Impact of severe drought associated with the 1997-1998 El Nino in a tropical forest in Sarawak

The impact of the unusually severe drought associated with the 1997-1998 El Nino on tropical forest dynamics in Sarawak, Malaysia was examined. Mortality during the non-drought period (1993-1997) in a core plot (1.38 ha) was 0.89 % y ^-1, while that during the drought period (1997-1998) in the same plot and a peripheral plot was 6.37 and 4.35 % y ^-1, respectively. The basal area lost in the drought interval was 3.4 times that of the annual incremental basal area in 1993-1997. Drought mortality was higher for the smaller trees, though it was less size dependent than the non-drought mortality. Dipterocarpaceae, which is the dominant family in the study plot, had a mortality 12-30 times higher in the drought than the non-drought period. There were no significant differences in mortality among the topographic types. From the results of a log-linear model (multi-factored contingency table), the death of trees was correlated with size class, indicating a change in the size-class structure of the forest. Thus, both the species composition and structure are totally affected by such an episodic drought even in a per-humid tropical forest.