Effects of "Selective" Timber Extraction on Rain Forest Structure and Composition and Some Consequences for Frugivores and Folivores

Abstract

Mechanized logging in tropical rain forest typically removes only a small percentage of timber trees, but incidental damage may be severe. In a West Malaysian dipterocarp forest, extracting 3.3 percent of trees destroyed 50.9 percent, and damage was spread equally among all tree taxa and all size classes. The overall availability of food sources for frugivores and folivores is thus drastically reduced, even where timber trees are not themselves used by animals. To some extent, the initial loss of food sources may be buffered by increased reproductive and vegetative activity of those trees that remain. SELECTIVE LOGGING HARVESTS THE MOST VALUABLE TREES from a forest area. Under sustained-yield conditions, the residual stand is then left to regenerate, either naturally or with certain management procedures designed to promote the reestablishment of commercially important timber species (Baur 1964). It is the commonest form of forest exploitation throughout the humid tropics (UNESCO 1978), although the techniques employed, the number of timber trees cut per unit area, and overall damage caused to the environment vary considerably. The environmental effects of typical logging operations have not been widely studied. Most available information concerns the commercially important timber trees and is directed toward improving forest productivity or other aspects of forest management (e.g., UNESCO 1978, Whitmore 1984). Damage levels resulting from mechanized logging in dipterocarp forest are generally estimated at 50 percent of the stand (Ewel & Conde 1976), although they are dependent on the extraction level (Nicholson 1958, Burgess 1971). It is the damage associated with logging that has the greatest impact on the rain forest environment and may be the crucial factor determining whether sustained-yield forestry is feasible (Skorupa & Kasenene 1984, Whitmore 1984). The effects of logging operations on rain forest wildlife also have not been widely studied, and almost all available information concerns vertebrate species (Johns 1983a, b, c, 1985, 1986a, b; Skorupa 1986). This paper examines the ecological effects of a logging operation in West Malaysian dipterocarp forest. It considers both the changing structure and composition of the forest itself, and how these changes affect the food resources available to frugivorous and folivorous mammals and birds. STUDY AREA The study was conducted in the Sungai Tekam Forestry Concession, Pahang, West Malaysia (4?10'N, 102?40'E). The main study site (Compartment 13C) was located on the edge of a ridge rising from the old floodplain of the Pahang River, and varied from 300 to 450 m ASL. The vegetation was primarily hill dipterocarp forest (see Symington 1943, Whitmore 1984), although it lacked the characteristic aggregations of the dipterocarp Shorea curtisii. Well-drained soils were dominated by other Shoreai spp. and by emergent legumes such as Koompassia spp. and Intsia palembanica; less well-drained soils supported lower-stature vegetation with a predominance of euphorbs and lianas. Species diversity was extremely high; 181 species of 110 genera were identified within sample transects of 2.08 ha total area (Johns 1983a). Selective logging began at this main study site after 14 mo of the 26-mo study period, and the process took 6 mo to complete. To supplement results from this main study site, botanical and zoological surveys were carried out in nearby forest (originally part of the same forest block) that had been logged 1-2 yr (C5A), 3-4 yr (CIA), and 5-6 yr (C2) previously. The of logging in use at Sungai Tekam is a selective forest management system (Canonizado 1978; for description see Whitmore 1984). Logging is highly mechanized, using both heavy bulldozers and winch rigs in which cables attached to ridgetop spar-trees are used to drag logs up steep slopes to loading areas. The relies largely upon natural regeneration of the forest, supplemented by replanting severely damaged areas with nursery-reared seedlings to enhance the timber yield in subsequent croppings. A fast-growing exotic species, Eucalyptus deglupta, is currently used for this purpose in preference to local hardwoods. Planting of Eucalyptus minimizes problems of destructive browsing by deer (Dykstra & Taylor 1976). BIOTROPICA 20(1): 31-37 1988 31 I Received 8 April 1985, revision accepted 11 September 1986. This content downloaded from 157.55.39.211 on Tue, 27 Sep 2016 03:59:52 UTC All use subject to http://about.jstor.org/terms TABLE 1. Causes of tree mortality in the main study area.% total number of trees