A review of dipterocarps - Center for International Forestry Research

Seedling Ecology of Mixed-Dipterocarp Forest
Does the simplification of mixed-dipterocarp forest by
the frequent use of various silvicultural release
treatments (weeding, cleaning, liberation) favour so few
commercial tree species that this may lead to greater
susceptibility to disease and/or herbivory of the forest?
Questions such as these need to be further tested.
Growth in Relation to Physiology and
Structure of Dipterocarps
Recent seedling experiments have focused on separating
the various abiotic and biotic factors that influence
seedling establishment and growth under controlled
conditions. Many studies have been investigating light
and the different effects of light quality, quantity and
duration. These experiments reinforced findings from
the earlier shade house studies but demonstrated that
forest understorey light quality can accentuate the poor
growth and survival of seedlings in deep-shade
conditions (Kamaluddin and Grace 1993, Lee et al.,
unpublished manuscript). Experiments that simulated
quality and intensity of light environments of a rain forest
also demonstrated that Shorea species allocate dry mass
proportions to roots, stems and leaves in different
amounts (Turner 1989, Ashton 1995). These results show
that the more shade-tolerant Shorea species allocate
proportionately more dry mass to root development than
to stem and leaves in forest understorey environments
whereas the reverse is true for more light-demanding
Shorea species.
The process of photosynthesis requires
photosynthetically active radiation, water and carbon
dioxide . The adaptations a seedling leaf can make to its
surroundings must accommodate all three. The
relationship among all three factors is so closely linked
that many of the leaf adaptations and adaptation responses
to environmental change are the same. Heat and
desiccation of leaves exposed to the full radiation of the
sun can promote leaves that have similar physiological
and anatomical adaptations as leaves that are droughtenduring.
Leaves that have grown in the shade often
resemble those of drought intolerant leaves. Alhough
much work has been done elucidating differences in leaf
anatomy and morphology between species of different
cladistic or successional groups for other forest regions
(Wylie 1951, 1954, Jackson 1967 a, b, Givnish 1988,
Lee et al. 1990), little has been done that examines these
relationships for mixed-dipterocarp forests. However,
there is some evidence that suggests the same leaf
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anatomical and morphological trends exist for mixeddipterocarp
forest.
For species belonging to the same cladistic group
or regeneration guild work has been equally negligible
in mixed-dipterocarp forest. In a seedling study of
Shorea by Ashton and Berlyn (1992) data show that
differences in net photosynthesis (P N ), transpiration (E),
and stomatal conductivity (g) can be associated with
differences in the anatomy of Shorea species. General
trends indicate that in experimentally controlled
conditions maximum P N rate was a good measure of the
light tolerance of Shorea. The shade tolerant species had
maximum P N rates at relatively lower light intensity
compared to that of more light demanding species. Ratios
between rates of P N and E of species at their maximum
P N light intensities can also suggest trends in water-use
efficiency. This can reveal some indication of species
order in relation to drought tolerance in controlled
environments. Differences in physiological attributes
also suggest that the greatest plasticity of response to
differences in availability of light was exhibited by the
most light-demanding species and the least by the most
shade-tolerant. At a regional scale, Mori et al. (1990)
showed similar patterns with dipterocarps. Those from
more seasonal climates having greater rates of P N and E,
and higher levels of plasticity than dipterocarps from
aseasonal everwet climates.
An array of anatomical characteristics can, in
combination, partly determine the physiological light and
drought tolerance of Shorea species in relation to their
associates. Patterns suggest stomatal frequency is a
factor differentiating Shorea species, with the most
tolerant having fewer and smaller stomates than the most
intolerant forms. Differences in thickness of the whole
leaf blade and the leaf cuticle among species appear
similarly related to both light and drought tolerance; with
sun loving species having thicker dimensions of both
characters than shade tolerant or demanding species.
These results elucidate some of the relationships between
the distribution patterns of Shorea species across the
topography and their differences in light and drought
tolerance. They also show that an important period
determining site specialisation of a dipterocarp species
occurs during regeneration establishment. Another area
of study related to the anatomy and physiology of
seedlings is tissue chemistry (foliar nutrients, secondary
compounds). Although little work has examined tissue
chemistry, investigations along these lines would tie in
closely with studies on soil fertility, seedling herbivory