Effects of Logging, Drought, and Fire on Structure and Composition of Tropical Forests in Sabah, Malaysia

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Abstract
Extensive tracts of tropical rain forest were burnt in Borneo during the El Nino drought of 1983. Severe droughts have occurred previously but without causing such extensive fires. This extensive burning is a result of forests becoming more fire-prone after disturbance by logging. Rates of tree mortality after drought and fire ranged from 38 to 94 percent in logged forests and from 19 to 71 percent in unlogged forests, while for saplings rates of mortality exceeded 80 percent in both forest types. Secondary succession after logging was truncated by fire with the result that the post-fire condition of a forest logged six years before fire was similar to that of a forest logged two years before fire. The impact of the fire differs from other natural or man-made gap forming processes in that most pre-existing seedlings and saplings are killed by fire. This left the regrowth in burnt forest depauperate in species diversity and in regeneration of upper-canopy species. Logged and burnt forests suffered severe canopy loss and the ground cover was dominated by grasses (e.g., Imperata cylindrica) or woody creepers (e.g., Eupatorium odoratum). In burnt primary forest, however, canopy loss was less severe and there was a low density of grasses. The prospects for recovery of forest structure appear to be good in burnt primary forests although species composition may be permanently altered. In forests logged prior to the fire, however, prospects for recovery of forest structure are not good, especially if further burning occurs. The recovery of these forests depends heavily on the ability of the secondary tree species to shade out the vigorous grasses, the continued presence of which may herald the conversion of the forest to unproductive grassland as has occurred widely elsewhere in the tropics after over-intensive shifting cultivation.