A review of dipterocarps - Center for International Forestry Research

Seedling Ecology of Mixed-Dipterocarp Forest
microenvrionments (Gunatilleke and Ashton,
unpublished).
These studies have focused investigations on the
spatial availability of light at the forest groundstorey and
its relationship to seedling survival and growth. Findings
also suggest that the seasonal variation in soil water
availability, scaling up from different groundstorey
microsites to across the landscape (ridge to valley), can
be another factor that affects the survival and growth of
dipterocarps. Ashton (1992a), Brown (1993) and
Palmiotto (1993) observed seasonal periods of water stress
that may play a critical role in determining seedling
composition of canopy gap regeneration. Transplant
experiments suggested soil water availability, related to
topography (slope, ridge, valley etc.), affects the survival
and growth of dipterocarps. The transplant experiments
also suggested seedling survival and growth allocation
was affected by interaction between soil water availability
and radiation. For example, some species showed fivefold
decreases in root mass between seedlings growing
in the understorey of a ridgetop site as compared with
those seedlings in the understorey of a valley site.
Although understorey PAR was comparable between the
two sites the poor root development on the ridge
predisposes these species to drought. Studies by Brown
and Whitmore (1992) and Ashton et al. (1995) suggested
that seedlings of more light demanding dipterocarp
species have larger leaves and that more shade tolerant
species have smaller leaves and are more sensitive to heat
stress. This is contrary to most other literature for example,
Givnish (1988) which has mostly described the sun shade
dichotomy for mature trees that are from temperate forest
regions. There are only a few studies (Ashton and Berlyn
1992, Strauss-Debenedetti and Berlyn 1994) that have
investigated the sun shade dichotomy for seedlings of
the moist tropics.
Other studies are also providing evidence that
dipterocarps are affected by soil characteristics related to
the underlying parent material. Surveys by Baillie et al.
(1987) and Ashton and Hall (1992) suggest both
concentrations of total and available magnesium and
phosphorus to be particularly important in determining
species-site associations. However, no fertiliser studies
of seedlings using field experiments have clearly
demonstrated that these factors affect the establishment
stage of forest development (Turner et al. 1993, Burslem
et al. 1995) although some studies that are in progress
are suggesting differences may occur (Gunatilleke et al.
93
1996, Palmiotto, in preparation). In these experiments
different soils are being investigated to understand
nutrient use efficiency of dipterocarp species whose
distribution is restricted to very different levels of soil
fertility. These kinds of studies are beginning to provide
the basis for the development of new silvicultural
regeneration methods and the refinement of currently used
methods. These studies on light, soil moisture and fertility
are providing knowledge for a better mechanistic
understanding of regeneration dynamics of forests. In
some cases they have contradicted previous understanding
of forest dynamic patterns based only on observation and
census methodologies. An example would be the recent
findings that show discrete differences in the sitespecialisation
among species of Shorea section Doona.
These species were formerly assumed to be very similar
in their site requirements and therefore their silvicultural
treatments were the same.
In addition, there are many biotic interactions that
can moderate or accentuate patterns in the establishment
of seedlings within the physical environment. For
example, although no studies substantiate this, host
specific ectomycorrhizae could accentuate the differential
exploitation of soil nutrient resources among closely
related assemblages of dipterocarp species. Studies by
Becker (1983) and Smits (1983) suggest that
ectomycorrhizae can play important roles in dipterocarp
seedling establishment and growth. Mycorrhizal infection
was found to be greater for seedlings located in small
clearings than for those seedlings located beneath forest
canopy. These results suggest that seedling regeneration
of dipterocarps will respond more vigourously to
overstorey removal if pre-release treatments create higher
light environments in the understorey. In addition, Lee
and Lim (1989) found that foliar phosphorus contents of
Shorea seedlings growing on either phosphorus deficient
or phosphorus rich soils were the same - indicating a
difference in uptake efficiency that was attributed to
ectomycorrhizae (for more detail see Chapter 6 on
nutrition and root symbiosis).
Herbivory is another biotic effect that has had little
investigation. Becker’s (1981) studies of seedling
populations found less herbivory on the leaves of a late
successional, more shade-tolerant Shorea species as
compared to more light-demanding Shorea species.
However, no studies followed up on this work. More
investigation should be done, particularly on the role of
non dipterocarp tree species in mixed-dipterocrap forest.