Resistance of a lowland rain forest to increasing drought intensity in Sabah, Borneo

Occasional strong droughts are an important feature of the climatic environment of tropical rain forest in much of Borneo. This paper compares the response of a lowland dipterocarp forest at Danum, Sabah, in a period of low (LDI) andaperiodofhigh (HDI) drought intensity (1986–96,9.98y;1996–99,2.62y).Meanannual drought intensity was two-fold higher in the HDI than LDI period (1997 v. 976mm), and each period had one moderately strong main drought (viz. 1992, 1998). Mortality of ‘all’ trees≥10 cmgbh (girth at breast height) and stem growth rates of ‘small’ trees 10–< 50 cm gbh were measured in sixteen 0.16-ha subplots (half on ridge, half on lower slope sites) within two 4-ha plots. These 10–50-cm trees were composed largely of true understorey species. A new procedure was developed to correct for the effect of differences in length of census interval when comparing tree mortality rates. Mortality rates of small trees declined slightly but not significantly between the LDI and HDI periods (1.53 to 1.48% y−1): mortality of all trees showed a similar pattern. Relative growth rates declined significantly by 23% from LDI to HDI periods (11.1 to 8.6mm m−1 y−1): for absolute growth rates the decrease was 28% (2.45 to 1.77mm y−1). Neither mortality nor growth rates were significantly influenced by topography. For small trees, across subplots, absolute growth rate was positively correlated in the LDI period, but negatively correlated in the HDI period, with mortality rate. There was no consistent pattern in the responses among the 19 most abundant species (n≥50 trees) which included a proposed drought-tolerant guild. In terms of tree survival, the forest at Danumwas resistant to increasing drought intensity, but showed decreased stem growth attributable to increasing water stress.