Seedling Growth of Shorea (Dipterocarpaceae) Across an Elevational Range in
Southwest Sri Lanka
C. V. S. Gunatilleke; I. A. U. N. Gunatilleke; P. M. S. Ashton; P. S. Ashton
Journal of Tropical Ecology, Vol. 14, No. 2. (Mar., 1998), pp. 231-245.
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Journal of Tropical Ecologl, ( 1 9 9 8 ) 1-1:231-215. it'ith 3 figures
Copyright O 1998 Cambridge University Press
Seedling growth of Shorea (Dipterocarpaceae)
across an elevational range in Southwest Sri
Lanka
C . V . S . GUNATILLEKE,* I . A. U . N. GUNATILLEKE,:".
ASHTON?' and P. S . ASHTON$
RI. S .
*Department ofBotanq; Faculty of Sczence, Cnztlerszty of Peradenya, Srz Lanka ?School of Forestry and Envzronmental Studzes, Yale Unzverszty, Aew Haven, C T 06511, USL4. $Departnzent of Organzsnzzc and Evolutzona~Bzology, Harvard Unzverszty, Cambrzdge, M A 02138, USA. (Acceflted 6 October 1997) ABSTRACT. Performance of seedlings of seven rain forest, canopy dominant
Shorea species was studied in a transplant experiment in forest sites a t three different elevations (low, mid and high) M-ithinthe humid zone of southlt-est Sri Lanka.
Five species generally inhabit lo~v-to mid-elevations, one at mid- and lo~vermontane elevations, and one exclusively at lower montane elevations. Temperature,
rainfall and cloudiness varied M-ithelevation. For each site seedlings were grolvn
in pots under partial shade conditions using similar soils and evermoist conditions.
All gro~vthmeasures sho~veddifferences among elevation sites, among species and
in the interaction bet~veenspecies and elevation sites. Performances of species
collecti\~elysho~ved(i) decline in height and leaf number M-ithincrease in elevation,
(ii) higher dry mass at lo~v-and mid-elevation sites compared to that at highelevation and (iii) a higher mass of single leaves at the mid-elevation site than at
the high-elevation site. Rank order of species changed across elevations for both
height and dry mass. Dry mass declined with elevation in four of the seven species
studied. S.gardlzeri, the only exclusi\~elyIon-er montane species, increased dr)- mass
with elevation. Height declined with elevation for six of the species with only S.
gardneri showing no change. Changes M-ithelevation in the rank order of species
for total leaf number and mass of single leaves lvere small. Hom-ever, total leaf
number and masses of single leaves differed among species and among elevations.
S.megistofih~llaand S. disticha had a few leaves with high individual masses, lvhile
S. gardizeri, S . a f i ~ z i sand S.trapeczfolia had many leaves lvith less mass per individual
leaf. O n e group of species sho~vedrelatively little change in leaf number per seedling and large changes in mass of single leaves. The other group varied more in
leaf number but mass of individual leaves remained constant. Grom-th allocation
' Author
for correspondence
to leaf production versus individual leaf size appears related to the successional
division of Shorea section Doona. Also all species grew better at the lo~v-elevation
site irrespective of their natural ranges except S. gardneri, whose natural range is
restricted to high elevations, and exhibits markedly lo~vergro~vthresponsiveness
than the other M-iderranging species.
m Y TVORDS: Asia, dipterocarps, rain forest, regeneration, seedling morpholog),
Sinharaja.
INTRODUCTION
We aim to examine causes of the distinct ecological ranges manifested by suites
of related tree species in tropical rain forests. O n e aspect is to clarify the role
of competition by comparing ranges in nature ~ v i t hmeasures of performance
in pot experiments, in ~vhichthe species are groLvn across separate gradients
of light, soil moisture, nutrients and elevatioil Lvith other factors held constant.
Later, results will be cornpared Lvith trailsplant experiinents in forest gaps,
including mixed-species plantings across gradients of combined en\~ironrnental
factors in the forest. I11 this paper we present results from pot experirneilts in
~vhichseedliilgs of a series of related species have been grolvn at three elevations and temperature regimes, but with soil coilditions and light held constant.
Several distinctive rain forest communities have been described for an elevation gradient that asceilds from moist coastal lo\vlands to the lobver rnontaile
hill ranges of southwestern Sri Lailka (De Rosayro 1942, Gunatilleke & Ashton
1987). Across this gradient tree species in the family Dipterocarpaceae codominate the forest canopy along with species in the Clusiaceae, Bombacaceae,
Sapotaceae and Myrtaceae (Greller et al. 1987, Gunatilleke & Ashton 1987,
Gunatilleke & Gullatilleke 1985). A clade of nine partially sylnpatric tree species in the genus Shorea ~vithinthe endemic section Doona (Dipterocarpaceae)
is distributed across this elevation gradient (Ashton 1977), and occur at lobver
and higher altitudes than other members of the genus. Species diversity is
conceiltrated at middle elevations, perhaps because only a f e ~ vforest fragments
nobv survive at l o ~ velevation.
Rain forests of the coastal plains, valleys and lolver slopes up to 300 m elevacommunity (De Rosayro 1942, Gunatilleke &
tion belong to the Dipterocarh~~s
Ashton 1987). S . a&nis ( T h ~ v . )Ashton, S . congestflora (Th~v.)Ashton, and S .
cordfolia (Thbv.) Ashton occur in this community. The i14esua-Shorea community
occupies steeper upper slopes and middle elevations from 300-900 m but also
descends to low elevatio~lson shallobv soils over siliceous rocks (De Rosayro
1942, Gunatilleke & Ashton 1987). In this elevation zone eight of the nine
Shorea section Doona species can occur (h'lerritt & Ranatunge 1959; Gunatilleke & Gunatilleke 1981, 1985) as well as five other Shorea species, in two other
sections. In lo~vermontane rail1 forests at 900-1600 m, S. gardneri ( T h ~ v . )
Ashton dominates forests either singly or in mixture with species in other
families (Ashton 1977, Greller et al. 1987). Both S . qylanica (Thbv.) Ashton and
5'. trapezfolia (Thbv.) Ashtoil extend into the lower elevations of this zone.
Seedling growth of Shorea (Dipterocarpaceae) in Sri Lanka
233
The distribution of dipterocarp species in Sri Lanka follobvs general patterns
that have been reported else~vherein the Far East (Ashton 1964, Ashton &
Hall 1992). Research in the mid-elevation zone of the .Vlesz~a-Shoreacommunity
suggests that Shorea section Doona has undergone speciation in relation to local
topographic differences in available soil moisture (Ashton 1992, Ashton et al.
1995), soil nutrition (Gunatilleke et al. 1996, 1997), and in irradiance regimes
at the forest groundstorey (Ashton 1992, Ashton & B e r l ~ n1992, Ashton 1995,
Ashton et al. 1995). Ho~vever,we are abvare of no ecological stud) that has
compared the performance of closely-related tropical tree species within and
outside their natural elevation ranges.
S T U D Y S I T E S A N D SPECIIES
The study Lvas made at three different elevations in south\vestern Sri Lanka,
representing the Dipterocarpus cornmunity, the :Wesun-Shorea cornmunity, and
the S. gardneri association. The sites selected lvere Indikada h'Iukulana (125 111)
in Kalutara district, and Sinharaja field station (580 m) and Sooriyakanda
(1060 m), both in Ratnapura district. Indikada hIukulana is located 60 km to
the northbvest of the Sinharaja field station while Sooriyakanda is 25 km to the
east. The climate at all three sites is aseasonal, receiving precipitation from the
southwest monsoon from May to July, the northeast monsoons from October to
December, and convectional rains betbveen the monsoon periods. The mean
annual rainfall at Indikada 'Iukulana is 2370 mm with a mean annual temperature of 26.6 "C. During February Indikada Mukulana receives an average rainfall of 160 mm and is the only month that receives <200 mm. The Sinharaja
field station receives an average annual rainfall of 3904 mm lvith no months
receiving <200 mm. At Sinharaja average monthly temperatures range
betlveen 25 and 27 "C with a mean of 26 "C. The high-elevation site at Sooriyakanda has a mean annual temperature of 18 "C and receives annual rainfall
of 2200 mm, with only February receiving <200 mm. The experimental plantings at all three sites were in large clearings adjacent to rain forest because
these conditions provided suitably uniform environments across elevation.
These will be referred to as the lolv-, mid- and high-elevation sites hereafter.
Poor fruiting in tlvo of the Shorea species, one of which is uncornmon (S.
zeylanica), restricted this experiment to seven of the nine species. The species
examined were S. aflnis, S. cordfolia, S. distichs, S. gardneri, S. megistofihylla, S.
trapezfolia and S. worthingtonii. These species reach the canopy of mature phase
forest with the exception of S. aznis and S. cordfolia which are usually subcanopy species. S. aflnis and S. trahegfolia are known locally as 'thiniya', S. gardneri
is called 'ratu dun', and all three species are light hard~voodsthat produce
highly resinous fruits. The remaining species are heavy hardbvoods kno\vn locally as 'beraliya' and produce larger edible fruits (Ashton 1977, Gunatilleke &
Gunatilleke 1991, Trimen 1892).
234
C . 1'. S . G U K A T I L L E K E E T A L .
Mature fruits were collected from several populations for each of the six species
from the mid-elevation site and for S . gardneri from the high-elevation site.
Insufficient seed was available from lobv-elevation sites because little mature
rain forest no\\. exists that is not in a degraded state due to past land clearance
and logging that has preferentially removed most mature Shorea trees. Shorea
aflnis, S . cordfoolin, S . distichs, S . megistohhylla and S . worfhingtonii fruited during
January-February 1989, while S . trahezzfolia and S . gardneri fruited May-June
1989. As all these species have recalcitrant seeds, they \\.ere immediately germinated in nursery beds after collection. Seedlings lvere grolvn under partial
shade in a nursery at the Sinharaja mid-elevation site until the experiment
commenced in September 1989.
At the start of the experiment, seedlings from the nursery lvere transplanted
into black polythene bags, each 15 cm in diameter and 30 cm in height containing 2.5 kg of soil (5.3 1 volume). Large polythene bags lvere used to pot
seedlings from the start to avoid transplanting into larger pots during the
experimental period. The topsoil lvas taken from 0-25 cm depth of a 10-y old
Pinus caribaea plantation adjacent to the boundary of the Sinharaja rain forest
and near the mid-elevation site. The plantation lvas established on abandoned
slvidden land that \\.as originally cleared of rain forest 30 y previously. In a
separate experiment, the growth of these Shorea seedlings, in soil from this
Pinus plantation, lvas shobvn to be comparable to that in their original rain
forest soils of valley and midslope (Ashton et al. 1997; Gunatilleke et al. 1996,
1997). h'lacro-nutrient analyses of these soils have been given in Gunatilleke et
al. (1996, 1997). A handful of soil taken from a composite soil sample from
natural forests in the lo\\.-, mid- and high-elevation sites \\.as also added to each
pot to serve as rnycorrhizal inoculum.
Previous work on four of the species (Ashton & Berlyn 1992, Ashton et al.
1995) indicated that, although maximum grobvth rates for each species \\.ere
attained at different irradiance levels, the level closest to the maximum for all
species was 50% of full sunlight. The shade houses lvere therefore constructed
in open areas free of surrounding vegetation and receiving direct light for
> 8 h per day. Shading lvas provided by over-laying a coir (coconut fibre) mesh
material with 1-cm' mesh-size on the roof and sides of the houses. This material allowed approximately 50% of sunlight of normal red: far red ratio to
filter through as measured at noon on sunny days in October 1989 using PPFD
sensors attached to a data logger (LI 190SZ sensors, LI 1000 data logger;
LICOR Inc., Lincoln, Nebraska, USA). All seedlings in shade houses received
natural precipitation through the coir material during rainy periods, but during
relatively dry periods they were irrigated. Seedlings were not fertilized during
the period of the experiment.
The experiment comprised planting seedlings of seven different Shorea spp.
lvithin shade houses at low-, mid- and high-elevation sites. Twelve potted seedlings of each species lvere randomly arranged in one of four groups within each
Seedling growth of Shorea (Dipterocarpaceae) in Sri Lanka
235
shade house. In total 1,008 seedlings (48 seedlings x 7 spp. x 3 sites) liere
monitored over a 24-mo period from September 1989 to October 1991.
Seedling height from the soil surface to the apical leader, to the nearest
0.5 cm, and leaf nurnber per seedling were measured 1 mo after transplanting
and then at intervals of 3 mo for 2 y. At the end of the experiment, dry mass
of the lihole plant and that of its constituent parts (stem, leaves, tap root and
fine roots) of a randornly selected subsarnple of three seedlings per group per
species per elevation site Lias recorded after harvesting, washing them free of
soil particles, and oven-drying them at 85 O C for 48 h. No record Lias made of
leaves and other stem and root tissues that abscised over the experimental
period.
Because of some seedling mortality over the extended period of the study, data
were subjected to an unbalanced AILOVA using the GLM procedure of SAS (Ray
1982). ANOVAs liere done using logarithmically transformed data for each of
the attributes measured to compare performailce of species grolin at the three
different elevations. For analyses, groups within each shade house liere pooled
and a tlio-way ANOVA performed comparing species, elevation and their interaction. Multiple cornparisons arnong means of different species liere carried out
using Tukey's hoilestly significant difference method (P 5 0.05).
RESULTS
Overall mortality was loli and varied little betlieen species. T h e plants greli
well and excavation for biomass estimation at the end of the experiment
revealed no evidence of pot-binding. T h e main treatments (species, ele\lation)
and their interactions for each measure of grobvth were significant at the 0.001
level, with four exceptions ~ i h e r esignificance lias at the 0.01 level, and three
exceptions lihere significance was a t the 0.05 level (Table 1).
Table 1. Values for the F-statistic and the levels of significance for height (cm), leaf number and dr) mass
attributes (g) from the tLvo-way analyses of variance of seedling performance for the seven Shoreti species
grown at thrcc elevations (sites).
Treatment effects
Sites (2)
Species (6)
Sites x Species (12)
Treatment effects
Sites
Species
Sites x S ~ x c i e s
*** P<0.001,
Height
353,09***:
23,0&:$"*
12,2+***:
Stem dry mass
44,03:$:$*
6.7 7**:$
4.54*
Total dry mass
31,70:$:$*:
Leaf number
G.&,61""*
6,84***
5,40:$"
13G,31**:
10,85:$"*
Leaf dry ~ n a s s
T a p root dry mass
18,07***
6,93:$":$
5,66:$*:
23,11**:$
11,6j:$**
.&,69:$*
Mass of single leaf
+.9G**
11.&,.&0"*"
+.+a*
Fine root dry mass
52,2g**:$
~,37***
2.65*
"*:P<0.01, *P<O.O5, ns = not significant
Growth: height and d~ mass
Species varied in their rank of height and dry mass in three liays. Firstly,
for S. cordzfolia, S. trtrpe~zfolia,and to some extent S. worthingtonii, rank order of
height Iias the same as that for dry mass at all three elevations (Figure 1). For
236
C . V . S. G U N A T I L L E K E ET AL.
these species, height and dry mass are both inversel) related to ele\~ation.
Secondly, S . a z n i s Iias among the three top ranked species with respect to
height. Its corresponding position ~ i i t hrespect to dr) mass lias sirnilar for the
lobv- and mid-elevation sites but Lias considerably lolier for the high-ele\~ation
site (Table 2). Seedlings of S. nztzis at the lo~i-elevationsite Liere taller than
those at mid-elevation (Figure 1). This species has the ability to increase its
height without correspondiilg increases in dry mass. S . nzegistophylln and S.
worthingtonii shobv a growth pattern sirnilar to that of S. aznis. Thirdly, liith
increase in elevation, height of S. gnrdneri shobved no change, but there Lias a
much greater variation in total dry mass (Figure 1). In this species, the heaviest
seedlings were observed at the mid-elevation site lihilst those at loli-elevation
Iiere the lightest. At the high-elevation site height growth of all species Lias
similar except for S. megistophylla, which greLi least (Table 2).
For S. disticha and S. megistophylla, mass values of stern, leaf and tap root did
not vary among the different elevations (Table 3). For S. nznis and S . trahe~ifolia mass values of stem, leaf and tap root at the l o ~ i -and mid-ele\lation sites
were higher than those at high-elevation. S. worthingtonzi responded similarly,
except that its tap root mass did not differ among sites. S. cordfolia and S.
gardneri tended to differ from the trends in masses across sites s h o ~ i nb) the
other species. In particular, leaf and root masses of S. gnrdneri Liere greatest at
the mid-elevation site as compared to the l o ~ i and
high- elevation sites (Table
3).
At the lo\\-elevation site, a greater proportion of total plant dry mass Iias
allocated to the above-ground compoilents (67-84%) by all species as compared
to those plants groLin at mid- and high-elevations (58-70%) (Figure 2).
Correspondingly, the total plant dry mass allocation belo~i-groundincreased
from the loli- to mid- and high-elevations, sholiing little difference betlieen
the latter tlio elevations in three of the species (Figure 2, Table 3). S. nznis
and S . worthingtonii contributed most to their above-ground parts, and S . gardneri and S. nzegistophylln the least.
For all species proportional allocation to fine roots lias greatest for the midelevation site follo~iedin decreasing order by those at high- and low-elevations
(Figure 2, Table 3). This proportional increase in mass for fine roots at midelevation appears associated with a concomitant decrease in leaf mass in all
species except S. gardneri and S. megistophylln, for ~ i h i c hit was at the expense
of the stern and tap root. Proportional mass allocation to leaves, on the other
hand, Iias greater in seedlings grown at l o ~ i -and/or high-elevations in five of
the species (Table 3). I11 the t ~ i oremaining species, S. gardneri and S . megistophylln, the least allocation to leaves was for those plants grown at the l o ~ i ele\lation site.
Len ties
Among individual species leaf number for S. distichn and S. megistophylln did
not vary significantly among the three sites, lihile S. gardneri, unlike all the
Seedling growth of Shorea (Dipterocarpaceae) i n S r i Lanka
60
237
(a) Height
a
1
Figure 1. Mean height (a) and total dry Inass (b) of each of seven S'l~orea species grown in shade houses at
lo~v-,mid-, high-elevation sites in southwestern Sri Lanka. Letters signify statistical differences in the performance among sites of each species separatel), based on Tuke)'s honest significant difference. Significant
differences (P<0.05) are indicated b) different letters. Sites with the same letter are not sigilificantl) different from each other. Analyses Lvere made on log-transformed data.
Seedling growth of Shorea (Difiteroca$aceae) i n S r i Latzka
239
Table 3. Significant differences (shown by small letters, a, b and c, at the P<O.O5 level) among three sites
(elevations, columns) for each of seven species. Analyses Lvere done using Tuke)'s honest significa~ltdifference method on log-transformed data.
Stem mass
S. ofinis
S. cordfolia
S. disticlia
S. gnrdneri
,S, megirtopiylla
S.trnpez$lia
,S.ztorthingtonii
Leaf mass
S. aj7nis
S,cordfolia
.C. disticira
S , gnrdneri
S. megistopiplln
,S. trapezfolia
S. worthingtonii
Tap root mass
S. ofinis
S. cordfolia
S. disficha
5'. gnrd~zeri
,S. megistophjlln
S. t r n ~ e s f o l i a
S.zuortlri~rgtonii
Fine root mass
.S.aflnis
S . cordfolin
.S. distici~a
S. gardneri
S. nzcgistopl~rlla
S. trapezij~lia
S.ztortlri~ietonii
Lo~v
(125 rn)
Elevation
Mid
(580 m)
High
(1060 m)
a
a
a
a
a
a
a
a
b
a
a
a
a
a
b
c
a
a
a
b
b
a
a
a
b
a
a
a
a
b
a
a
a
ab
ab
b
b
a
ab
a
b
b
a
a
a
b
a
a
a
a
a
a
a
a
a
a
b
b
a
b
a
b
a
b
a
b
b
b
ab
b
a
a
a
a
a
a
a
b
b
b
ah
b
b
ah
irradiance and the warmer temperatures at the low-elevation site. Overall,
our seedling results support those of Ashton & Hall (1992) whose census of
trees from Sarawak indicate a decline in performance of climax species with
elevation. This is contrary to the 'cool nights theor)' (for evidence see
;\/fergen et al. 1974), based on temperate conifer seedlings, which claims
that night temperatures decline more rapidly with elevation and leads to
increased productivity with elevation. Other climatic changes than temperature ma) occur in the humid tropics with elevation. For example, because
of the lower rainfall at the low elevation site, as compared to the sites at
mid- and high-elevation, we suspect that the low elevation site also has
reduced cloud cover and more sunny days than the other sites. More cloud
cover and cooler day temperatures have been shown to reduce photosynthesis
and plant growth (Hart 1988, Jones 1983, Larcher 1995).
Climatic affects on soil development could be another influencing factor on
S. disticha
Idow Mid High
::
C
u
LUU
,
S, cordifolia
S. trapezifolia
S. affinis
Lon Mid High
Idow Mid High
Altitude
S. gardneri
S. worthingtonii
Idon Mid High
S. rnegistophqlla
7
J 701
Leave?
- Stem
)Tap root
Lateral? R
fine roots
$
Lo\% Mid High
Lo\% Mid High
Altitude I,on Mid High
F i q i ~ r e2. Compariiori of tile proporiiori of dr) mass allocatecl to different beedling parts as a percentage
of the total dr)- mass for seven ,Y/iorrn sprcirs grolvn in shade housrs a t lo\\-, mid- and high-elcvaiion sites
i n Southu.est i r i Lanka.
seedling grotvth. For example, although the same soil mixture tvas used for
potting seeclliilgs at all three elevations, it is likely that release rates of nutrients (nitrogen ill particular) into solution declii~eivith decreases ill ambient
temperatures. There ma) also be local, temperature-induced differences in
ambient CO, concentratioi~near the ground.
Among the species, S. megistophjlln is knotvn to perform best under higher
light conditions than that provided in the experiment, thus restricting its
but it is
potential height increases (Ashtoil 1995. Ashton Pr B e r l ~ i1992).
~
unclear hoiv this could influence its relative performance at different eleva-
Seedling growth of Shorea (Dipterocarpaceae) in Sri Lanka
40
24 1
(a) Leaf number
a
(b) Mass of single leaf
0.5
o
Figure 3. hleali leaf number per seedling (a) and mass of single leaves per seedling (h) of each of seven Siioren
species grown in shade houses at lo~c-,mid- and high-elevations in South\cest Sri Lanka. Letters signify statistical differences in the performance amollg sites of each species separately, based on Tukefs honest significant
difference. Sig~lificantdifferences (P<0.05) are indicated hl- different letters. Sites with the same letters are
not significantly different from each other. Analyses xcere made on log-transformed data.
tions. The only true lower montane species in this study, S.gardneri, performed
best at the mid-elevation site. S. gardneri could be maladapted to higher temperatures (respiration) and amounts of irradiance (photo-inhibition) at lowelevation, while its absence in nature at mid-elevation is likely due to competition. T h e poor performance of S. gardneri at low elevation was correlated with
a reduction in leaf number (Figure 3) and a reduction in proportional allocation to leaves (Figure 2). Both trends that would lead to reduced gro~vthunder
non-limiting temperature and irradiance conditions. Although it possessed the
equal highest allocation to roots among all species (Figure 2) this factor alone
is unlikely to be responsible for its poor performance because S. megistofihj'lla
showed a similar allocation pattern and did not suffer in the same wav. Further
studies are needed to examine this.
T h e ratio of above- to below-ground components in all species decreased
more from lo~v-to mid-elevation, than from mid- to high-elevation, except in
S. xorthingtonii and S. megistofihylla where they were similar. All species can be
divided into two general groups based on the trade-offs between the proportion
of resources allocated to fine roots as compared to leaves or stem. For S . disticha, S.trpezz;folia, S. a f i n i s and S. worthingtonii the change in proportion of
fine roots with elevation was associated by a n inverse change in leaf mass.
However, for S.cordzfolii S.gardneri and S. megistophylla this change in fine root
proportion with elevation was associated by a n inverse change in stem mass.
Species varied little in rank order of leaf number and masses of individual
leaves among sites. Only relative positions of closely ranked species interchanged. An exception to this Lvas S. tr@ezz;folia which ranked first, fourth and
fifth at the lo~v-,mid- and high-elevation sites. Based on these traits of leaf
size and number, species can be loosely organized into three kinds leaf morphologies. The large-leafed S. megistofilylla and S. disticha showed relatively small
or no change in leaf number but greater change in leaf size, reflected in the
mass of single leaves. In contrast, the small-leafed species S. g a ~ d n e r i ,S . afJinis
and S.t~ajezzfolzashowed a decrease in leaf number with increase in elevation
but relatively small variation in individual leaf mass. T h e third group comprises
S.zoo~thzngtonzz and S.cordzfilza mhich show intermediate variation in both leaf
number and leaf size between the other two groups.
Comparison of peforn~ancezn the etjerz~izentwzth speczes' natural elecatzonal ranges
The recorded elevational range of each species is shown in Table 4. In general, seedling height and dry mass responses to elevation failed to shom differences among species consistent with differences in their natural ele\7ational
ranges. This inight be because this study did not select enough sites. Comparative performance across sites demonstrated that seedlings of most species, in
fact, gro\v in height better at lower elevations irrespective of their natural
ranges. This lvas most noticeable for S . gardneri which performed best at the
mid-elevation site as compared to within its natural range at the high-elevation
site. T h e absence of most of these species in forests at the lomest elevation,
Seedling grote'th of Shorea (Dipterocnrpncene) in Sri Lnnka
243
Table 1. Elevational rallges in nature of the seven Siiorea species studied. Vertical lines indicate the elevation
of the shade house sites.
Indikada
hlukulalla
Sinharaja
Soosiyakunda
S.gnrdneri
S.trajez$o/ia
,S. zrorthitigtonii
S. disticiia
,S. cordjfilio
,S. me,~istoi,h>//a
,S', n r n i ~
0
300
GOO
900
1200
1500
1800
Elevation (rn)
therefore, must be attributable to factors other than the direct influence of
temperature. It appears related to the superior competitiveness of other species in occupying the gro~vingspace of the forest, although historical reasons,
including deforestation cannot be excluded.
T h e confinement of S. gardneri above 900 m, and most of the other species
belo~vthat eles~ation(Table 4), hoxvever, reflects their respective confinement
to lo~vermontane and lo~vlandforest, respectively. The abrupt ecotone betxveen
these formations, ~vhichgenerally occurs at c. 900 m in the aseasonal tropics,
accompanies a rather sudden increase in soil organic matter and a qua1itatis.e
change in soil arthropod fauna, notably a decline in termite numbers and
dis~ersity.Thus temperatures may have a n indirect influence on eles~ational
distributions through soil not simulated in our experiment.
Pres~iousstudies have sho~vna n influence of soil moisture (Ashton et al.
1995), soil nutrition (Gunatilleke et al. 1996, 1997) and light climate (Ashton
1995, Ashton & Berlyn 1992). They have also sho~vnthat these factors vary
primarily with topography (Ashton et al. 1995). The dis~ersityof topography is
greatest at middle elevations, explaining xvhy the greatest diversity of Shorea
species occur there. Also S. megistojhj'//a, the species among those studied which
extends to the lo~vestelevation, is the highest light-demander, achiesing highest gro~vthrates ill the largest gaps ~vhichare concentrated in the lower slopes,
valleys and bottom-land habitats (Ashton 1995, Ashtoil et nl. 1995). T h e occurrence of S. megistojlylla at lo~verelevatioils than the others appears due nlore
to its higher light saturation point than any direct response to temperature.
Species differed in growth responsiveness anlong the eles~ations.For measures both of height and dry mass S. col-dfolin, follo~vedby S. afi~zis and S.
trapezflia, sho~vedgreater differences among sites than the other species. S.
gardneri sho~vedthe least difference in height and dry mass among the sites. We
interpret these effects to result from a combination of factors. O n e important
244
C
V
5 . GUNA TILLEKE ET AL.
consideration is that tree species that are usually in the subcanopy (e.g. S .
cordfoolin, ,S. nznis) exhibit greater physiological and groxvth responsis~enessin
relation to the environment than true canopy and emergent species. Also, midsuccessional species that comprise the light hardwoods (S. aztzis, S. tra/~ezifolia)
exhibit greater physiological and groxvth responsis~enessin relation to the environment than the late-successional species that comprise the sloxver groxving
heaxy hardxvoods (S. cordfolia, S. distichs, S. megistoph~lla,,S. worthingtonii); and
lastly, ,S. gnrdtzeri, xvhose natural range is restricted to high elevations exhibits
markedly less groxvth responsis~enessthan the other species that have xvider
ranges and that are predominantly found at lower elevations.
Species differences are also apparent for leaf number and mass of single
leaves among elevations. In this case, hoxvever, species variation in leaf mass
and number among sites appear most related to the successional dis~isionof
Shorea section Dootza into txvo subgroups (beraliya, thiniya). In general S. afinis,
S. trnpe~folin (thiniya) and S. gnrdneri (ratu dun) shoxved greater s~ariationin
leaf production among sites xvhilst the remaining species (beraliya) shoxved
greater variation in mass of single leas~esamong sites. Shoren cordzfolia xvas a n
exception showing significant variation in leaf number but not leaf size. T h e
thiniya group of species tend to reduce their leaf numbers xvith increasing
elevation more than the beraliyas.
This study has shoxvn that ecological diversification among most species xvas
not apparent across a large scale eles~ationgradient. The exception to this xvas
S. gardtzeri that does better in relation to the other species of the group at
higher elevations. These interpretations might be misleading because a greater
number of sites at various elevations ~vouldallow for a more refined analysis,
and because xve failed to include both S. cotzgest$ora, a low elevation species,
and S. zglatzica, a nledium to high elevation species, because of their poor
fruiting. We therefore conclude, first, that although the natural elevational
ranges of Shoren (section Doona) species are partly mediated by competition,
the indirect influence of the temperature elevation gradient, as well as other
factors associated xvith topography, on the elevational distribution of soils are
also important. Second, xvhen pres~iousstudies are taken into account we conclude that these species have diversified primarily in relation to the local topography in mid-elevation forests, xvhere the majorit). of species are concentrated.
This investigation xvas supported by a grant from USAIDRSTC to Harvard
Unis~ersity,USA and the University of Peradeniya, Sri Lanka. The field facilities and permission to xvork in Sinharaja and Wags xvere given by the Forest
Department and in Sooriyakanda by the chief incumbent priest of the Sinharaja Prantha temple. Special thanks are due to Eddie Tapiero for statistical
and computer assistance, and to both the Arnold Arboretum of Hars~ardUnis~ersityand the Smithsonian Tropical Research Institute for felloxvships to
Seedling growth
of Shorea (Dipterocnl;bnceae)
in S r i L n n k n
245
I.A.U.K.G. and C.V.S.G. during ~vhichtime a part of the manuscript Lvas prepared. llTe~vouldalso like to thank Dal~idBurslem, Peter Becker, Ian Turner,
David Keubery and one anonymous re~.ietverfor cominents and suggestions on
pres,ious manuscript drafts of this paper.
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