A review of dipterocarps - Center for International Forestry Research
A review of dipterocarps - Center for International Forestry Research
A review of dipterocarps - Center for International Forestry Research
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Conservation <strong>of</strong> Genetic Resources in the dipterocarpaceae 48<br />
Table 3. Outcrossing rates <strong>of</strong> Dipterocarps.<br />
Species Outcrossing Rate tm<br />
(± standard error)<br />
tropical tree species (Murawski and Hamrick 1990,<br />
1991). Apparently, the low density <strong>of</strong> trees in logged<br />
stands reduces the inter-tree movement <strong>of</strong> pollinators<br />
and promotes self-pollination. Selfing, in part, may be<br />
aided by a weak self-incompatibility system.<br />
Kitamura et al. (1994) compared outcrossing rates<br />
<strong>of</strong> Dryobalanops aromatica in primary and secondary<br />
<strong>for</strong>ests but found no significant differences.<br />
Self-incompatibility as well as mating system studies<br />
suggest that <strong>dipterocarps</strong> are predominantly outcrossed.<br />
Outcrossing in large populations can allow populations<br />
to harbour considerable genetic variation. In <strong>dipterocarps</strong>,<br />
mass flowering is also likely to enhance outcrossing by<br />
allowing exchange <strong>of</strong> gametes among a very large number<br />
<strong>of</strong> individuals. Not surprisingly, there<strong>for</strong>e, populations<br />
<strong>of</strong> <strong>dipterocarps</strong> show considerable genetic variation (see<br />
below).<br />
Although the analysis <strong>of</strong> mating systems shows that<br />
the rates <strong>of</strong> outcrossing are high, it is also clear that there<br />
is a considerable potential <strong>for</strong> selfing in almost all species<br />
examined so far. Moreover, apomixis has been reported<br />
in several species (see below). While outcrossing<br />
continuously generates new genetic variation, potential<br />
<strong>for</strong> self-pollination and apomixis allows occasional new<br />
variants to spread in the population or colonise new sites,<br />
and thereby promote differentiation <strong>of</strong> taxa.<br />
Pollen and Seed Dispersal<br />
Pollen dispersal influences the mating system, and both<br />
pollen and seed dispersal affect population genetic<br />
structure. Limited dispersal results in inbreeding, small<br />
effective population sizes, and a high level <strong>of</strong><br />
differentiation among populations. Extensive dispersal<br />
has the opposite effect.<br />
Outcrossing in <strong>dipterocarps</strong> is achieved through a<br />
wide variety <strong>of</strong> pollinators that differ in their <strong>for</strong>aging<br />
References<br />
Dryobalanops<br />
0.794 (±0.059) – Kitamura et al. (1994)<br />
aromatica<br />
0.856 (±0.063)<br />
Shorea congestiflora 0.874 (±0.021) Murawski et al. (1994)<br />
S. megistophylla 0.860 (±0.058) Murawski et al. (1994)<br />
S. trapezifolia 0.617 (±0.033) Murawski et al. (1994)<br />
Stemonoporus<br />
0.898 (±0.022) Murawski & Bawa<br />
oblongifolius<br />
(1993)<br />
ranges and there<strong>for</strong>e disperse pollen<br />
over varying distances. Appanah and<br />
Chan (1981) implicated thrips as<br />
pollen vectors <strong>for</strong> several species <strong>of</strong><br />
Malaysian species <strong>of</strong> Shorea, section<br />
Muticae. The thrips breed in flower<br />
buds <strong>of</strong> the species they pollinate and,<br />
as flowering progresses, they multiply<br />
in number. The adult thrips feed on<br />
stamens and petals. As the petals <strong>of</strong> the<br />
flowers are shed from the tree the<br />
thrips fall on the ground and then move<br />
to a new cohort <strong>of</strong> subsequently opened flowers. The<br />
distances over which thrips move are not known but,<br />
because <strong>of</strong> their relatively small body size, they<br />
apparently do not fly over long distances. It is presumed<br />
that their restricted movement is not a drawback in their<br />
effectiveness as pollinators because the species they<br />
pollinate are relatively abundant.<br />
Dayanandan et al. (1990) present evidence <strong>for</strong><br />
pollination <strong>of</strong> Shorea megistophylla (section Doona) and<br />
Vateria copallifera by bees (Apis spp.). They also<br />
observed a wide variety <strong>of</strong> other insect floral visitors<br />
including thrips. However, the thrips acted as flower<br />
predators rather than pollinators, particularly in V.<br />
copallifera.<br />
More recently, Momose et al. (1994) have presented<br />
evidence <strong>for</strong> pollination <strong>of</strong> Dryobalanops lanceolata, a<br />
large canopy tree species in Sarawak, by medium sized,<br />
stingless bees (Trigona spp.). They also noted the presence<br />
<strong>of</strong> many other types <strong>of</strong> flower visitors (Coleoptera and<br />
Diptera). Momose et al. suggest that medium sized,<br />
stingless bees constitute an important group <strong>of</strong> pollen<br />
vectors <strong>for</strong> canopy and subcanopy trees in Sarawak (see<br />
also Chan and Appanah 1980).<br />
Clearly, the <strong>dipterocarps</strong> are pollinated by a wide<br />
variety <strong>of</strong> insects. The three detailed studies, respectively<br />
by Appanah and Chan (1981), Dayanandan et al. (1990),<br />
and Momose et al. (1994) have revealed three different<br />
classes <strong>of</strong> pollinators. Ashton (1982) in his extensive<br />
<strong>review</strong> also lists beetles and moths as flower visitors,<br />
although their role in pollination has not yet been<br />
demonstrated. Among the pollinators implicated so far,<br />
all, except thrips, are capable <strong>of</strong> moving pollen over long<br />
distances. The extent <strong>of</strong> gene flow via pollen (and seeds)<br />
is further discussed in subsequent sections.<br />
Seed dispersal in most <strong>dipterocarps</strong> is by wind<br />
(Ashton 1982). In most species, sepals are modified into