A review of dipterocarps - Center for International Forestry Research

Seedling Ecology of Mixed-Dipterocarp Forest
(Nicholson 1960, Ng 1978). However, it was shade house
investigations (Mori 1980, Sasaki and Mori 1981, Ashton
and de Zoysa 1989) that clearly demonstrated that most
dipterocarp seedlings require greater amounts of radiation
than that received at the groundstorey of a closed canopy
dipterocarp forest, but less than the amount of radiation
received when exposed to full sun.
Field research on disturbance regimes of mixeddipterocarp
forest supports evidence from shade house
experiments and studies of seedling population dynamics
in the forest. Natural disturbances documented in mixeddipterocarp
forest are varied but most are of a kind and
scale that promote the survival and release of an existing
seedling groundstorey. Disturbance types include
lightning strikes (Bruenig 1964), insect defoliation of
canopy trees (Anderson 1964), single and multiple tree
falls (Anderson 1964), and cyclones (Whitmore 1974,
1989). All allow the groundstorey vegetation to remain
largely unharmed. Disturbances in mixed-dipterocarp
forest that have been observed to destroy groundstorey
vegetation include landslides, flooding and fire (Day
1980, Leighton and Wirawan 1986, Tagawa and Wirawan
1988). This might be one reason why dipterocarp
regeneration does not establish well in parts of a forest
landscape subject to lethal disturbance of the groundstorey
such as steep slopes, flood plains and swidden agriculture.
Current investigations focus on refining our
understanding of the regeneration microenvironment of
tree species in mixed-dipterocarp forest. Compared to
other tree species of mixed-dipterocarp forest,
dipterocarps have some general autecological
characteristics that allow for their categorisation in the
same regeneration guild (Table 1). Although dipterocarps
have the same general autecology there are also
differences among them, however, these differences are
small compared to other regeneration groupings.
Important questions are: what degrees of difference exist
and why do they occur among species belonging to the
same congeneric group. The answers are particularly
relevant to understanding dipterocarp dominance in
mixed-dipterocarp forests and will provide the silvical
information for the fundamental treatments imposed on
these forests for management purposes.
One such topic that merits attention is the site
specialisation of dipterocarp regeneration. How site
specific is advanced regeneration of dipterocarp species?
Recent field studies demonstrate that forest gaps of
different size exhibit considerable spatial (Ashton 1992a,
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Table 1. Silvical characteristics of canopy tree species
belonging to genera assemblages (e.g. Shorea) that dominate
the mature phase of mixed-dipterocarp forest. These
characteristics should be interpreted broadly as exceptions
will exist (Ashton 1992b).
Reproduction
• Pollination vectors are small insects
(hymenoptera, hemiptera)
• Seed is with storage tissue
• Seed is dispersed by gravity (often aided by
territorial animals such as rodents)
• Fruiting time is more or less supra-annual with
distinctly different amounts of seed at each fruiting
(masting)
• Seed shows no classical dormancy
Establishment and Growth
• Seed requires partial shade protection for
germination and early survival
• Seedlings require an increase in light (as
compared to understorey conditions) for
satisfactory establishment and growth
• Seedling survival and establishment is usually site
specific, according to particular biotic,
microclimatic and edaphic characteristics
Brown 1993) and temporal (Raich 1989, Torquebiau
1988) variation in forest groundstorey microclimate.
Changes in size of small canopy openings can greatly
influence the overall amount of radiation received at the
groundstorey of the opening centre (Brown 1993).
However, larger canopy openings provide a greater range
of microclimates at the groundstorey of the opening
(Ashton 1992a). Studies that monitored pre-established
seedlings and new recruits (Raich and Christensen 1989,
Brown and Whitmore 1992) showed that there were
significant differences among dipterocarp species in
survival and growth at these different microsites. Studies
by Ashton et al. (1995) that controlled age and spacing
of dipterocarp seedlings supported these findings.
However, investigations by Turner (1990a, b), who
monitored pre-established seedlings, suggested mixed
results of dipterocarp seedling survival and growth in
relation to light availability at the scale of the microsite.
Studies are now investigating the competitive relationship
between species for regeneration growing space through
the monitoring of long-term self-thinning trials located
on different sites and within different