Drought-induced mortality of a Bornean tropical rain forest amplified by climate change

Drought-related tree mortality at a regional scale causes drastic shifts in carbon and water cycling in Southeast Asian tropical rain forests, where severe droughts are projected to occur more frequently, especially under El Nino conditions. We examine how the mortality of a Bornean tropical rain forest is altered by projected shifts in rainfall, using field measurements, global climate model (GCM) simulation outputs, and an index developed for drought-induced tree mortality (Tree Death Index h) associated with a stochastic ecohydrological model. All model parameters have clear physical meanings and were obtained by field observations. Rainfall statistics as primary model forcing terms are constructed from long-term rainfall records for the late 20th century, and 14 GCM rainfall projections for the late 21st century. These statistics indicate that there were sporadic severe droughts corresponding with El Nino events, generally occurring in January-March, and that seasonality in rainfall will become more pronounced, e. g., dry (January-March) seasons becoming drier and wet (October-December) seasons becoming wetter. The computed eta well reflects high tree mortality under severe drought during the 1997-1998 El Nino event. For the present, model results demonstrate high and low probabilities of mortality in January-March and October-December, respectively, and they predict that the difference in such probabilities will increase in the future. Such high probability of mortality in the dry season is still significantly high, even considering the beneficial effect of increased soil water storage in the wet season (which is projected to increase in the late 21st century).