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.

Automated summary

Due to climate change, extreme droughts have become more frequent, causing widespread tree mortality and significant changes in community structure and function in forests. Hydraulic failure is a major cause of tree death during droughts. However, the effects of hydraulic failure on tree dynamics in tropical forests with different soil water availability during repeated extreme droughts are poorly understood. In this study, the mortality rate and abundance change of 29 tree species were surveyed in a humid tropical ravine rainforest (TRF) and an adjacent dry tropical karst forest (TKF) between 2004 and 2015, during which two extreme droughts occurred. Leaf and stem hydraulic-related traits were measured, and trait-demography relationships were analyzed. The results showed that repeated extreme droughts increased tree mortality and significantly affected changes in species abundance in both forest communities. Tree species with higher stem embolism resistance were more likely to survive extreme droughts and increase in basal area. The two forests exhibited contrasting hydraulic strategies in response to extreme drought, with the TRF having larger leaf and stem hydraulic safety margins, while the TKF had a more negative leaf hydraulic safety margin and stronger vulnerability segmentation. This study enhances our understanding of the hydraulic mechanisms used by tropical tree species to cope with extreme droughts under different substrate conditions and future climate scenarios.