Drought-induced mortality of a Bornean tropical rainforest amplified by climate change: stochastic ecohydrological modelling

Southeast Asian tropical rainforests are among the most important biomes in terms of global carbon and water cycling. Drought-related tree mortality at a regional scale causes drastic shifts in these ecosystems, where severe droughts are projected to occur more frequently, especially under El Niño conditions. This study examines how drought-induced mortality of a Bornean tropical rainforest is altered by projected shifts in rainfall, using a combination of field measurements, global climate model (GCM) simulation outputs, and an index developed for drought-induced tree mortality (Tree Death Index η) 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 Niño events, generally occurring in January-March (JFM), and that seasonality in rainfall will become more pronounced, e.g., wet (JFM) seasons becoming wetter and dry (October-December: OND) seasons becoming drier. The computed η reflects well high tree mortality under severe drought during the 1997-1998 El Niño event. For the present, model results demonstrate high and low probabilities of mortality in JFM and OND, respectively, and they predict that the difference in such probabilities will increase in the future. Such increased probability of mortality 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).