Stem water content predicts drought‐induced mortality in tropical semiarid species

Hydraulic Failure is one of the primary mechanisms underlying plant mortality during drought. Hydraulic failure occurs when the plant's xylem, under water stress, crosses a critical percentage of loss of xylem hydraulic conductivity (PLC) threshold, which causes the plant's tissues to desiccate and die. The PLC threshold that triggers mortality varies between different taxa and functional types and remains largely undefined for plants from tropical seasonally dry formations (TSDF)—biodiverse and drought‐prone ecosystems. Recent studies have proposed that stem Relative Water Content (RWC) thresholds might be more useful predictors of drought‐induced mortality due to their direct link with cell mortality. This implies that a different set of functional traits, which control plant tolerance to dehydration, also plays an important role in plant drought‐induced mortality. This study investigated whether PLC or RWC thresholds predict drought‐induced mortality more accurately for two TSDF species with contrasting leaf phenology. We conducted a drought experiment with two TSDF species, the drought‐deciduous Cordia oncocalyx and the evergreen Cynophalla flexuosa . Our results reveal that both species can survive PLC >99%, suggesting that extreme embolism alone does not always lead to mortality. However, the evergreen species succumbed when its stem Relative Water Content (RWC) declined close to 37%. The deciduous species withstood a PLC over 99% for more than 20 days and recovered rapidly after rehydration, likely due to its higher stem water storage capacity and quick leaf shedding. Our findings show that maintaining a high RWC, rather than only avoiding hydraulic failure, is key to the survival of TSDF species during drought. The deciduous species' leaf shedding and higher stem water content allowed it to sustain its RWC for longer during drought, making it less vulnerable to drought‐induced mortality. The RWC thresholds identified in this study provide a key parameter to improve model projections of TSDF vegetation mortality during future droughts. Read the free Plain Language Summary for this article on the Journal blog.