A review of dipterocarps - Center for International Forestry Research

Plantations 166
are removed. Another type of thinning is the thinning from
below, where particularly the dominants or selected
dominants are favoured and a varying proportion of other
trees is removed. Grade of thinning is a degree of thinning
based on dominance, crown and stem classes, and the
extent to which these classes are removed at any one
thinning.
With the exception of Shorea robusta no thinning
regimes have been developed for dipterocarp plantations.
Krishnaswamy (1953) and Mathauda (1953a) studied the
effect of thinning intensities on height and diameter
development, stand basal area and volume increment of
Shorea robusta. The conclusions from this thinning trial
were: that the thinnings should be carried out every 5
years up to an age of 20 years and thereafter at larger
intervals; and the maximum volume production is
obtained under C/D-grade (heavy to very heavy low
thinning as per standard definition of the terms adopted
in India). In the C/D grade the dead, moribund, diseased
trees, whips of co-dominant and dominant trees, defective
co-dominant and dominant trees and a small proportion
of sound co-dominant and dominant trees are removed.
Thinning according to the C/D grade was found to be
best for the production of both fuelwood and timber.
Wyatt-Smith (1963a) assumed that in dipterocarp
plantations a thinning cycle of 5 to 10 years would be
adequate. Suri (1975a) developed a quantitive thinning
model for Shorea robusta forests in Madya Pradesh, India.
Based on the correlation between crown diameter and
stem diameter a thinning model was formulated and stem
density regimes for different crown disengagement levels
determined. It was concluded that quantitative thinning
grades can be developed for different species by studying
their crown diameter/bole diameter relationship. The
crown disengagement in younger stands was sometimes
carried out as so-called stick thinning, i.e. starting from a
selected crop tree any tree growing within a defined
distance (e.g., six, nine or twelve feet) from the selected
crop tree was removed for example, in a naturally
regenerated, more or less even-aged stand of
Dryonbalanops aromatica (Anon. 1948b). An important
conclusion from this trial is that it is not advisable to make
heavy thinnings before the overwood has been removed,
since the young crop can be overtaken by climbers and
secondary species benefitting from increased light. The
heavily thinned treatments suffered severely from
climbers and weed species, while trees damaged by the
falling overwood had no neighbours to replace them.
Thinning is usually done with a bush knife (smaller
trees), an axe or a saw but if the tree is not to be utilised,
girdling or poison-girdling may be applied. Often girdling
alone is unsuccessful and poison-girdling is recommended
(e.g., Wyatt-Smith 1963b, c, Agpaoa et al. 1976). The
trees to be removed are frill-girdled and the poison is
applied into the frill. Effective chemicals have already
been mentioned in the section on weeding and cleaning.
Thinning interventions require some kind of
classification of the stems in the stand to be thinned.
Krishnaswamy (1953) presented a detailed stem
classification which is based on dominance position and
within each position on vigour, soundness, crown
development and other characteristics. It resembles the
classification of Kraft (1884), but includes reproduction
or regeneration and overmature trees (e.g., standards).
Any thinning, except for schematic interventions, requires
that all trees in the stand are judged according to their
function. Potential final crop trees (PCT) are distinguished
from non-crop trees (NCT). The PCT are those trees
which owing to their straightness and evenly formed
crowns are to be retained as crop trees and released from
competition. NCT may have different functions. There
are harmful trees that damage the crowns or stems of the
PCT and should be removed. There are useful NCT which
enhance growth form and branch-shedding of the PCT
or have important ecological functions. There are
individuals for which their future development and
function is not clear and they have to be spared from
thinning until the necessity for removal is beyond doubt.
In the Malaysian context Watson (1931/1932e) has
classified the most common trees in Peninsular Malaysia.
He classified the species into the following categories:
• quality timber trees,
• utility timber trees,
• subsidiary trees,
• insignificant trees (fillers only),
• poles,
• cover or nurse trees, which are harmless species, and
• weed trees, which are undesirable.
This classification was made for natural forests and
is not really applicable for plantations.
Although there is no experience available on the
tending and thinning of dipterocarp plantations outside
India, some inferences can be made from tending and
thinning experiments and from observations in naturally
regenerated dipterocarp forests, which lead to more or
less even-aged and fairly regular stands. Such stands may