1. Dipterocarp Biology as a Window to the Understanding of Tropical Forest Structure
Author(s): Peter S. Ashton
Reviewed work(s):
Source: Annual Review of Ecology and Systematics, Vol. 19 (1988), pp. 347-370
Published by: Annual Reviews
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2. Ann. Rev. Ecol. Syst. 1988. 19:347-70
Copyright ? 1987 by Annual Reviews Inc. All rights reserved
DIPTEROCARPBIOLOGY AS A
WINDOW TO THE
UNDERSTANDING OF TROPICAL
FOREST STRUCTURE
Peter S. Ashton
Professorof Dendrology, Departmentof Organismicand EvolutionaryBiology, Har-
vard University Cambridge, Massachusetts02138
INTRODUCTION
My aim is to show, by one example, why systematics must provide a
foundation equal to that of ecology if the structureof tropical forests, and
particularlytheir floristic structureand often extraordinary
species richness, is
to be understood.Species composition accounts for much of the geographical
variationin structure thatoccurs among forests in differentregions sharingthe
same habitat. Our knowledge of rain forest structurehas advanced from the
descriptionof whole plant communities, or often their tree component alone,
to a contemporaryinterest in the demographyand genetics of species pop-
ulations, and to the physiological ecology of individuals. Always, though,
systematic as well as ecological relationships must be understood if this
knowledge is to contribute to understandingof the structureof the whole
forest.
The biological attributes shared among the individuals of species, and
which decide their success in interspecific competition, determine both the
ecological guild to which they belong and, in part, their systematic rela-
tionships. Systematics aims to classify species into a hierarchyof categories
of increasing rank. The hierarchy is aimed to reflect evolutionary rela-
tionships, and it thereby provides inferential evidence for the origin of the
major species groups constituting tropical rain forest communities. I argue
that an understandingof the floristic structureof tropical forests, and more
347
00664162/88/1 120-0347$02.00

3. 348 ASHTON
specifically of the way species invade and survive in mixture in these com-
munities, of the way the communities vary in species composition and of
why, in particular,these forests vary in species richness-all these can be
achieved only by understanding how the hierarchiesof systematicand ecolog-
ical relationshipsare intermeshed.
My vehicle for analysis is the Dipterocarpoideae,the species-rich Asian
subfamily of a pantropical family that, because its trees dominate both
internationalhardwoodmarketsand also the emergent canopy of most low-
land rain forests in Asia, has been the subject of more-and a broaderrange
of-research than any otherrain forest taxon of comparablesize. It will soon
become obvious though that dipterocarpsare unique in many important
ecological and systematicrespects, and it is only partiallypossible to general-
ize from them. This underlineshow important is to apply the same approach
it
to other groups.
This review is structuredby systematic categories, startingwith the sub-
family itself and terminatingwith a more elaboratediscussion of the species.
It serves to show how fragmentary knowledge still is and where research
our
must now be directed.
The Dipterocarpoideae
Ashton (10) has elsewhere reviewed the systematic and ecological literature
on the group. Dipterocarpsare trees; their simple penninervedleaves have
articulatedstalks. Their bisexual and star-shapednodding flowers have five
-merous perianthsin which the sepals and petals, respectively, overlap. The
sepals enlarge and become winglike in fruit. Petals and fruit sepals are
generally twisted. The single ovary has three cells, each of which generally
contains two ovules; it generallyripens a single seed in a dry indehiscentnut.
The pollen grains are ellipsoid, slightly sticky, with smooth thin walls.
Dipterocarpsare insect pollinated. Fruit dispersal, if it occurs at all, is by
wind or more often by gyrationalone. Rats commonly scatter-hoard nuts the
but are thought not to carry them far (L. Curran,personal communication).
Asian dipterocarpsare distinguishedfrom others by their resin canals, which
are abundantin many tissues, and by details of wood anatomy, pollen, and
stamens. Two tribes are recognized, and these differ in details of the fruit
calyx and wood anatomy, and in basic chromosomenumber;any ecological
implications of these differences are unknown.
Though there are only 12 dipterocarpgenera in Asia, some 470 species
with at least 100 infraspecific entities are recognized. This makes them
particularlysuited for this review. Thoughspecies diversityis now centeredin
Borneo and surroundingregions, systematic analysis suggests that the sub-
family originally invaded Asia by way of the Indian fragmentof Gondwana
(10). Total species diversity is markedlyinfluenced by historical biogeogra-

4. DIPTEROCARPBIOLOGY 349
phy, there being only 26 species east of Wallace's line, and 287 in Borneo as
against 7 in Sulawesi, which is only 80 km east of it at the same equatorial
latitude.
The absence of a towering emergentcanopy dominatedby dipterocarpsin
New Guinea gives the structureof lowland forests there an aspect quite
differentfrom those of Asia, and similarin many respectsto that of neotropi-
cal forests. Many other families fill the gap and are more abundantin the
canopy there, though tall emergentsare either more scatteredthan in diptero-
carp forests or absent. The southwest of Sri Lanka, in an area some 180 km
square, possesses 45-55 dipterocarpspecies, all but one endemic. The re-
latively low numberin this case must have an island biogeographicexplana-
tion (17). These regional differences in species diversity indicate major
differences in forest floristic structure.It is clear from the start, then, that
historic accident has been a major influence on the role of dipterocarpsin
Asian forests.
Dipterocarpsare exclusively components of the forest maturephase, only
gradually invading successional forests after a closed canopy is fully es-
tablished. Most are confined to wet climates, with a dry season not exceeding
four months. All but a handful are evergreen. Dipterocarpspecies (but not
individuals) are dramaticallymore abundantin aseasonal than seasonal cli-
mates. Thus, of the 155 occurringin peninsularMalaysia, only 27 occur also
in seasonal Indo-Burma,whose total dipterocarp flora is only 58 species (53).
In the Far East, the majorityare confined to the lowlands, with a few species
endemic to higher altitudes. Curiously, this is not so in Sri Lanka, where a
majority of the large endemic genus Stemonoporus occur in hill or sub-
mountaneforest, and the family is best representedin moist premontane[in
the sense of Holdridge (37)] forests at 500-800 m (8). The overall species
richness of forest tree communitiesseems to reach a maximumat this altitude
in Sri Lanka(33), althoughelsewhere in the Asian tropics the maxima are in
the lowlands. This could reflect periodsof aridityin the geological past, when
the South Asian' mixed rain forests might have been restrictedto an area
immediatelybeneathand within the cloud belt on the mountainslopes, as they
are today in central peninsularIndia.
In seasonal climates, dipterocarpsare confined to relatively infertile soils.
Thus, in IndiaShorea robusta forests are concentratedon graniteand schists,
and are replaced by deciduous teak forest on basalts and other calcareous
rocks. In Indo-Burmathe Dry Dipterocarpforests are confined to podsolic
sands and laterites, and mixed deciduous forests replace them on mesic sites
(23).
In the aseasonalhumidtropicsof the FarEast, abundanceof reproductively
'PeninsularIndia and the island of Ceylon (Sri Lanka)

5. 350 ASHTON
maturedipterocarpsis greateston yellow and red lowland soils (5). Not only
is the family as a whole generally dominantin the emergent stratum,there
comprising 80-100% of all individualson yellow-red soils (e.g. 21, 5), but
single species dominancemay be approachedboth on highly fertile (10, 15)
and highly infertilesites (1, 58). Dipterocarpspecies populationsare general-
ly aggregatedinto loose clusters of reproductiveindividuals, with juveniles
concentratedaroundthem (6).
The ecological characteristics the majortree families of Far Easternrain
of
forests differ to a greateror lesser extent. Species richnessin Myrtaceaevaries
in ways similarto Dipterocarpaceae relative to soil for instance, but there are
few emergent species and clustering is less evident. Myrtaceae seeds are
mostly animal dispersed. Meliaceae and, to a lesser extent, Rubiaceae and
Euphorbiaceae broadlyincreasein species richness with increasingsoil nutri-
ent status (5). Dipterocarp ecology, however, remarkablyrecalls that of
Fagaceae.
Fagaceaereachtheirlimits of species diversityin the submontaneforests of
eastern Himalaya, Indo-Burma,southernChina and western Malesia.2 They
are represented,thoughmore poorly in species and abundance,in the lowland
forests of the Far East. Like dipterocarps, few Fagaceae appear east of
Wallace's line. Fagaceae are absent from peninsularIndia and Sri Lanka,
presumablyfor historicalbiogeographicalreasons. The altitudinal distribution
of the two families is suggestive of interfamilialcompetitive exclusion.
Asian Dipterocarpaceae and Fagaceae are both unusual among rain forest
tree families in being apparentlyobligately ectotrophicmycorrhizal(52; 54,
55). Trees of both families possess abundantmats of superficial fine roots.
These are best developed on less fertile soils. Smits (55) has evidence that
many basidiomycete mycorrhizalspecies are symbiotic with dipterocarps.
Dipterocarps,like many othertropicaltrees (19), experiencehigh mortality
between anthesis and fruit set. Mortalityis extended throughoutthis period.
Many fruit with fully extended winglike sepals apparentlyripen, but the
carpels of most are empty. Dipterocarps (including saplings) flush in-
termittentlyonce or twice a year. In most species inflorescences are both
terminaland axillary, andthey arise from the same positions from which leafy
shoots would otherwise originate. Ashton (12) has therefore suggested that
the retentionof sterile fruits until full sepal extension may serve to increase
photosyntheticarea at a time when energy demandis high but when the tree
lacks its usual leaf complement. It would be interestingto observe whether
fruit survivorshipis the same among species with winglike fruit sepals as
among those many species in which all the sepals remain short. Whether
dipterocarp sepals function in the way suggested or not, it seems that
the and west of SulawesiandBali.
2Malaysia, Philippines Indonesia

6. BIOLOGY
DIPTEROCARP 351
reproductionimposes severe stress on the trees. Primack& Chai (47) showed
that dipterocarpsexperience dramaticarrestment growth in fruitingyears.
of
Dipterocarpsand tropicalFagaceae also sharereproductivecharacteristics.
Both have poorly dispersed, one-seeded fruits. Both have seeds of high fat
content, lacking dormancy but nevertheless scatter-hoarded rodents. In
by
both, the seeds are chemically defended by phenolic compounds and are
enclosed in a woody pericarp,though the dipterocarp pericarpis also richly
furnishedwith resin-canals.In both, the seeds are predatedby beetles before
maturationand by vertebratesafterwards.Both are notorious for mast fruit-
ing, though this does not seem to occur in the same years in the two families.
This lack of fruiting synchrony between the families appears largely to
explain the celebratedmigrationsof hordes of wild boars (Sus barbatus) in
Borneo, and formerlypresumablyelsewhere in WesternMalesia (22). Janzen
(40) suggested that mast fruitingof the whole family is an adaptation avoid
to
predation, among poorly defended seeds, by means of predator satiation.
Ashton et al (13) have provided furtherstrong inferentialevidence for this
hypothesis. Dipterocarpsflower annuallyin the seasonal, and at intervalsof
about five years in the aseasonal, tropics. They found a correlationbetween
dipterocarpflowering years and El Ninloclimatic events, during which pro-
longed depression of minimum night temperature correlatedwith onset of
is
inflorescence development. Unusually heavy fruiting occurs at similar in-
tervals in the seasonal tropicsalso, and intensityof seed predationis very high
there except in these years. Ashton et al (13), elaboratingon Janzen (40),
nevertheless suggest that the seed predatorsatiationachieved by intensifica-
tion of the mast fruiting event is occasioned in the aseasonal tropics by the
absence of an annual climatic cue to flowering. This may partially explain
how the Dipterocarps, as putative invaders, have become so successful
there.
The period between germinationand establishmentis critical for diptero-
carps. The invasion of mycorrhiza has never been studied. Ashton (10)
speculatedthatthe rarityof fruitingamong tropicalforest basidiomycetesmay
be associated with poor spore dispersal and low spore dormancy in the
windless moist and equable rain forest understory.If this is so, and fungal
host specificity is variablebut high in some cases (e.g., see 36), fungal spread
may be principally through the soil, and this could partially explain why
dipterocarpsare clustered in forests at both species, genus, and also family
level (Figure 1; see also 6, 10).
Dipterocarp seedlings once established are generally able to survive in
shade, and many persist in the absence of canopy gaps between mast fruiting
years (28). This capacity appears to be limited by water stress (20). In
seasonal evergreen dipterocarpforests, few seedlings of dipterocarpsor of
other mature-phasetree species survive beyond a year on this account (see

7. 352 ASHTON
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8. DIPTEROCARPBIOLOGY 353
references in Ashton-12). This indicates a fundamentaldifference in the
dynamics of rain forests under seasonal and aseasonal climates. In the
aseasonal rain forest, gap regenerationis dominatedby establishedregenera-
tion except where the soil is disturbed,whereas pioneers play a more promi-
nent role in seasonal forests. There, dipterocarpsestablish through later
invasion, following a mast year and the establishmentof a pioneer canopy.
Predationon Dipterocarpseedlings is surprisinglylow in comparisonwith
that on the seeds, though fungal pathogens may be an importantsource of
mortality(20). Low seedling and saplingpredationmay be associatedwith the
presence, possibly ubiquitous, of sugar-secretingglands on the upper, and
sometimes lower, leaf surface and stipules (29). These may attractants that
may defend the seedling againstpredators,thoughthis has not been observed.
With the exception of Dry Dipterocarpforest species, dipterocarp trees are
remarkablefor their poor capacity for reiteration(12, 35), a charactershared
by many gymnosperms. As seedlings, they have high capacity for shoot
reiteration, often through formation of accessory buds (45). By maturity,
however, the capacityto form new branchesby reiteration becomes limited or
lost. Also, whereas seedlings and saplings respond to increased light with
increasedgrowth, this capacity apparentlydeclines as the plant matures(48).
Thus, the need for self-repairapparentlydeclines as maturityis approached,
and selection may thus decline and even cease once the tree reaches full
height. Some evidence (e.g. 46) suggests that dipterocarpsare not unique
among trees of the maturephase in these respects, which, in view of their
of
implicationsfor understanding floristic and dynamicstructure the forest
the
and the relationshipbetween ontogenesis and changing selection pressures,
merit more study.
In summary, Dipterocarpoideaeare seen to be as ecologically distinct a
group in Asian tropicallowland evergreenforests as they are systematically.
Their usually dominantpresence distinguishesthese forests from those else-
where. Their success as a family there is apparentlydue to their ability to
maintainabundantseedling populationsthrough satiation of seed predators,
possibly through offering rewards to predatorsof seedling herbivores, and
through their ability to enhance nutritionon leached infertile soils by sym-
biosis with ectotrophic mycorrhizae. Dipterocarpsappearto trade abundant
photosynthatefrom theiroften emergentcrowns for cheap defense and superi-
or nutrient acquisition in a limiting environment. Analysis of the cladistic
relationshipsof Dipterocarpaceae with other families has not been attempted
but would be valuable. Ashton (10) argues for including them in Malvales.
Thoughmany Malvales are emergentforest trees with the same architecture as
emergent dipterocarptaxa (which is unusual and perhaps unique; this is
discussed below), none share the family dominance and gregarioushabits of
dipterocarpoids.None have seed and seed dispersal characteristicsof dip-

9. 354 ASHTON
terocarpoids, though some have small winged seeds. Some, such as many
Durio (Bombacaceae), may fruit supraannually the wild, but fecundity is
in
low in these large-seededtrees. Rather, it is the Fagaceae, a quite unrelated
family within the angiospermsand one with a quite separatebiogeographic
history (49, 43, 10), with which dipterocarpoidshave most in common
ecologically and with whom, it seems, they survive at a familial level through
competitive exclusion. Here, then, is evidence that taxa of familial rankmay
manifest niche specificity within the teaming forests of the humid tropics.
Genera and Sections
Though only 13 genera exist among Dipterocarpoideae,the large genera
Shorea andHopea are divisible into 14 sections and4 subsections,respective-
ly, and Anisoptera is divisible into 2. These sections approachthe status of
genera on structuralgrounds in some cases, but others distinguishedon the
same set of character-statesare less distinct. This is the reason for their
infrageneric status (4). All these taxa represent natural groups within the
family. Each containsan averageof 16 species, thoughthereis much variabil-
ity in size, and 5 are monotypic.
These entities, which for simplicity I shall term quasigenera, possess
distinct geographical distributions. In some, such as Vateria, Vateriopsis,
Stemonoporusand Shorea section Doona, which are endemic to southern
India and some Indian ocean islands, their distributionmay be explained by
historicalbiogeographyalone. The ranges of one section (and of anotherbut
for three species) of Hopea, of Cotylelobium,and of Shorea sections Riche-
tioides, Ovales, BrachypteraeandMutica co-terminateat the sharpboundary
between aseasonal and seasonal climates at the Isthmus of Kra in southern
peninsular Thailand (See 58). All are confined everywhere to aseasonal
climates. Upuna and threefurthersections of Shorea are confined to aseason-
al regions in the Far East west of Wallace's line. In all, this concentrationof
quasigeneragreatly enhances dipterocarpspecies richness in this zone. The
reasons for these coincident distributionswithin a continental area must be
ecological but remain entirely unknown.
Quasigeneradiffer consistently in one or more characters,including wood
and often bark anatomy, leaf venation, and in certainaspects of morphology
for which there are not obvious adaptiveexplanations.Each quasigenushas a
distinct architecturalform, afterthe models of Halle & Oldeman(34, 35), and
these models in some measurepredicatethe size at maturityof its members.
Thus Vatica, and apparently also Cotylelobium, early become sympodial
throughterminalflowering of the plagiotropicbranchesand orthotropiclead-
er. Both nevertheless continue to extend through apposition. Both genera
apparentlyconform to Kwan-Koriba'smodel. Most Vatica are understorey
trees, whereas Cotylelobiumoccupies the main canopy at reproductivematu-

10. DIPTEROCARPBIOLOGY 355
rity. In Stemonoporus,many species conform to the model of Troll, whereby
the leader is initially vertical with spirally arrangedleaves but eventually
bends over, apparently under its own weight, when the leaves become
distichous. Height is furtherextended through axillary shoots, which arise
from the upper side of the leaning or horizontalstem and these successively
repeat the process. Stemonoporusspecies rarely exceed 15 m in height.
Most emergent dipterocarpoid quasigenerahave a distinctive growth pat-
tern that appearsto be sharedby otheremergentMalvaliantrees including, in
Asia, Sterculia, Pterygota (Sterculiaceae), Pentace (Tiliaceae), and
Durioneae (Bombacaceae). Apical dominance is maintained in juveniles,
when the leader bears spiral leaves while branches are plagiotropic with
distichous leaves. At this stage, the branches may be arrangedin pseudo-
whorls correspondingwith each extension of the leader (Massart'smodel), or
they may be arrangedin an irregularspiral (Roux' model). These forms
permit leaf surfaces to be spread diffusely in a horizontalplane, which will
maximize capture of sunflecks, but in such a mannerthat height increment
and the mechanicalstrengthof the eventual tree are not compromised.When
the stem apex emerges above the canopy, the subsequentbranchesof all but a
few species are orthotropic,with ascending trajectoryand spiral leaves, and
they are thus equivalent to the leader itself (33a). At that time, flowering
commences from terminalas well as axillary inflorescencesso that branching
becomes partially sympodial. Surprisingly, neither Vatica, Stemonoporus,
Vateria or Vateriopsis, (regardedas the most primitive genera in the sub-
family) nor African and South American dipterocarpshave this malvalian
characteristic.
Quasigenera frequently possess distinct embryology. Shorea section
Doona, for instance, differs consistentlyfrom others in possessing one fleshy
cotyledon that remains hidden in the pericarpand is a food store, and one
minute, bract-like photosynthesizing cotyledon that emerges with the plu-
mule. But there is much variabilityin the morphologyof the matureembryo
within some quasigenera,particularly Vatica and Stemonoporus
in (44), some
of which may be adaptive at the species level and none of which has so far
been shown to correlate with other, independent, characters of the tree.
Similarly, the presence or absence of extended wing-like sepals has for long
been given supraspecifictaxonomic recognition, but it is uncorrelatedwith
variation in independentcharacterstates and is therefore generally only of
value in species delimitation (4, 10).
There is some, still far too limited, evidence that quasigeneramay con-
sistently differ in some of the complex secondaryhydrocarbonswith which
dipterocarp exudatesare richly endowed [for referencessee Hegnauerin (10),
pp. 273-74]. If correct, some herbivores and seed predatorsmay be host-
specific at a quasigeneric level. Evidence for this latter has recently been

11. 356 ASHTON
found in some genera, such as Dipterocarpus, but not in most sections of
Shorea (R. Toy, personalcommunication,L. Curran,in preparation).Some
specificity at this level may occur between dipterocarp species and mycorrhi-
za, which may invade in response to the specific cue of chemical root
exudations (36). Both interdependencies,if they do exist, could help explain
how related quasigeneracan co-occur in rain forest communities, through
density-dependenteffects on the one hand (40, 27) and through resource
allocation (56, 57) on the other.
Much is known of pollinationbiology and the functionalmorphologyof the
flower, thanksto the work of S. Appanahand H. T. Chan (2, 3, 24-26) and
C. V. S. and I. A. U. N. Gunatilleke(personalcommunication,in prepara-
tion). One majorgroup of dipterocarpoid quasigenerasharesfloral characters
that include large elongate yellow antherswith short appendages,a columnar
style that protrudes beyond them, and white petals that open widely at
anthesis. These include Dryobalanops, Vateria, Neobalanocarpus, Shorea
section Doona, Stemonoporus,Parashorea, Cotylelobium,and Vateriopsis.
This is not a monophyleticgroup;the membersareequally represented both in
tribes of the subfamily. The flowers as well as other parts differ in other,
taxonomicallyimportant,characters.The flowers in this groupare diurnaland
last one day. The first four are known to be principallyvisited by honey bees.
The patternof visitation each morningis ostensibly the same in all cases (S.
Appanah, personalcommunication;C. V. S. and I. A. U. N. Gunatilleke,in
preparation),with the large Apis dorsata, and smallerA. indica var. cerrana
visiting first and then meloponid bees and various other insects. I have seen
Stemonoporusbeing visited by Trigonabees (Meliponidae),while no visitors
have been observed on flowers of the others. With the exception of Ceylon
quasigeneraShorea section Doona and Stemonoporus,species in these taxa
contain few species. With the exception of species in section Doona, two or
more congeners rarelyco-occur in the same communitytype. These taxa also
often flower more frequently between mast fruiting seasons than do other
dipterocarps.
Dipterocarpus has large flowers whose large yellow anthers with long
appendages and columnarstyle are enclosed in large pink and white petals.
The pollinatorsremainunknown. All other quasigenerapossess small cream-
colored anthers in flowers in which the petals, which are white, pink, or
yellow, remain strongly contorted at the base, thereby concealing the an-
droecium and gynoecium. These flowers differ at a quasigenericlevel in the
shape of the stamens and of the connectival appendage, in the length of the
style, and in the presence or absence of a stylopodium.
Appanah & Chan (24, 26, 3) made a thorough study of six species in
Shorea section Mutica. One tree can produce four million flowers over two
weeks. They found thatthe flowers open at night and last less thana day. The

12. BIOLOGY
DIPTEROCARP 357
corollas fall with stamensattached.Thripslay theireggs between the petals in
the young bud. The juveniles feed on the petals. Up to five hundredadult
thrips can thereby originate from one female over three generations, or four
thousandjuveniles over four, by which time anthesis begins. The thrips get
trapped during anthesis by the phalanx of connectival appendages, which
bend back against the corolla so that the anthers open inside a chamber
therebycreated. There, the adultsfeed on pollen grains. Many escape though,
carryingone or a few of the sticky grains. Many others, includingjuveniles,
remain with the connate corolla and stamens, which together gyrate to the
forest floor at terminationof anthesis. Juveniles can survive on the fallen
corollas for up to five days, and pollen can remain viable for this period.
Adults, some bearing pollen, fly up to the canopy each evening.
Trees in this section are outbreeders(25). Thrips can be transportedon
convectional wind eddies, but they land directionally.No other floral visitors
have been observed during exhaustive studies in this section. Shorea section
Mutica, whose abundantindividualsproduce millions of flowers briefly and
at long intervals, achieve pollinationthroughattraction a fecund short-lived
of
pollinatorwhich rapidlybuilds up numbersby feeding on the buds of the trees
that they will pollinate. Thrips are almost ubiquitousin dipterocarpoid flow-
ers. Among other pale antheredquasigenera,Appanah(in Ashton et al, 13)
found bugs (Hemiptera:Miridae:Decomia) to visit flowers of Shorea pauci-
flora (section Brachypterae)and plant hoppers (Homoptera:Cicadellidae:
Varicopsella)to visit S. seminis (section, subsectionShorea). Neitherof these
quasigenerais abundantly represented,by individualsor species, in individual
forest community types. In particular,studies of pollination are needed in
Shorea section Richetioides (which, after section Mutica, can occur in the
largest co-occurring species series) and in Dipterocarpus.
It does seem likely that competition for the services of pollinators is
avoided at a quasigeneric level. Pollinator specialization cannot, however,
provide a density controlling factor for the population of these taxa, and
cannot thereby explain their co-existence let alone the co-existence of the
multitudeof othergenera in the mixed rain forests. In the searchfor equilibri-
um explanations for these coexistences we likely need to know more about
patternsof predatorand herbivorespecificity at this taxonomic level, and the
role of specific secondary metabolites.
The Species
With 12 genera and 470 species, Asian dipterocarpsserve as an example of
those several families with large genera, the ecological range of whose
members is the principal determinantof variationin the species richness of
tropicalevergreenforest (15). We have seen that the species of Dipterocarp-
oideae are naturally grouped into some 30 generic and infrageneric

13. 358 ASHTON
taxa. Many of these species, as will be explained, are subdividedinto distinct
geographicalor edaphic subspecies.
Within quasigenera,habit is relatively uniform, though maximum stature
can vary between and within species, according to site conditions, and in
particular accordingto soil depth (e.g. 5). Stemonoporusand Vatica can vary
substantiallyin habit, even among taxa closely related on the evidence of
othercharacters.In Hopea several species in two, and most species in a third,
of the four infrageneric groups are understorey trees. They differ from canopy
members in coming into flower while still in the monopodialjuvenile habit,
which they maintainthroughoutlife. Flowers in these neotenous species are
borne solely on the plagiotropiclateralbranches.In Stemonoporusand Vatica
the differencein habitbetween understorey canopy species is less extreme
and
owing to the sympodial habit of the understoreyspecies, but distichous leaf
arrangements not give way to whorled in the understoreyspecies. In these
do
two genera neoteny can even differentiateinfraspecifictaxa. Vatica oblongi-
folia Hook. f., for instance, consists of four ecotypically differentiatedsub-
species, two of which first flower only after reaching the canopy while two
are understorey taxa. Stemonoporus canaliculatusThw. occurs as two distinct
subspecies, sometimes recognized as species; one is a canopy tree of high
ridges, the other an understoreyshrub of lowland yellow podsolic soils. In
Shorea too, subtle differences of habit can occur among subspecies. For
example, S. macropteraDyer has one subspecies macropterifoliathat is an
emergent tree while its other three subspecies do not emerge above the main
forest canopy.
In some quasigenera, notably Shorea section Richetioides and Vatica, a
correlationbetween habit and fruit charactersoccurs (6, 10). In these and
most other groups a significantminorityof species lack winglike fruit sepals.
This wingless characterhas evolved many times, at least within the more
advanced quasigenera, and pairs of taxa differing only in this respect are
frequent. In Shorea section Richetioides most emergentspecies have winged
fruit, while all main canopy species have wingless (6). This implies that
winglike sepals actually reduce survivorshipin species that drop their fruit
directly into the windless subcanopy. This is quite likely, as the wings can
entangle fruits among the boughs as they fall. Often, wingless fruitedspecies
possess largerthan averageseed size, and the largestdipterocarp seeds are all
wingless. These characters,also sometimes correlatedwith distinctcharacters
of the embryo (44), may be adaptive, enhancing successful regenerationin
shady habitats. Wingless fruit also occur among riparian, water dispersed
species.
The overwhelmingsourceof differencesamong species, however, is in leaf
morphology, principally in leaf shape, size, and number of veins, and in
presence or absence, length, and color of indumentum.Among canopy and

14. DIPTEROCARPBIOLOGY 359
emergentspecies these differences become most manifestat maturity,though
enough are possessed by the seedling for all ontogeneticstages to be generally
identifiable. Seedlings of related taxa are, however, generally more similar
thanare reproductively matureindividuals;this implies thatselection for these
charactersmay continue up to maturityand that it may shift duringontogeny
from mainly intra- to mainly interspecific (6). These leaf charactersmay be
expected to influence waterrelations,but comparativestudiesof the physiolo-
gy of closely related species which differ in these respects are still awaited.
Large-leafed species generally also possess large buds, flowers and fruit,
and stout twigs. Frequently,taxa differ in leaf and twig size, and occasionally
numberof nerves, but sharedistinctiveleaf and twig shape, indumentum,and
other characters in common; these are almost always allopatric and are
recognized as subspecies (10). Occasionally, as in Shorea macroptera (6)
subspecies may co-occur. In this example, no morphologicallyintermediate
individualsoccur in the mixed stands, suggesting that even when distinctions
are reduced to this level of subtlety the taxa may function as biological
species.
A remarkablefeatureof most dipterocarp taxa, therefore, is their morpho-
logical constancy. This enables clear and consistent differentiationbetween
entities that may differ only, for instance, in leaf size and twig diameter,but
which are generally also geographically or ecologically allopatric. Notable
exceptions are those few species confined to the savannawoodlands, called
Dry Dipterocarpforests, of continentalAsia, and to a lesser extent also some
species of Seasonal EvergreenDipterocarpforests. Here variation,particular-
ly in the presence, distribution,and density of indumentum,is common both
within and between populations (10). Populations in the driest habitats are
generally the most tomentose.
In Shorea section Pachycarpa, with ten species all endemic to the con-
tinental island of Borneo, populationsmorphologicallyintermediatebetween
two species occur in at least one locality in every species, though all co-occur
with othersthroughout much of theirrangewithoutthis evidence of hybridiza-
tion. The high uniformity within these hybrid populations is curious. An
experiment, not completed, suggested that hybridizationis possible (24).
Evidence for interspecifichybridizationis otherwise scanty among diptero-
carps (see review and reference in 10). It is most abundantbetween related
species occurringin different, adjacenthabitats.Most examples are between
species of Seasonal Evergreenand Dry Dipterocarpforests, suggesting recent
differentiation.However, these taxa are usually strikinglydifferent in habit:
those of the latter forest type are short trees frequently with larger, more
in
coriaceous leaves, more diffusely arranged the crown;they also have thick,
fissured bark. Occasionally, species of the Mixed Dipterocarpforests of the
aseasonal Far East occupying different edaphic ranges may hybridize in

15. 360 ASHTON
the ecotone between. The best known case is thatof Shorea leprosula Miq. of
clay lowland soils and Shorea curtisii Dyer ex Brandisof xeric ridges, both in
section Mutica. Groupsof individualsmorphologicallyintermediatebetween
these species are known from several localities in peninsularMalaysia. In one
standthe inferenceof hybridorigin was supportedby isozymal evidence; one
fruiting hybrid individual there failed to set viable seed (30). Jong (in 10)
provided evidence for hybrid origin of the tetraploid apomict S. ovalis
(Korth.) Bl., a species monotypic to its section yet occurringas three geogra-
phic subspecies. All evidence presently available to us, which is morpholo-
gical and isozymal, suggests that speciation among dipterocarpsis as a rule
geographically or at least ecologically allopatric (6, 7, 10, 11, 12a).
Biogeographical and paleontological evidence implies that dipterocarps
have been invadingthe Far East from the west since the late Eocene. Several
species endemic to the aseasonal southwestof Sri Lankaappearmost closely
allied to taxa in the Far East. Curiously,the FarEasterntaxon in nearlyevery
case occurs entirelyor principallyin SeasonalEvergreenforest. It is no longer
possible, on biogeographicgroundsto determinewhetherdipterocarps origi-
nally arrivedin Asia, which they almost certainlydid by way of the Deccan
plate (10, 17), principallyor entirely as denizens of the mixed forests of the
aseasonalzone, althoughthe threearchaicgeneraendemic to the Deccan plate
and Seychelles are thus restricted.Many taxa of the aseasonal Far East also
have seasonal evergreen forest vicariants. If dipterocarpsinvaded from the
west, the original Asian dipterocarpsmay have been aseasonal mixed rain
forest species. They likely speciated into Seasonal Evergreen forests and
spread eastwards. Some of these furtherspeciated into the aseasonal mixed
forests of the Far East, formingthe progenitorsof the rich modem flora there
(7). Independent flora of the
evidence for a seasonal origin for the dipterocarp
aseasonalFarEast is providedby the climatic cue for flowering which appears
to be a drop in minimum night temperature.This occurs annually in the
seasonal tropics, but at several year intervals in the aseasonal tropics (13).
The Dry Dipterocarpforest species all seem to be of recent origin, however,
as already implied by the evidence of hybridization.
Wallace's Line, and the Torres Straits, have been formidablebarriersto
dipterocarp spread.Only one species is sharedbetween Borneo and Sulawesi,
and that is a riparian species with water-dispersedfruit. Dipterocarpsare
unknown in Australiaand the BismarkArchipelago. A strikingexception to
these restricted patterns is provided by Vatica odorata (Griff.) Sym. ssp
mindanensis(Foxw.) Ashton of the Philippinesand northernBorneo, which
also occurs in Hainanand Kwangsi; and by Shoreafalciferoides Foxw. of the
Philippines which is closely allied to a species so far known only from the
Quilon peninsula of coastal central Vietnam, S. falcata Vidal. The most
parsimoniousexplanationfor such distributions,in the absence of a common
continental shelf between the two regions is typhoon dispersal, westward.

16. BIOLOGY
DIPTEROCARP 361
Within the aseasonal Far East, west of Wallace's Line, dipterocarps mani-
fest three patternsof distribution: widespread,which includes several that
(a)
also occur in the Seasonal EvergreenDipterocarpforests of Indo-Burma;(b)
island-wide endemics; and (c) local endemics and disjuncts (7). The third
category includes most taxa with wingless fruits, whereas only one wing-
less fruited species, Shorea multiflora(Buck) Sym. falls into the first cate-
gory.
Local endemism is unusual among dipterocarpsin the forests of the Far
East. Hopea contains a number of local endemic and disjunct, apparently
relict distributionsin the Seasonal Evergreenforest isolates of Indo-Chinaand
southernIndia. In Sri Lanka, Stemonoporusis unique in the subfamily in the
absence of any widespreadtaxa. All species are more or less local endemics,
sometimes apparentlyvicarious on the main mountainmasses but sometimes
also geographically, though not apparentlyecologically, sympatric(17). To
this we return, but I now wish to addressa more general fact-that the vast
majority of dipterocarpswithin the aseasonal zones are restricted in their
distributionby their edaphic and altitudinalranges.
Dipterocarpspecies, particularly the aseasonaltropics, almost invariably
in
occupy narrowrangesof soil fertility. Distributionis correlatedwith a number
of soil factors, but primarilywith magnesiumand then phosphorus(18, 14).
Widespreadspecies are confined to widespreadsoils, endemics and disjuncts
to soils (and rock substrates)themselves of local and disjunct distribution
(10). No dipterocarpsare endemic to limestone or to ultramaphicrocks,
though morphologicallydistinct forms occur, particularlyon the latter (10).
Within a landscape on uniform rock, soil fertility is often correlated with
physiography, and congeneric and consectional species may occupy distinct
but overlappingranges along the catena (5). Ashton et al (15) have shown that
dipterocarpspecies richness within individualhabitatsincreaseslinearly with
total mineral soil magnesium concentrationup to a thresholdof about 1300
ppm Mg, above which close correlationis lost and there is a sudden drop in
species richness. In Borneo this represents a range of between 9 and 57
dipterocarp species in samples of 1000 trees. Species richness of Mixed
Dipterocarpforest is also, and independently,correlatedwith within-sample
variation in soil fertility up to but not above the same total magnesium
threshold, a point to which I return.Above the fertilitythreshold,dipterocarp
species richness continues to fall erraticallyas soil Mg and P values increase,
but the abundanceof the commonest species markedly increases. The soil
specificity and center of edaphic distributionof dipterocarpsis compatible
with their apparentlyobligate association with ectotrophic mycorrhiza, and
with the possibility that some of these symbiontsmay be host specific (54, 55,
personalcommunication).This implies that some secondary, species-specific
metabolites may act as cues to mycorrhizalinfestation. The patternof tree
species-richness overall in Mixed Dipterocarpforests is similar to that of

17. 362 ASHTON
the dipterocarpsthemselves, even though some large individual families
includingMeliaceae, Rubiaceae, and Euphorbiaceae most species-richon
are
the most mesic fertile sites (5).
Up to 11 consectional Shorea species are known to co-occur in a single
forest (Figure 1). An understandingof how these populations of closely
related taxa are maintainedin mixture is central to our understanding the of
maintenanceof species richness in rain forests, while their cladistic rela-
tionships may throw light on the evolution of species richness (51 and in
press). Cladisticand genetic analyses of interspecificrelationsbetween clades
are still awaited. Ecological andbiogeographicpatternsamongsmallergroups
of species, closely related on morphological and isozymal evidence, and
among interspecific taxa suggest that speciation is generally but perhapsnot
always geographically or ecologically allopatric (6, 11, 31).
Host specific mycorrhizacould allow differentialexploitationof soil nutri-
ent resources among consectional species. The ecological distributionof the
ten species of Shorea section Doona in Sri Lankacan be distinguishedalong
gradientsof altitude, soil fertility (and the catena), and light response (Figure
2). Shorea trapezifolia(Thw.) Ashton and S. congestiflora (Thw.) Ashton in
section Doona, Shorea leprosula (section Mutica) and Dryobalanops lan-
Figure 2 Schematic ecological range, in the wet zone forests of Sri Lanka, of the species of
Shorea, section Doona. Key: a. Shorea cordifolia b. S. gardneri c. S. zeylanica d. S.
affinis e. S. trapezifolia f. S. megistophylla g. S. disticha h. S.worthingtonii i. S. con-
gestiflora

18. DIPTEROCARPBIOLOGY 363
ceolata Burck differ from other membersof their groups in their exceptional
maximal growth rates (16; P.M.S. Ashton, in preparation).Differences in
shade tolerancesamong membersof quasigeneraare well known to foresters.
All dipterocarpsappear, remarkably,to have low compensationpoints, and
the differences appear to be principally attributable differences in max-
to
imum rates of photosynthesis(A. Moad, in preparation) to rates of dark
and
respiration(R. A. Sunderland,unpublished).
The species with highest maximum growth rates overall are confined to
fertile soils. Dryobalanops lanceolata and also Parashorea malaanonan
(Blco) Merrill are unusual in combining high maximum growth rates with
high shade tolerance as juveniles (A. Moad, personal communication). P.
Hall (personalcommunication)has found evidence from Borneo that seedling
survivorshipof D. lanceolata is greaterin the deep shade of Mixed Diptero-
carp forest on basalt-derivedsoil than is that of D. beccarii in the relatively
open light conditions within forests on infertile leached yellow sands. The
ratios of adults to saplings to seedlings, estimated in the same year, were
1:5.2:87.6 in D. lanceolata, 1:1.5:46.3 in D. beccarii. High juvenile
survivorshipin shade has also been noticed among some Shorea species on
volcanic soils in the Philippines (21). It is possible, where available soil
nitrogenis high, thatspecies can maintainhigh enough photosyntheticrates in
shade to compensate for night respiration rates (F. A. Bazzaz, personal
communication).This may explain why D. lanceolata is amongthe dominant
species on fertile soils and, conversely, why species dominance is un-
correlatedwith species richness throughoutmost of the range of less fertile
soils.
In most quasigenera, floral morphology is ostensibly identical among
closely related species. Chan & Appanah (26) have shown that six co-
occurring members of Shorea section Mutica in Pasoh forest, peninsular
Malaysia, flower in overlappingsequence. Casual observationssuggest that
this happens in other dipterocarpgroups where several species co-occur (6).
Chan & Appanah found that flowering is very highly synchronizedwithin
populations, but that the individualsof each succeeding species flower over
successively longer periods. Interestingly,the two-andone-half monthperiod
over which all six species flowered is no longer than the flowering period of
Shorea robusta Roxb., a single species dominantof Dry Dipterocarpforests
in northeastIndiain which individualsare more loosely synchronized.Ashton
et al (13) demonstrated that this sequentialflowering is significantly nonran-
dom. Dipterocarps,includingthe species in question, are outbreeders(25; C.
V. S. and I. A. U. N. Guantillekein preparation).Sequentialflowering has
presumablyarisenas a consequence of lower fruit set duringtimes of flower-
ing overlap among species sharing a common pollinator. Co-occurrenceof
such species series is only possible where barriers hybridizationare strong.
to
Extrapolatingback on the basis of the length and time of the flowering

19. 364 ASHTON
periods of the six species, Ashton et al identifiedthe time when inflorescence
developmentprobablybegan. They discovered that over a 20-year period, a
20 C depressionof minimumnight temperature occurredfor several days at
this same intervalbefore each general flowering, and at no other time. They
furtherfound a broad correlationbetween mass flowering seasons in east-
facing lowlands and the El Nifio climatic event over the Pacific, when the
aseasonal region of the Far East may experience prolonged periods of clear
skies and relatively low humidity, hence lower than average night tempera-
tures. This can explain why dipterocarpsflower annually in the seasonal
tropics, and why mast fruiting has not been observed in the Mixed Diptero-
carp forests of Sri Lanka, outside the main zone of El Nifio influence. The
evidence of Ashton et al also implies that dipterocarpswere preadaptedby
their flowering physiology to become mast fruiters when they invaded the
aseasonallowlands of the Far East. It is remarkable the fruitingof the six
that
species thatflowered in close sequencecoincide with one anotherand with the
other species in the family. This lends furtherstrong supportto Janzen's (40)
hypothesis and suggests that new species immigrantsto a forest community
become entrained through selective predation to fruit synchronously with
those already established (13).
In Semengoh forest, Sarawak,East Malaysia, 11 species of Shorea section
Mutica co-occur (Table 1). Some of the most abundant species are common to
both Semengoh and Pasoh, which is 1000 km to the west across the South
China Sea. S. acuminata Dyer and S. quadrinervis Sloot. are vicariants.
Otherssuch as S. dasyphyllaFoxw. and S. parvifolia Dyer change their rank
order. We believe this is due to edaphic differences between the two sites,
because the most abundant species areremarkably constanton the same soil in
different localities in northwest Borneo (15). The difference in number of
consectionalspecies at the two localities is attributableentirelyto the different
numberof species of low populationdensity in each, and this has also been
found to explain differences in forest species richness overall (15). Un-
derstandinghow species of low populationdensity (which comprise the vast
majority in species rich forests) are maintainedis crucial to understanding
how species richness is maintainedoverall.
Some of the rare species in the Semengoh example, such as S. slootenii
Wood ex Ashton, S. rugosa Heim andS. hemsleyanaBrandisseem always to
be rare, occurringas scatteredindividualsor clumps. Othersclearly arenot. A
comparisonof the populationsof Shorea dasyphyllain Pasoh and Semengoh
forests provides one example (Table 1). Some species such as Dipterocarpus
gracilis Bl. and Anisoptera costata (Korth)Bl. are abundantin the seasonal
partsof theirrangebut generallyoccur as far scatteredindividualsin aseason-
al Borneo. Low populationdensity may reducefecundityin outbreedingtrees,
and Chan (3) found evidence of this in the low density population of S.

20. DIPTEROCARPBIOLOGY 365
Table 1 Relative densities of reproductiveindividualsShorea, sec-
tion Mutica, species in two lowland mixed Dipterocarpforests
Pasoh Research Forest Semengoh Forest
PeninsularMalaysia Sarawak
Shorea acuminata 3.0/ha Shorea quadrinervis 4.7/ha
Shorea parvifolia 2.8/ha Shorea scabrida 4.7/ha
Shorea leprosula 2.7/ha Shorea macroptera 4.6/ha
Shorea macroptera 2.4/ha Shorea dasyphylla 3.5/ha
Shorea lepidota 1.6/ha Shorea slootenii 1.1/ha
Shorea ovata 1.0/ha
Shorea parvifolia 0.9/ha
Shorea dasyphylla 0.3/ha Shorea leprosula 0.2/ha
Shorea rubra 0.1/ha
Shorea rugosa 0.1/ha
Shorea hemsleyana 0.07/ha
dasyphylla and in isolated individuals of S. leprosula. Chan (24, 11) found S.
dasyphylla to be the only species, among the 6 members of Shorea section
Mutica whose flowering phenology he studied, whose flowering period en-
tirely overlapped with those of other species. It strains the imagination to
believe that all 11 species at Semengoh possess distinct flowering times. The
phenology and reproductive biology of the rarer species there would repay
study.
It is curious that dipterocarps of the seasonal tropics tend to have higher
pollen/ovule ratios than do those of the aseasonal tropics (9). In those few
quasigenera where staminal number varies, such as Shorea sections Shorea
and Anthoshorea, and most particularly in some understory Hopea and Stemo-
noporus, a broad albeit inconsistent tendency exists for local endemics to
have reduced number of stamens. In Stemonoporus this is accompanied also
by a reduction of the number of ovules to four. It is unknown whether this
trend is accompanied by increases in self-compatibility.
These patterns would appear to be consistent with the presence-to a still
largely unknown extent but on inferential evidence probable in many
genera-of apomixis through pseudogamous agamospermy (41, 42). Aga-
mospermy has been confirmed in one species of Hopea and two of Shorea and
inferred, on the presence of polyembryony or triploidy, in some 10 of 70
species examined cytologically to date. Apomixis has been found or inferred
in abundant and widespread species such as S. ovalis and S. macroptera, but
also in one local endemic of specialized habitat (Hopea subalata Sym.), in
one gregarious species of river banks (H. odorata Roxb.), and another of Dry

21. 366 ASHTON
Dipterocarpforest (DipterocarpustuberculatusRoxb.). S. macropterais the
first of its section to come into flower. Pollinatornumbersmay not yet be at
full strengththen, and it may be supposedthatS. macropteraexperiencesthe
greatest vicissitudes in securing successful cross-pollination. Certainly,
adventive embryony provides a means of maintaining fecundity without
reductionof heterozygosity,but a cost is paid in reducedgenetic variabilityat
population level.
The proportion of seeds with multiple and perhaps apomictic embryos
varies between species, and in some cases between individuals within pop-
ulations (24, 13). Isozymal analysis of population samples of reproductive
individualsof Shorea species indicatesthat genetic variabilitycan be remark-
ably high (31, 32, D. Buckley in preparation).In a population sample of
almost matureindividualsof S. trapeziflora,variationconformedwith Hardy-
Weinberg expectations. Whether this is due to outcrossing patterns or to
differential mortality between the progeny of selfed and cross-pollinated
flowers is so far unknown (D. Buckley in preparation).Gan (30) and Chan
(25) had evidence that isozymal and leaf morphological variation within
populationsamples of S. leprosula increases with distancesbetween trees up
to 200 m, and wi-thin-population genetic variability can exceed variability
between populations 100 km distant. On the other hand Gan (30) found
sibling seedlings to be isozymally uniformin the apomicticS. ovalis, though
thi'sresult could have been an artifactof tetraploidy.
CONCLUSION
I arguethattoo much emphasishas been put on the species as an independent
unit to test for niche specificity in species-rich terrestrialplant communities
such as tropicalrain forests. I have endeavoredto show that dipterocarp taxa
in all levels in the systematic hierarchypossess charactertraits upon which
selection may be expected to act uniformlyin nature.Families, I suggest, are
in competition with one another. The Dipterocarpoideaeseem to com-
petitively exclude otherfamilies undercertainconditions, and not necessarily
those in which dipterocarp species richness is maximalmore thanunderother
conditions. Selection acts on different charactertraits at different taxonomic
levels. Hubbell & Foster's (39) search for significant positive or negative
association between the spatial and temporal patterns of different species
populations, and between them and habitatvariation,as a test of the equilibri-
um-hypothesis of communityspecies composition, shouldbe extendedto taxa
of higher rank (1 1). Association can be expected to be manifest also between
taxa of differingrank. So far though, the relationshipbetween phylogeny and
ecology is largely unknown above the species level of the taxonomic
hierarchy.

22. BIOLOGY
DIPTEROCARP 367
Patterns of species richness of Dipterocarpaceaeamong different forest
types sharing a common climate conform extraordinarily closely with Til-
man's (56, 57) equilibriummodel for the relationshipbetween species rich-
ness and resource availability. The data currently available, though in-
ferential, do suggest thatthe ecological range and performanceof dipterocarp
taxa are influencedby a wide arrayof climatic and edaphicvariables(see e.g.
18). In combination,it could easily be arguedthat every species in the mixed
rain forest may possess unique ecological requirements.But to furthersug-
gest, in the case of sessile organisms of large size like trees, that their
ecological differences have evolved as a consequence of direct interspecific
competitionis patentlyabsurd.The high local genetic variability,yet regional
uniformity, of morphologicaland genetic variationthat seems to characterize
some of the more abundantdipterocarps,such as Shorea leprosula and S.
trapezifolia, suggests that they are panmictic species in which (curiously, in
view of the biological heterogeneityof theirhabitats)selection acts unifornly
throughouttheir geographical ranges. W. Smits (personal communication)
has suggested that poor seed dispersal in combinationwith stringentrequire-
ments for maintainingspecies-specific mycorrhizalsymbionts could result in
the pattern of variation observed, and these might also explain the sharp
though subtle discontinuities in variation that differentiate taxa, and the
apparentrarity of hybridization. Some of the scant population genetic data
available and the existence of adventive embryony do cast doubt on natural
selection, that is, equilibriumphenomena, as the sole mediator of floristic
composition and species richness (15). Apomixis may serve to sustainfecund-
ity in populationsof low density and may also allow survivalof taxa occupy-
ing habitatfragmentsof limited area, where space precludesthe full species
complement of a community type and thereforewhere species composition
and abundances will be unpredictable and substantially determined by
stochastic processes of immigration and extinction. Selective processes
appear to determine maximum population density, but extinction in these
species-rich communities, stable over time, may essentially be a stochastic
process if fecunditycan be maintained(13). The key to understanding role
the
of selection in determining species composition will be the knowledge of
patterns of change in gene frequency which take place as sibling cohorts
develop to reproductivemat, rity.
I believe that the principles that govern the role of dipterocarpsin the
structureof tropical rain forests are general to all terrestrial
rainforestplants
and perhapsalso to epiphytes. Obviously, differences of detail exist between
taxa and guilds. Tree families known to be mycorrhizal,such as Myrtaceae,
show patternssimilar to those of dipterocarps(5). Agamospermyis known
among many of these also (12). Others such as Meliaceae and Sapindaceae
differ in both species distributionand breeding systems (11, 13). The role of

23. 368 ASHTON
non-equilibrium,island biogeographicprocesses may be expected to be larger
among epiphytes than terrestrial organisms. In every case, a fuller un-
derstandingcan only be reached by combining systematic with ecological
approaches.
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