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