Spatial variations in the net ecosystem productivity of a primeval tropical forest in a 50-ha plot in Pasoh, Peninsular Malaysia

We assessed the spatial variations in net ecosystem productivity (NEP) using a biometric method based on the dynamics of coarse woody organs in a primeval lowland tropical rain forest in Pasoh, Peninsular Malaysia. The average NEP and carbon stocks in biomass and coarse woody debris over 15 years from 1990 to 2005 were -0.75 t C ha y, 205 t C ha, and 22 t C ha, respectively. However, there were large spatial variations among the five communities, which were correlated with edaphic and topographical features. A mature carbon cycling system with high biomass (where NEP ≈ 0) was observed in a subcommunity on west-facing slopes of the hill community. Over the course of 15 years, a drought and storms were the major agents causing forest disturbances. These factors affected the spatial variations of NEP through not only spatial distribution patterns in species with different sensitivities to stress but also differences in the extent of stress depending on topography. We modeled the disturbance regime as elapsing from 1907 to 1947 basing on observed values of net primary production and biomass of five communities during the 15 years. When we extrapolated high mortalities of trees under this regime, our simulation suggested that a 50-ha plot will remain a carbon-source system for the atmosphere at an average rate of NEP=-0.95 t C ha y until 2060, with different biomass decreases among these five communities.