Bole
To the observer on the forest-floor bole-form is usually more or less columnar, at least up to the lowest limbs, and he feels as though in a lofty greenroofed cathedral. In fact there is usually some degree of taper, as can readily be seen on felled trees and for which allowance must be made when constructing volume tables for forestry purposes. Buttress-height, spread, thickness, and surface form are generally fairly constant within a species and therefore, like crown-form, buttresses are a valuable guide to forest identification. A simple practical classification was introduced by Wyatt-Smith (1954b; see also Whitmore 1966a). Within dipterocarp lowland evergreen rain forest in Malaya 40,7 per cent of t8 067 'timber-size' trees measured had buttresses reaching higher than 1'35 m ; the range was from 87'8 per cent of Intsia palembanica stems down to only 0,8 per cent of Calophyllum (Setten 1953). For Koompassia malaccensis incidence of buttresses varied from 3; per cent (Kemahang) to 93 per cent (Bukit Mambai), and buttress-height increased with girth in this species, also in Dryobalanops aromatica, Shorea leprosula, and S. parvifolia but not in S. curtisii (Setten 1954a,b). |
Tree form
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There is singularly little evidence from which to judge the truth or otherwise of the common generalization (for. example, of Corner 1940, Richards 1952) that trees with deep tap roots do not form buttresses, and vice versa. Henwood (1973) has recently demonstrated by the analysis of the stresses to which a tree is subject, using a simple model and a formulation borrowed from engineering, that buttresses are indeed structural members which help to support trees on substrates that offer poor anchorage, as has long been suspected. They act as tension members which reduce the strain on the roots on which they occur (and indeed they resonate when hit with a bush knife).
They thus make these roots less susceptible to withdrawal or breakage under strain. Flowering plants are known to form tension wood rather than compression wood. Buttresses develop on the windward side, Where there is a prevalent wind direction, on the uphill side on slopes (for example, on all trees in an extensive stand of' Shorea curtisii at Semangko forest, Malaya (Burgess)).
In trees with asymmetric crown or a heavy asymmetric climber load, buttresses can he expected to form on the side opposite to the hulk of the crown or climbers. The different incidence of buttresses in different species can he ascribed to intrinsic differences in construction of the root system and in the tensile strength of the root wood. Differences between sites and individuals reflect substrate and loading differences. The incidence and size of buttresses diminishes with altitude (Table 16.1, p. 199), part of the explanation must be that smaller trees have a lesser need for support.
Bark
It is still a common fallacy that all or most rain forest trees have pale, thin, and smooth bark. This is far from the case. In fact, within rain forest there is a rich range of colours and shades from black (Diospyros) to white (Tristania), through fawn and bright rufous brown (Eugenia). It is only trunks exposed outside the forest microclimate, such as are possessed by isolated trees and at forest fringes, which are bleached to a uniform pale grey. Saplings and small poles do have thin, smooth bark.
Trunks over about 0,9 m girth exhibit a great diversity of surface sculpturings, crudely summarized as fissured , scaly , or dippled, and some are smooth . After leaf hark surface characteristics and the appearance in section (`slash' or `blaze') are the most important aids to forest recognition of species (Whitmore _ 1962d, 1972c) and may have significance for taxonomy (Whitmore 1963). Some families are fairly homogeneous in bark (for instance, most Lauraceae are smooth) and others show the whole gamut of patterns (for example, Dipterocarpaceae (Whitmore 1963)). It is now known that smooth barks are slowly growing (Whitmore 1962a,b,c, 19720 with the surface long-persistent, and these are the barks on which lichens grow, often as conspicuous splashes of colour .
Went (1940) reviewed by Richards (1952) and van Steenis (1972) demonstrated that in the mountain forest at Cibodas in west Java, different tree species have different epiphytes and that this is most likely due to water-soluble chemicals in the bark. By contrast, epiphytes growing on humus were far less specific in their hosts. This interesting work has still not been followed up or extended; some bark microlichens appear to be specific and aid, by their colour, the forest recognition of particular species, for example, the dark-green lichen on Diospyros.
REFERENCE :
Whitmore, T.C, 1975, Tropical Rain Forests of the Far East , 1st Edition, Oxford University Press, Oxford.
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