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