Drought and heat-induced mortality of conifer trees is explained by leaf and growth legacies

An increased frequency and severity of droughts and heatwaves have resulted in increased tree mortality and forest dieback across the world but underlying mechanisms are poorly understood. We used a common garden experiment with 20 conifer tree species to quantify mortality after three consecutive hot, dry summers and tested whether mortality could be explained by putative underlying mechanisms, such as stem hydraulics and legacies affected by leaf lifespan and stem growth responses to previous droughts. Mortality varied from 0-79% across species and was, surprisingly, not affected by hydraulic traits. Mortality increased with the species’ leaf lifespan, probably because leaf damage caused crown dieback and contributed to carbon depletion and sensitivity to bark beetle damage. Mortality also increased with lower growth resilience, which may exacerbate the contribution of carbon depletion and bark beetle sensitivity to tree mortality. Our study highlights how ecological legacies at different time scales can explain tree mortality in response to hot, dry periods and climate change.