PRIMARY LOWLAND DIPTEROCARP FOREST AT DANUM-VALLEY, SABAH, MALAYSIA -SPECIES COMPOSITION AND PATTERNS IN THE UNDERSTOREY

The spatial distributions of species of tree greater than or equal to 10 cm gbh were examined in two 4 ha plots and related to the local var iation in topography and soil chemistry. The plots were similar in the ir species composition, particularly in terms of the densities of smal l trees, and they showed very similar edaphic characteristics. Size cl ass distributions varied little within and between plots. Ordination o f 0.25 ha subplots highlighted parallel gradients in the vegetation of both plots when the densities of trees greater than or equal to 10 cm gbh were considered. Focusing on understorey trees in the 10-<50 cm g bh class at the 0.04 ha subplot scale showed a similar vegetation grad ient in both plots closely associated with change from lower slope to ridge. No relationship with soil chemistry was found. On the ridges a special group of understorey species formed clumps and these species c ontributed importantly to the ordinations. Borneo has a regional histo ry of occasionally severe droughts. It is suggested here that the obse rved patterns in the understorey are due to differential responses to low soil water supply, the ridges probably tending to dryness more tha n the lower slopes. Within the large and diverse family Euphorbiaceae, which dominates the understorey at Danum, there may be ecophysiologic al groupings of species. The long-term effects of disturbance interact ing with local edaphic factors on forest structure and composition are discussed.