A review of dipterocarps - Center for International Forestry Research

Management of Natural Forests
were introduced. Following the Second World War, there
was a surge in logging for rebuilding the country, and the
only management control was a ‘diameter limit’ of 50
cm for cutting trees. Despite the limit, mechanisation
of logging led to almost clear-cutting due to high
stocking.
The above ‘diameter limit’ cuttings brought about the
development of the Philippine Selective Logging System
(PSLS), which is a modification of the Selection System
used to manage old growth hardwood forest in North
America. Under this system, 60% of the healthy
commercial residuals in the 20-70 cm dbh classes are
to be retained as growing stock for a future harvest
(Reyes 1968). This has since been raised to 70% of all
the commercial residuals in the 20-60 cm dbh classes.
The selective logging amounts to removing mature,
overmature and defective trees with minimum injury to
an adequate number of healthy residuals of commercial
species to guarantee a future timber crop. Also
incorporated into the system is a timber stand
improvement (TSI) guideline which consists of
treatments before and after the major felling to ensure
the stand attains maximum timber quality and growth
(Uebelhoer and Hernandez 1988). The TSI appears to be
yielding results. Preliminary results indicate that
liberation from crown competition results in increase
in diameter: a removal of 33% basal area, resulted in up
to 10% increase in basal area of crop trees in ten years.
The Philippine forests are generally very rich in
dipterocarps. Therefore, the PSLS is regarded as the best
silvicultural system for their forests. If logging damage
is contained, and residual forests protected and post
logging treatment given, another economic cut is
possible after 30-45 years. While the system looks good,
overcutting and bad implementation has led to
degradation of vast areas of forests. Today, there is
concern for the quality of the second cut.
Indonesia
From historical times, teak forests in Java have received
most interest from silviculturists in Indonesia. After
1966, changes in forest policy took place and the
dipterocarp forests in the other islands were opened for
large scale exploitation. At first it was merely a timber
felling operation. Sustained management efforts began
in the 1970s when a simplified variation of the PSLS
was introduced for lowland dipterocarp forests
(Soedjarwo 1975). The original version, the Indonesian
Selective Cutting System, locally known as the TPI
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(Tebangan Pilih Indonesia), relies on leaving behind an
adequate number (25 stems/ha or more) of sound
commercial species of 20 cm dbh and above. With this
minimum guaranteed, everything above a certain diameter
limit may be harvested. If the putative residuals could be
met, the TPI system allowed for a short felling cycle of
ca 30 years. If these were not present, the option was to
harvest on a Uniform System rotation of ca 60 years.
There was also a further option to clear cut and replant,
although not necessarily with dipterocarps.
Compared to the PSLS, the TPI is a much simpler
system. It is therefore cheaper and easier to monitor.
Liberation thinning is prescribed to release residuals and
nucleus trees for reseeding. Planting of seedlings to
enrich the stand may be carried out if followed by
subsequent tending and liberation thinning.
Pre-felling inventories in Indonesia however suggest
that stands rarely have sufficient residuals of commercial
species (Burgess 1989). Therefore, the second cut may
have to be delayed. The TPI was subsequently modified
to the TPTI (Tebang Pilih Tanam Indonesia) which
resorted to the necessity of planting if the selecting
fellings failed. This resulted from the conviction that it
is possible to easily plant up large areas with
dipterocarps (see Enrichment Planting). Unfortunately,
the impression from this decision is that uncontrolled
logging can be done without serious consequences, as
enrichment planting can overcome the problems. Caution
should be exercised here until evidence for the success
of enrichment planting is clear.
Growth and Yield
One of the biggest difficulties for sustained management
of dipterocarp forests is in getting reliable data on growth
and yield. The data are a prerequisite for determining
harvesting volumes and cutting cycles. In this respect,
there is much scepticism about the growth rates being
used for managing many forests in the region. A quick
glance of the data from the everwet region, based on only
a few sites, gives some clue to how dipterocarps are
growing.
From studies in Peninsular Malaysia, Sabah, Sarawak,
Philippines and Kalimantan, the following
generalisations can be made. In undisturbed, virgin
forests growth rates are relatively much lower compared
to logged ones, and the best growth is achieved in
plantation conditions (e.g. mean growth rate (diameter
increment) of Shorea spp. in Sarawak: primary forest,