A review of dipterocarps - Center for International Forestry Research

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