SPATIAL DISTRIBUTION OF SOIL MOISTURE AND ITS INFLUENCE ON STAND STRUCTURE IN A LOWLAND DIPTEROCARP FOREST IN PENINSULAR MALAYSIA

In tropical rainforests the pattern and quantity of rainfall influence various ecosystem functions, as well as forest stand structure by controlling soil water status. In the present study, spatial distribution of volumetric soil water content (VSWC), measured over 1 year, was compared with stand structure in a Southeast Asian tropical rainforest. Forest surface-layer VSWC was determined by the relative elevation and soil physical properties. The patterns of spatial variation in surface-layer VSWC and residual VSWC during dry period suggested that drier surface soil areas had developed forest soil texture with larger pore size. There were no relationships between VSWC and tree mortality or number of trees for all soil layer, but a significant negative relationship was found between surface-layer VSWC and basal area, and thus also biomass. This could be due to the preference of trees for drier surface areas with larger median pore size. The positive relationship between mean diameter at breast height (dbh) and deep layer soil water content might indicate the preference of deep layer water by big trees. On the other hand, the number of trees in Dipterocarpaceae, as well as of emergent species trees, was negatively correlated with SWC in deep soil layer.