Hydraulic determinants of drought-induced tree mortality and changes in tree abundance between two tropical forests with different water availability

Because of climate change, extreme droughts have become more frequent and caused widespread tree mortality, and thus significant shifts in community structure and function in many forests. Hydraulic failure is a major cause of drought-induced tree mortality. However, its effects on tree dynamics during repeated extreme droughts in tropical forests with different soil water availability are poorly understood. In this study, we surveyed the mortality rate and change in abundance (basal area) for 29 tree species from a humid tropical ravine rainforest (TRF) and an adjacent dry tropical karst forest (TKF) between 2004 and 2015. During this period, two extreme droughts occurred. We measured leaf and stem hydraulic-related traits and analyzed the trait-demography relationships. Our results showed that repeated extreme droughts induced an increase in tree mortality, and thus significantly affected the change of species abundance in the two forest communities. In both forests, the tree species with higher stem embolism resistance were more likely to survive extreme droughts, and increased in basal area during the census period. Moreover, the tree species in the two forests exhibited contrasting hydraulic strategies to deal with extreme drought. The lower mortality rate was associated with larger leaf and stem hydraulic safety margins in the TRF, but with more negative leaf hydraulic safety margin and stronger vulnerability segmentation (larger difference in embolism resistance between leaf and stem) in the TKF. This study expands our understanding of the hydraulic mechanisms to cope with extreme droughts in tropical tree species grown in different substrates under the present and future climatic conditions.