A review of dipterocarps - Center for International Forestry Research

Plantations 167
have resulted from, e.g., Regeneration Improvement
Systems or from Uniform Shelterwood Systems, as they
were, for example, applied in Malaysia. Wyatt-Smith
(1963b) gives a thorough review of the thinning
experience up to that time. His recommendations for
thinning more or less regular crops were:
• removal of climbers of above 2.5 cm diameter, although
the limit can be lower if smaller climbers prove
to be damaging the crop trees,
• removal of all weed trees; also those that are going to
overtop the PCT until the the next intervention,
• removal of all malformed stems of commercial species
provided a stem of better form is adjacent,
• removal of all wolf trees,
• removal of co-dominants of inferior timber value,
• selective thinning of co-dominants of equivalent
silvicultural and timber value that compete strongly,
and
• thinning to a maximum basal area of about 1/2 to 3/4
of the expected carrying capacity of the site.
In the context of regeneration operations within the
Regeneration Improvement Systems Durant (1940) was
confronted with the criticism that opening the canopy
would lead to luxuriant ‘secondary growth’ (what we
would call today secondary forest) consisting mainly of
Randia scortechenii, Pasania sp., Barringtonia sp.,
Girroniera nervosa, Trema ambionensis, Macaranga
spp., Endospermum malaccense and various fast-growing
trees of other families. It was feared that the young
dipterocarps might be suppressed by these species and
frequent and expensive cleanings needed. Three
experimental plots were set up. Two plots were
established in stands where the canopy over young
regeneration had been removed by regeneration
improvement fellings and one plot was laid out in an area
where the canopy over young regeneration had almost
completely been removed by a heavy storm. The
treatments in the first plot were: (i) untouched control,
(ii) cleaning (cutting back all growth other than Shorea
spp.), and (iii) cleaning and respacing (‘thinning’ of the
Shorea spp. to an average distance of 1.83 m leaf to leaf).
The treatments in the second plot were: (i) untouched
control, (ii) cleaning (cutting back everything except
saplings of the desirable species), and (iii) mainly climber
cutting with minimal cutting of undergrowth. In the third
plot only a cleaning in favour of saplings and small poles
was carried out. The objective of the first two plots was
to investigate the effect of the secondary forest vegetation
on survival and diameter growth of sapling-size natural
regeneration of Shorea spp. The third plot tested whether
larger regeneration (large saplings, small poles) was out
of danger from its competitors. After establishment, the
plots were left unattended for four years and then
enumerated again.
From Durant’s experiment, inferences were made
concerning the regeneration of S. leprosula:
• However severe the opening of the canopy, provided
adequate seedling regeneration is present, S. leprosula
can tolerate competition with other vegetation up to
the sixth year.
• Cleaning and thinning after the second year will secure
an even distribution of stocking and will increase
the growth rates. Complete omission of tending up to
the sixth year is not fatal (which is in agreement with
other authors e.g., Walton 1933, 1936a, Wyatt-Smith
1949b, 1958, 1963b).
• Serious competition from secondary forest species
is probably due to a comparatively few species, and, if
these can only be recognised and eliminated, a considerable
reduction of cleaning costs should be possible.
(The species recognised as responsible for suppression
were Endospermum malaccense,
Elaeocarpus stipularis, Macaranga spp., Paropsia
varediformis and Quercus lucida).
• With sufficient initial opening of the canopy, good
stocking of Shorea leprosula can be expected to survive
up to the 14th year. At this stage the crop reaches
pole size, and adequate assistance can be given very
cheaply by the poison-girdling of competitors around
individual trees.
The conclusions are important for the tending of
young naturally regenerated and more or less even aged
stands originating either from natural stands or from
plantation stands under the Shelterwood System. The
findings of Durant (1940) can, however, not be applied
without some restrictions to young plantations of
dipterocarps. The initial number of stems in plantations
is usually so low that omission of early tendings
(weedings, cleanings) will probably entail high losses
endangering stand establishment.
Strugnell (1936b) tried three treatments (only
dominant trees retained; dominant and dominated trees
retained; dominant, dominated and suppressed trees
retained) in a young natural pole stand of Shorea leprosula
and S. parvifolia. He found that the basal area of the 50
largest trees/acre was highest for the medium