紅 ChiangM氾，百四land
(18054'N，9 8051'E; 770 m elevation). This region supports am ixed deciduous forest
community with nualrainfall average of about 1200 mmt hat falls mostly between
組組
May and November.τbe forests near the study site at the garden bum on average every
1t o 3y ears (RUNDEL & BOONPRAGOB，1 995). Thes tudy took place between May 16 and
25，1 997，d uring the transition from dry to wet season.
Study Species
Macaranga denticulata (Bl.) M.-A. and Mallotus paniculatus (Lmk.) M.-A. are two
common colonizing species of Northeast Thailand. Macaranga and Mallotus etwo related
訂
genera within Euphorbiaceae; they are grouped in the位ibeCrotoneae and the sub回be
Mallotinae (P阻MACK& LEE，1 991). Both species are evergreen and have moderately to
deeply dormant seeds with widely distributed seed banks in North Thailand (CHEKE ETA L.，
1979).
Macaranga denticulata (hereafter refered to as Macaranga) is as mall tree，u p to 20
m in height，t hat is widely distributed in open woods and clearings from the Eastem
Himalayas and Southem China to Sumatra and Java. It is ac ommon member of secondary
forests up to 1400 m in elevation (RIDLEY，1 967). Thel eaves訂ethin and住iangularwith
round peltate or cordate bases. Thep etioles egenerally 7-13 cm long and young leaves
紅
and petioles are rust-colored.
Mallotus paniculatus (hereafter refered to as Mallotus) is similar in size to the
Macaranga species，a lso reaching 20 m in height (AIRY SHAW，1 971)， and ranges from
Southem China throughout Malaysia to New Guinea and Queensland. It is commonly
found in evergreen or dry deciduous forest at low and medium altitudes (up to 1500 m)
(RIDLEY，1 967). Leaves are altemately a gedand ovately shaped，of ten tricuspidate or
汀叩
trilobed. The leaves have whitish，re f1ective undersides with long，f1 exible petioles that are
qui旬f1exibleand may flip over to reveal the white leaf surface in high light，w ater stress，
high leaf temperature，o r a combination of these factors. Mallotus paniculatus
characteristically has large macular glands at the base of the leaves.
Photosynthetic Rates and Light Response Curves
Photosynthesis measurements and light response curves were obtained using aL iCor
“
6200 portable photosynthesis system (Li-Cor; Lincoln，N ebraska，U SA) and ab attery
powered light source (100 W halogen light). By putting al eaf in as ealed cuvette and
measuring the drawdown of carbon dioxide (C02)i nside the cuvette with an infrared gas
analyzer，th e Li-Cor system provides ag ood approximation of carbon dioxide assimilation
(A) into the leaf，a nd this is am easure of photosynthetic activity. Assimilation is thus
reported asμmol CO2m -2.s-1. A 450 cold mirror was positioned to reduce cuvette and leaf
temperatures. Leaf temperatures for both species ranged from 2TC to 34-C when
photosyn出eticmeasurements were recorded. Wire screens controlled ambient light levels
234 CATHERINE KLEIER. LISA AINSWORTH. AND PHILlPR UNDEL
dab lack drop cloth was used for dark photosynthetic readings. Photosynthesis was
加
measured in stepwise increments beginning in full (2000μmol m-2.s-1p hotons) light. At
each subsequent lower light level the leaf was allowed at least ten minutes to acclimate to
the new light intensity. One leaf at at ime was measured at all light increments，a nd
measurements were taken from at least two leaves on each individual plant. Additionally，
numerous photosynthetic ineasurements were recorded at various naturallight levels (ranging
from 400 to 1600μmol photons m-2.S- I). TheC O2c oncentration in the cuvette was
maintained at approximately 375 p紅白 permi1lion. Photosynthetic measurements were
taken in the moming (0900ー1200h) to avoid attenuation due to water or heat s回 ss.
Light curves were fit according to血enon-rectangular hyperbolic equation using Sigma
Plot version 2.01 (Jandel Scientific，F airfax，C A). The light saturated maximum of
photosyn曲目isand the maximum CO2a ssimilation rate (Amax) were both calculated with
Photosyn Assistant，V er. 1.1.2 (Dundee Scientific，D undee，U K). The light saturation
maximum of photosynthesis estimates the light level at which photosynthesis is no longer
limited by light，b ut is instead limited by carboxylation activity (LAMBERS ETA L.，1 998).
Additionally，t he maximum CO2a ssimilation rate (Amax) is the light saturated rate of net
CO2a ssimilation. Both Amax and the light saturation maximum of photosynthesis are
estimated from the asymptote of the light curve.
Leaf Sampling and Measurements
Leaf and petiole samples were removed from six Mallotus and eight Macaranga. Leaf
samples were taken from trees，y ounger seedlings，a nd saplings. Leaves were chosen
haphazardly from the bottom ptof the c opy.On individual trees in both species，th e
紅 叩
shortest petioles belonged to the youngest leaves，w hich were easily identified by rust
coloration. Extremely young leaves were not used. At least two leaves were removed from
each plant. Petiole lengths were measured with am ilimeter ruler，an d leaf and petiole eas
訂
were measured with ap ortable leaf area meter (Li-Cor 1600，Li ncoln，Ne braska). Weights
of petioles and leaves were measured with 10 and 100 gp esola balances. Alllength，ar ea，
and weight measurements were taken within 12 ho f collecting the samples.
Isotope Analysis
ln order to determine water use efficiency (WUE)，fi ve leaves of each species were
collected，dr ied，a nd placed in sealted plastic bags. Upon retum to UCLA，th e leaves were
finely ground into powder and 2m gs ub-samples of each leaf were weighed. Samples were
combusted in an elemental analyzer coupled to an isotope ratio mass spectrometer ([Duke
University，Du rham，N .CふDataare presented relative to the intemational Pee Dee Belemnite
standard. Carbon isotope ratios (OI3)a re used as al ong-teロnindicator of plant water-use
efficiency (FARQUAR ETA L.，1 989). Discrimination again~! I3C02 occurs during diffusion
into the stomata and during carbon fixation. Since OI3C is proportional to stomatal
conductance and hence inversely related to WUE，it was expected that if there were
ecological differences pertaining to plant water use，t here would be ad ifference in o13C
levels.
SUNLIGHT FORAGING IN TWOP IONEER TREE SPECIES 235
Statistics
Student's t-tests were preformed to detect for differences泊leafand stem measurements
between Macaranga and Ma/lotus. Degrees of freedom are reported as subscripts to血et
value and the probability (P) of the difference being due to random ch ceis also reported.
佃
For relations between petiole and leaf measurements，li near regressions were done，a nd r，
the product moment correlation coefficient，wa s evaluated. R号gressionsfor leaf area against
petiole length were made using Origin version 2.8 (MicroCal，N orthhampton，M A).
SigmaStat，ve r. 8.0 (Jandel Scientific，Fa irfax，C A) was used for all t-tests.
RESULTS
Light Response Curves
τ
'hel ight curves presented are best-fit lines through the data using the rectangular
hyperbolic regression equation (Fig. 1). Light response c町vesfor Macaranga and Ma/lotus
have similar va1ues at very low light intensities; for photosynthetic photon fIux density
(PPFD) lower血an100μmol m-2.s-1，ph otosynthesis va1ues for both species were less than
lμmol CO ，.m-2.s-1. At PPFD levels from 100 to 800μmol m-2.S-I ，Ma caranga had higher
photosynthetic ra旬S出anMallotus. Above aP PFD of 1，000μmol m-2.S-I ，th e Ma/lotus
curve continues to increase while that of Macaranga approaches saturation. Light saturation
occurs where there is no longer an increase in assimilation of CO2w ith ac o汀'espond加g
increase in light level (PPFD). Photosyn Assistant calculated light saturation estimates of
742μmol m-2.S-1 photons for Macaranga and ah igher estimate of 1001μmol m-2.s-1
photons for Mallotus. Photosyn Assistant estimated maximum assimilation of CO ，.into the
leaf as 6.7μmol m-2.S-1 for Macaranga and 10.2μmol m-2.S-1 for Ma/lotus. However，白e
highest assimilation values measured in血isstudy were 7.1 !lmol m-2.s-1fo r Macaranga
and 8.5μmol m-2.S-1 for Ma/lotus.
Leaf Measurements
Macaranga and Mallotus both display plagiotropic branching. Branches develop
rhythmica11y from the trunk，bu t the intemode length between secondary branches decreases
dista11y. Leaves are arranged in clusters from the branches with variable petiole lengths in
MaLlotus. Leaves釘eoriented horizontally and have leaf angles between 1400 and 190。
from the petioles.
Petiole lengths of mature leaves did not differ between Macaranga and Mallotus
(t24 =--u .276，P = 0. 785)，bu t leaf areas were sig凶白cantlyhigher in Macaranga (t24 =_ -3.651，
P= 0 .001). Macaranga denticulata leaves varied in size from 203 to 1144 cm'" in our
Snpleswith larger leaves commonly seen in the field. Mallotus paniculatus leaves were
創
not as large or as variable in size，ra nging from 112 to 502 cm2，b ut the petiole lengths
were more unpredictable than the petiole lengths of Macaranga. Ther elation of petiole
length to leaf eav;ui.ed greatly between the two species. Leaf increaseslinearly with
紅 紅白
petiole length in Macaranga (日=0.82，P < 0 .001; Fig. 2). Although one petiole measured
45 cm，a nd this point may bec onsidered an outlier，M a/lotus leaves show no relationship
236 CATHERINE KLEIER, LISA AINSWORTH, AND PHILIP RUNDEL
10
•
Mallotus paniculatus
8
I!
•
....,.. -.,
6
I
Vl
N
I- s
4
0
],
< 2
0
10
Macaranga denticulata
8
• ••
...,... -.,
6
I
Vl
N
I s
4
0
§_
...._,
< 2
0
-2
0 500 1000 1500 2000
PPF.D (f.!mol m-2s-1)
Figure I. Light curves for Mallot!ts paniculatus and Macaranga denticulata performed in the morning with both
artificial and natural light. Points represent one of at least two leaves from ten individuals. Light
curves were fit to the rectangular hyperbolic equation. They-axis shows carbon assimilation (A) and
the x-axis shows photosynthetic photon flux density (PPFD).
SUNLIGHT FORAGiNG IN TWO PIONEER TREE SPECIES 237
1200
•
Macaranga denticulata
2
R = 0.81961492
1000
P<O.OOl
•
800
,-.-.
N ••
E
u •
'-'
".Q..'). 600 ••
"'
.......
"' •
Q)
......1
400 •
•
•
200
#
0
Mallotus paniculatus
1000 2
R =0.45119101
P=0.1585
800
,-.-.
N
E
u
'-'
".Q..'). 600
..."...'. •
"'
Q)
......1
400
200
0
0 5 10 15 20 25 30 35 40 45 50
Petiole length (em)
Figure 2. Linear regression for leaf petiole length to leaf area for Macaranga denriculata (top). Linear regression
for leaf petiole length to leaf area for Mal/arus paniculatus (bottom).
238 CATHERINE KLEJER，LJ SA AJNSWORTH，A ND PHILJP RUNDEL
= =
between leaf area and petiole length (r2 0.45，P 0.158; Fig. 2). Additionally，t he
coefficient of variation (CV =s tandard deviation/sample mean) was calculated for leaf area
and petiole length for both species. The CVr atio of petiole length to 1eaf area was greater
in Mallotus血佃泊 Macaranga，1.044 to 0.643 respectively，a nd this also supports more
variable petiole lengths加Mallotus.Mean petiole weights and petiole surface areas did not
differ between species. Average leaf weight of Mallotus was smaller than that of Macaranga
=
(t24 4.197，P < 0 .001). In addition to higher le~f area and leaf weight，Ma caranga also
showed significantly higher leaf mass area (gmー)(t =7 .342，P <0 .001)，in dicating that
24
Macaranga had thicker leaves than Mallotus.
Stable Isotope Analysis
Three samples of each species were processed for o13 carbon isotope levels. Them ean
for Macaranga was -28.481 :t 1.537，a nd the mean for Mallotus was -29.181 :t 1.515.
= =
百lerewas no statistical di百'erencebetween the two species (t24 0.562，P 0.604).
DISCUSSION
In this study，M acaranga and Mallotus exhibit the classic morphology for pioneer
岡崎(HALL邑ETAL.，19 78). They have branch units that釘'earranged in an umbrella-shaped
monolayer of large leaves with long petioles (HALLE ETA L.，1 978). Both Macaranga and
Mallotus can be characterized by Rauh's model，w hich is defined by monopodial growth
of the trunk with tiers of branches that form morphogentically identical to the trunk.
Branch growth is plagiotropic and“escape asymme句"can occur distally，ca using shorter
intemode lengths between secondary branches，出usminimizing self-shading. Although
phyllotaxy seems to be genetically constrained，fa ctors such as leaf morphology，al lometry，
and physiological plasticity c overridephyllotaxic constraints to increase light reception
組
and carbon gain (NIKLAS，1 988). In this study despite their similarities as pioneer species，
Macaranga and Mallotus display significant differences in their photosynthetic capabilities
and leaf morphologies. Although the similarities of overall morphology may be due to
sh edphylogenetic histories，th e photosynthetic and leaf morphology differences between
紅
these two species suggest that ag radient of niche selection may exist in pioneer habitats.
Macaranga and Mallotus showed am arked difference in light harvesting strategies via
leaf and petiole grow由.Macaranga shows al inear relationship between leaf size and
petiole length; such ar elationship is intuitive in ad evelopmental and biomechanical sense.
However，in Mallotus，t here is no correlation between leaf size and petiole length. L ge
紅
leaves with short petioles can be found on the same branch as smaller leaves with longer
petioles. Although there is relatively much less variation in leaf size in Mallotus than泊
Macaranga，th e former still has more variable petiole lengths，w hich may be controlled by
light availability rather than the size of the leaf.
TheR auh architectural model is found in trees that demand high light. Because Mallotus
shows highly variable petiole lengths白紙donot coηespond with leaf areas，th ere must be
another factor determining petiole length aside from age or size of the leaf. Our data
support the hypothesis that the determining factor for petiole growth is light availability，
and that petioles grow sufficiently to place leaves in easof high irradiance. Excess
訂
SUNLIGHT FORAGING IN TWO PIONEER TREE SPEClES 239
petiole growth should provide an advantage for the plant or carbon is wasted. Because
Mallotus can utilize higher light levels than Macaranga，th e excess light does provide
卸
advantage，a nd variable petiole growth in Mallotus represents an adaptive exarnple of
foraging for light.
Foraging has been shown in clonal plants by ramet production occurring in favorable
patches when grown in ah eterogeneous soil resour environment (EVANS & CAIN，1 995).
Similarly，Ma llotus extends leaves into favorable sun patches when grown in ah eterogeneous
light environmen .tIn Plantago major，f ull-sun plants developed widening leaf laminas，
whereas shaded plants devoted more resources toward lengthening the larnina (NIKLAS &
OWENS，1 989). By increasing larnina surface through producing longer leaf blades，a l eaf
has ag reater chance of reaching am ore favorable light environment. In ah eterogeneous
light environment，p lastic leaf petiole lengths would enable optimal foraging for light
patches. Such af oraging strategy would be expected in Mallotus only if it were able to use
the extra light resource，w hich was demonstrated by the higher A~oV in this species.
.u
Although the present study did not differentiate between light qantity and light spectral
composition，it has been reported that light quantity primarily affects growth and that
spectral composition (i.e. red to far-red ratio) mainly affects developmental processes
(LEE ETA L.，19 97) and plant morphogenesis (ST回目R& HUBER，1 998). Thus，th e differences
in growth between the two tree species were most likely due to light quantity rather than
light quality.
The maximum CO2 assimilation rates Amax for Macaranga and Mallotus are lower
thmmmyv alues forAmaxrepomd for pioneer species-For Cecropia，Ficus，叩dDidymopnax
一.
species，A ~oV values have been reported as high as 23.1 !!mol m S-l to 33.1 !!mol
m-2• S-1 (reyl長wedin Rundel&Gibson，1996).The highest Arylax reported for Mallotus was
14μmol m-2.s-1at PPFD of 500μmol m-2• S-1 measured in an Oevergreen forest in Singapore
(TAN，O NG，& TuRNER，1 994). The present study was conducted in al ess mesic，m ixed
deciduous forest，du ring the transition from dry to wet season，an d because rain fell before
and during our study，w ater stress is unlikely to have been an issue in our measurements.
Relatively low rates of maximum photosynthesis have also been found in other pioneer tree
species (RUNDEL ETA L.，u npublished data). Pioneers generally have the capacity to vary
rates of photosynthesis with light availability and moisture availability (BAZZAZ & CARLSON，
1982) (ISHIDA ETA L.，1 999)，s o perhaps the leaves that were measured were adapted to
lower light levels and thus had lower photosynthetic rates. Howerver，a s tudy of nine
MGcarangaspecies from Southeast Asia showed ar mge for Amax of7-13μmol CO
2
m-2.S- 1 (DAV IES，1 998).
Responses to light may be important in determining ecological niches of tropical forest
trees (BARKER ETA L.， 1997). Niche separation according to plant architecture may be more
important in understory trees where light is limiting (TrwA RI & SHUKLA，1 995). Variation
in shade tolerance is strongly correlated with life history traits (DAV lES，1 998)，a gain
supporting the importance of light in defining niche segregation. Foraging for light may
also be related to ability to forage for nutrients (HUANTE，E TA L.， 1998). Finally，a r ecent
review concluded that individual plant fitness，gr owth of plant populations，a nd the degree
of growth inequality arnong neighbors are all strongly dependent on sunlight foraging
(BALLA阻 ETAL.，1 997).百四， specific niches that pioneer trees are able to occupy may
be defined by the extent of plasticity those trees are able to display.
In conclusion，M acaranga denticulata leaves have al inear relationship between leaf
240 CATHERINE KLEIER，L lSA AINSWOR'四，AND PHILIP RUNDEL
紅eaand petiole length and a1so may be more tolerant of shade白血Mallotuspaniculatus
leaves. Light curves suggest白紙Macarangadenticulata leaves may have an enchanced
abi1ity to acc1imate photosyn也氏iccapacity to lower light levels，wh ich might explain why
Macaranga has aw ider distribution than Mallotus (AIRY SHAW，1 971).
ACKNO"江ADG勘ffiNTS
Thea uthors with to由ankDr. Kansri Boonpragob，th e Queen S註泳itBotanic Garden，
血.egovernment of Thailand，a nd UCLA who provided fmancia1 support for也iswork
出roughthe Field Biology Quarter program.
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