A review of dipterocarps - Center for International Forestry Research

Management of Natural Forests
terrain. In situations where it is not economically
equitable for the logger, the modified-MUS is prescribed
which imposes an arbitrary diameter of 45 cm dbh for
felling on a rotation of 50 years.
Sabah
Silviculture in Sabah followed a path similar to
Peninsular Malaysia. In the early 1930s, RIF were tried
on a limited scale (Fox 1968). In 1949 the Selection
Improvement Fellings were introduced, to assist the
pole-size trees of 10 cm dbh and above in areas logged
15 to 25 years before (Martyn and Udarbe 1976). The
method involved poison-girdling non-commercial
species and climber cuttings.
In 1956 a modified version of the MUS was
introduced for forest regeneration (Chai 1981). The
canopy was opened after felling by poison-girdling all
non-commercial species as well as defective trees of
commercial species down to 15 cm dbh. The next crop
is expected to come from seedlings, and advance growth
will be a bonus. This system became the standard
regeneration technique for dipterocarp forests in Sabah.
This modified MUS underwent further changes in
1971 to become a minimum girth limit system, the so
called Stratified Uniform System (Chai and Udarbe
1977). In this refinement, the advance growth for the
next crop is kept. The main elements of the system
include marking 25 preferred or desired trees/ha (25-
59 cm dbh) for retention, and poison girdling unwanted
and defective trees. Climber cutting and girdling of seedbearers
and relics is done in the 15th year.
Later, Chai and Udarbe (1977) expressed doubts on
the value of the girdling practices. They argued that since
logging intensity is high, much of the forest gets released
anyway without further treatment. Since then, only
climber cuttings are meant to be done. Furthermore,
girdling of weeds or non-commercials has been stopped
on account that such plants may become commercial in
the future, and moreover, the operation may be harmful
to the ecosystem.
Sarawak
The timber industry in Sarawak relied mainly on
extensive peat swamp forests, and moved into the hill
forests only in the late 1960s. Coming so late, Sarawak
tended to follow the systems developed in Peninsular
Malaysia (Lee 1982). At first the forests were selectively
logged. The relics left behind were defective and inferior,
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and seedlings/saplings unlikely to reach maturity before
70-80 years.
As a result, three UNDP/FAO projects (1974-1981)
were started to provide interim guidelines for managing
Sarawak’s dipterocarp forests (FAO 1981a, b). The study
evaluated three different treatments:
1. Overstorey removal only - All overmature non-marketable
trees left behind during harvesting were removed
by poison-girdling.
2. Malayan Uniform System evaluated - Following logging,
all other non-economical trees, which impeded
growth of the seedlings were removed. Such a treatment
was considered too drastic. The rough terrain
and shallow soil conditions are vulnerable to heavy
erosion. A modification to MUS was tried whereby
the advance growth of the desirable species were
saved. In this way the advance growth may be obtained
even before the seedlings mature, giving in effect a
polycyclic system.
3. Liberation Thinning - Desirable species were identified,
and liberated from competition including removal
of the overstorey to improve their growth. No
specific species or species groups were eliminated,
only those that restricted the growth of the selected
trees. Therefore, trees of non-commercial species
were left behind if they did not appear to hinder selected
trees.
Mild overstorey release was insufficient to release
the trees of desirable species. Both the Liberation
Thinning and the modified MUS resulted in increased
growth of the residuals (Hutchinson 1979), but the latter
resulted in elimination of a greater number of trees which
could have commercial value in the future. Despite the
potential loss in the future of commercial trees, for a
while liberation thinning held sway in Sarawak as the
appropriate silvicultural treatment (FAO 1981b). It lost
support subsequently, when Lee (1982) suggested that
the boost in initial growth is not sustained, the operations
are difficult, and cannot be kept up with the logging rate.
Since then, Liberation Thinning is being carried out for
a small portion (ca. 4%) of the forest logged annually
(Chai 1984). Otherwise, the practice has reverted to
selective felling based on diameter limits.
Philippines
Scientific management of dipterocarp forests began
during the American Regime. From 1900 to 1942
mechanised timber extraction and processing methods