A review of dipterocarps - Center for International Forestry Research

Conservation of Genetic Resources in the dipterocarpaceae 51
define the regeneration ‘niche’. Therefore, genetic
selection within taxa of the family can readily be moulded
by fine and coarse grain variation in the environment.
Thus, the inter-population differentiation observed in
Stemonoporus oblongifolius and Shorea trapezifolia is
consistent with the hypothesis that slight variation in the
habitat can allow genetic variants to differentiate along
environmental gradients despite low or moderate levels
of gene flow.
Summary of Diversification Processes
Most dipterocarps are outcrossed and diploid. Speciation
seems to have involved allopatric differentiation of widely
outcrossing populations; differentiation seems to have
occurred in response to differences in soils and habitats
(Ashton 1969). Aneuploidy, polyploidy, and hybridisation
may have also assumed a role in the spread of some
variants arising as a result of hybridisation and changes
in chromosome number. At the intraspecific level,
outcrossing maintains high levels of genetic variation in
populations. Mass flowering combined with abundance
of adults probably ensures large effective population
sizes. Nevertheless, despite extensive gene flow,
selection results in differentiation of populations over
relatively small scales.
Research Needs
Future research needs may be best examined in the context
of threats to diversity. Genetic resources are imperilled
by deforestation and forest fragmentation. Moreover,
selective logging often can lead to reduction in genetic
variation (Kemp 1992) and alter population structure with
concomitant changes in demography and genetics of
subsequent generations (Bawa 1993). Global climatic
change is also expected to influence plant populations,
but the potential effects, deleterious or beneficial, are not
well defined, particularly for the areas where dipterocarps
are dominant.
Deforestation and forest fragmentation may influence
diversity in several ways. Species or populations may
become extinct or severely endangered. At the population
level, once seemingly large, contiguous populations may
be broken into relatively small, remnant patches,
physically isolated from each other. Over time, gene
exchange among the remnant patches may be completely
eliminated and the small populations may be subject to
inbreeding. Habitat fragmentation can also increase
overall levels of variation if isolated populations diverge
from each other. The consequences of fragmentation
depend upon the degree and duration of isolation and the
size of the isolated population.
Fragmentation of habitats may have deleterious
effects on both the ecosystem dominants as well as rare
species. The ecosystem dominants may have very large
populations, and fragmentation may result in loss of
genetic diversity (Holsinger 1993). Rare species may face
severe reduction in population size following
fragmentation. Many species of dipterocarps have adult
population densities as low as 0.07 to 0.30 individuals
per hectare (Ashton 1988). Some of these species occur
in low population densities at more than one site and may
be particularly prone to inbreeding. In addition, there may
be selection for apomixis in such situations (P. Ashton,
personal communication).
Selective logging can also increase the potential for
inbreeding. Logging temporarily reduces adult population
densities. In many tropical tree species, inbreeding has
been shown to be a function of stand density (Murawski
and Hamrick 1990, 1991). In Shorea megistophylla, as
noted above, the rates of inbreeding are higher for trees
from logged stands than for trees in unlogged stands.
However, it should be noted that stands in properly
managed forests regenerate from seedlings established
prior to logging. In dipterocarps, the potential for
inbreeding is also increased by the fact that selfincompatibility
barriers are not strong; trees in many
species are capable of setting seeds after self-pollination,
but here again selfed seeds may be selected against in the
presence of outcrossed seeds in the same inflorescence.
The longevity of trees may not allow many of the
assumed deleterious consequences of forest fragmentation
and selective logging to be manifested for a long time.
Even in small patches, trees may set fruits and seeds and
regenerate without apparent ill-effects. Comparative
studies of reproductive output, mating patterns, and
regeneration processes involving trees in large contiguous
forests and small fragments may reveal the consequences
of habitat alteration.
Thus, in order to fully understand the effects of
deforestation, forest fragmentation, and forest
management practices on forest genetic resources of
dipterocarps, we need a better understanding of patterns
of diversity and processes that maintain diversity. Areas
of research that require immediate attention are outlined
below.