A review of dipterocarps - Center for International Forestry Research

Management of Natural Forests
and immature trees, for they need to be repeated
(Wyatt-Smith 1963). Following the initial burst,
growth slows down with onset of crown competition.
In the 1970s, such thinnings were introduced in
Sarawak under a different name, ‘Liberation
Thinnings’ (Hutchinson 1979). But the Department
reduced such treatments on the basis that the
increments are too small for the effort (Lee 1982).
However, liberation thinnings to forests following a
diameter limit cutting proved better (Chai 1984,
Primack 1987). This resembles more a MUS except
for the logging which was under diameter limits. With
this kind of confusion, opportunities for better
management were bypassed.
4. In other cases, the Shelterwood Systems have
degenerated into selective fellings. In the Indian
Irregular Shelterwood System, uncertainty of
regeneration led to retention of trees below a
specified girth as part of the future crop. This has led
to some confusion, and silvicultural treatments
benefit neither seedlings nor poles.
5. Most extreme is the case with Peninsular Malaysia.
The system introduced here to manage the hill forests
was called the Selective Management System (Mok
1977). One of three systems was to be applied
depending on the requirements. This included the
monocyclic MUS, polycyclic Selection System, and
cutting and planting. But unfortunately, the Selective
Management System in practice became a selective
felling.
In contrast with Shelterwood Systems, the Selection
System is based on maintaining the forest stand structure,
by extracting proportionate number of trees from
different size classes. It works well with species that can
tolerate some shade, and small gaps suffice for their
growth (Putz and Ashton unpublished). The system allows
frequent timber extractions, but substantial management
is required. Logging has to be carefully done to protect
young trees.
The selection systems are not truly practised in the
dipterocarp forests although the Philippines Selection
Felling System in theory has the necessary silvicultural
components to qualify as one. Elsewhere, Selection
Systems have degenerated in practice into selective
fellings based on diameter limit. This is not a silvicultural
system in the classical sense. Critics claim selective
fellings cannot fulfill the requirements of a polycylic
system (Wyatt-Smith 1987, Appanah and Weinland
144
1990), and that in reality it is merely a bicyclic system.
Its major difficulties are:
1. Seedling regeneration is not attended to, and this
might lead to a decline in the future crops;
2. Composition of future crops cannot be controlled;
3. The intermediate class (residuals) which is poorly
represented, may also be inferior, suffer much
logging damage, and subsequently succumb. Overall
their growth rates may also be below that forecasted;
4. The cutting cycles are over-optimistically short; and
5. The more frequent entries can damage the soil and
young regeneration.
Despite the criticisms, most of the seasonal and
aseasonal dipterocarp forests are selectively logged at
present. Perhaps the advantages of short felling cycles,
fewer tendings, and freedom from limitations of
seedling regeneration have led to such a preference.
Supporters nonetheless argue that the Selection System
is suitable for dipterocarp forests, many of which are
now in steep terrain, with spotty seedling regeneration,
and are relatively inaccessible. The weakness is in the
implementation. The test of course is with the second
cut, which will soon take place in Malaysia and Indonesia:
overall, a decline in yield is expected. The true danger
lies in temporarily overcoming the problem by reducing
girth limits and cutting cycles.
In the aggregate, both silvicultural systems have their
pros and cons. But trying to apply a workable silvicultural
system is not a simple matter. It has to ensure society’s
needs are met by harvesting the forest without degrading
it. Despite the many mistakes and miscalculations, more
has been done to develop management systems for
dipterocarp forests. Nonetheless, detractors may
emphasise that there is very little management in reality.
That aside, it must be stated that if ever management of
tropical forests is possible, the best chances are with
the dipterocarp forests. Their special attributes have
endowed them with several advantages in terms of easy
regeneration, fast growth, and a rich commercial timber
stand. So the silvicultural systems employed should
attempt to enhance and exploit the special attributes of
these forests.
As for the silvicultural system, no doubt we can argue
in favour of selection fellings for the existing
dipterocarp forests. The advantages include long
regeneration period for seedling recruitment, enhanced
biodiversity, guarantee of future crops from advance
growth that is retained, and retaining of species and grades