A review of dipterocarps - Center for International Forestry Research

Seedling Ecology of Mixed-Dipterocarp Forest
Work has been done on dipterocarp germination
mostly during the 1970s and 80s. Many studies of
dipterocarp species reported them to be recalcitrant
(Jensen 1971, Tang 1971, Tang and Tamari 1973, Tamari
1976). Chapter 3 gives a more detailed review of
dipterocarp germination. Other studies suggested that
many non-dipterocarp species, mostly pioneers, had
dormant seed buried in the soil that germinated with a
marked increase in radiation at the ground surface (Liew
1973, Aminuddin and Ng 1983, Raich and Gong 1990).
However, unlike the neotropics (see work by Vazquez-
Yanes and Orozco Segovia 1984, Garwood 1996), no
critical experiments have focused on this buried seed
phenomenon. Also, relatively few studies have
comprehensively evaluated patterns of germination for
the whole forest in relation to successional status and
taxonomy (Ng 1983). This is perhaps because past
research has focused on the autecology of individual
dipterocarp species. Future work should focus on
clarifying the germination mechanisms of mixeddipterocarp
forest tree species in general and the role
dipterocarp species play within it. Little work has focused
on the competitive interactions between dipterocarps and
non-dipterocarps and yet they are of direct relevance to
the maintenance of dipterocarps in a managed forest.
Early Survival and Establishment of
Dipterocarps
The main research objective early in this century was to
develop a method for evaluation of regeneration stocking
before logging (Wyatt-Smith 1963). The survey
techniques that developed were usually based on line
transects that assessed stocking by measures of
dipterocarp seedling distribution and number. Surveys
revealed that the abundance of regeneration was
associated with certain dipterocarp species and sites. In
many circumstances regeneration was absent particularly
on the slopes of hill forests and where competing
understorey palms, shrubs and herbs were present
(Burgess 1975, Wong 1981, Kusneti 1992). Though
measures of distribution are important to gauge adequate
and even coverage of seedling establishment within a
stand, measures of seedling number and density do not
necessarily predict successful establishment. A measure
that incorporates an estimate of seedling vigour is needed.
More recent studies have used different size classes and
estimates of leaf area to gauge vigour, promoting survey
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techniques that discard seedlings in the ‘less vigourous
classes’ for a representation of regeneration stocking
(Ashton 1990). These can be useful measures for most
dipterocarp species because they have poor ability to
sprout. Measures of their above-ground performance can
therefore be used to predict future growth and survival.
Studies by Nicholson (1960) and others (Fox 1972,
1973, Liew and Wong 1973, Tomboc and Basada 1978,
Appanah and Manaf 1994) elucidated the cyclic nature
of population recruitment and survival in the groundstorey
of a closed forest and demonstrated the importance of
advanced regeneration in the form of a seedling bank for
the successful establishment of new forest stands.
Conceptual models of the regeneration dynamic have
been developed that explicitly suggest the importance and
reliance of mixed-dipterocarp forest on advance
regeneration (see Fig. 1). This reliance is not only for
dipterocarps but also for late successional canopy trees
that are non masting, subcanopy trees and shrub species.
Forest management should therefore focus on advanced
regeneration of dipterocarp trees and similar associates.
These are the trees that are the canopy dominants during
the mid and late stages of forest succession. They,
therefore, create the basic forest structure beneath which
other strata exist, and reflect the changes in composition
associated with differences in site quality.
Studies have also shown that dipterocarp species
could be broadly categorised as shade-tolerant or lightdemanding
based on differences in frequency of
recruitment and rate of seedling death. Shade-tolerant
dipterocarps can have seedlings established beneath
closed canopied forest for long periods of time (> 10
years). Mast years for shade-tolerant dipterocarps can
therefore be fewer than relatively more light-demanding
dipterocarps but still provide adequate advance
regeneration establishment (Wyatt-Smith 1963, Fox 1972,
Gong 1981). In general, however, all dipterocarps have
cohorts of seedlings that continually replenish the seedling
bank from successful mast years. Over time, seedlings
die primarily from the very low light regimes of a closed
forest canopy (Ashton 1995). Groundstorey levels of
photosynthetically active radiation (PAR) beneath the
canopy of a mixed-dipterocarp forest have often been
recorded as less than 1% of that received in the open
(Torquebiau 1988, Ashton 1992a).
Other studies have also suggested the importance of
an increase in amounts of PAR that promotes only partial
shade for dipterocarp germination and early survival