A review of dipterocarps - Center for International Forestry Research

Plantations 169
leprosula, S. macrophylla and Dryobalanops aromatica
stands at about 20 years age. Additionally, plantation
stands of some other species (e.g., Dryobalanops
oblongifolia, Shorea macroptera) have established
regeneration. However, the exact stand age at first
flowering has not been recorded.
Little information is available concerning
preparatory operations. Chong (1970) reported the effect
of Eugeissona triste (a stemless palm) control on
regeneration of Shorea curtisii. The experiments
showed that a pre-felling treatment with a light girdling
and Eugeissona triste control undertaken after a heavy
seed fall prior to felling had a beneficial effect. The
operation not only increased the vigour of the established
regeneration but also created conditions on the forest
floor conducive to recruitment of new individuals. Raich
and Gong (1990) found that seed germination
demonstrates clear patterns of shade tolerance or
intolerance identical to those long recognised for tree
seedlings. Among the species tested were Dipterocarpus
grandiflorus, Shorea multiflora and Vatica nitens. They
germinated in the understorey as well as in the gaps
(typically 20-30 m in diameter) but failed to germinate
in larger clearings. So, if preparatory canopy openings
are prepared, these openings should not exceed normal
gap size.
Preparatory fellings have never played an important
role. Treatment of seed trees in the natural forests to
improve their crowns is unneccessary because being
emergents they have already fully developed crowns.
More information is available on the manipulation of the
old crop over existing regeneration (regeneration fellings
and final fellings). Although strictly applicable only to
natural forest conditions, the basic findings should also
be valid for plantations. Based on closely controlled
experiments in the Wet Evergreen Forests of Sri Lanka,
Holmes (1945) found that canopy conditions under
seeding fellings most conducive to regeneration seem
to be gaps of 20-30 m diameter evenly distributed and
separated from one another by not more than one row of
dominant trees. While raising the canopy gradually
upwards, an ultimate canopy density of about 0.5 will be
achieved. Zoysa and Ashton (1991) found that the
germination of Shorea trapezifolia seeds planted on
forest top soil with litter was little affected by partial
shade or exposure to full sun. Watson (1931/1932c)
discusses ‘preparatory’ fellings (strictly speaking they
were regeneration fellings) for fostering natural
regeneration within plantations. He states that seedlings
of commercial species would establish better after
opening the forest canopy, provided care is taken to
prevent intrusion of weed species. He recommends
removal of the lower forest canopy layers and cleaning
of the undergrowth. But no fellings of this kind should
be done in the absence of natural regeneration. Based on
experiments of girdling understorey and upper storey
trees, it was concluded that improvement systems should
ensure adequate regeneration while retaining the canopy
in such a condition that the lower storey is shaded
preventing growth of competing vegetation (Walton
1933, 1936a, b). Only after regeneration is abundant
should any drastic opening of the canopy be undertaken.
The vigorous response of seedling regeneration of
Shorea spp. to full light indicates that treatment should
aim at removing the canopy as rapidly and completely as
is considered safe. The extent of canopy opening,
however, should depend on the light demand/shade
tolerance of the species. Strugnell (1936a) investigated
the effect of suppression on young regeneration of
Shorea leprosula, S. parvifolia and Neobalanocarpus
heimii. Removal fellings should not be delayed for too
long in light-demanding species as mortality will be high
and growth responses weak. Shade tolerant species may,
however, react vigorously even after a long time of
suppression. In some species sudden exposure on canopy
opening might lead to shoot borer attack as in
Neobalanocarpus heimii (Durant 1939). Qureshi et al.
(1968) emphasise that, before commencing tending
operations on the regeneration, the canopy density has
to be reduced to ensure sufficient light for the young
plants. This was tested on natural regeneration of Shorea
robusta under a planted parent stand. In mixed stands
smaller gap sizes will favour shade tolerant species and
larger gap sizes light demanding species. This is an
important consideration if mixed stands of shade tolerant
and light demanding species are to be regenerated (e.g.,
Raich and Gong 1990).
The design of the regeneration system for
dipterocarp plantations depends, apart from the
production goal, on several other factors, e.g., the species
involved, the stand condition, the regeneration behaviour
and site factors. A uniform shelterwood system could,
for example, be applied to Dryobalanops aromatica
stands (Zuhaidi and Weinland 1993). They usually carry
a fairly dense regeneration that is evenly distributed over
the stand area. The canopy of the old crop is distinctly