Tropical tree species vary in how they regulate stem water status, which leads to differential mortality under extreme drought
To prevent drought mortality, trees must maintain their stem water potential (WP) above levels that would cause hydraulic failure (i.e., ~80% loss of stem hydraulic conductivity; P80). In ecosystems with periodic droughts, such as seasonally dry tropical forests, P80 ranges widely among species, suggesting a diversity of drought response strategies. However, species converge in that they maintain similar safety margins between WP and P80. As trees approach P80 during droughts, they are predicted to maintain WP by slowing water loss through stomatal closure and deciduousness and by relying on stored water to buffer WP against incidental water loss. We tested (1) whether species close their stomata and shed leaves as they approach P80 (2) whether trees experience declines in WP after shedding leaves, and (3) whether deciduousness is effective for maintaining WP under extreme drought.
In two seasonally dry tropical forests, we tracked saplings of species that vary in widely P80 for stomatal conductance, leaf area, stem water content, and WP. We also subjected potted saplings to an extreme drought and assessed their response in terms of leaf shedding, stem water loss, root dieback, and mortality.
Among species, the WPs at which trees closed stomata and shed leaves were correlated with P80, suggesting that these are convergent water-use behaviors among seasonally dry tropical forest trees. In the field, most species maintained stabile WP after shedding leaves, whereas one species, Genipa americana, declined to WP < P80 after shedding leaves, putting it at risk of hydraulic failure. This suggests that deciduousness is not universally effective at regulating WP, even during average dry seasons. Under extreme drought imposed on potted saplings, three species (Bursera simaruba, Cavanillesia plantanifolia, and Cedrella odorata) experienced zero mortality. These species shed their leaves, maintained high WP, lost relatively little stem water, and experienced lateral root dieback. In contrast, three species (Cojoba rufescens, G. americana, and Hymenaea courbaril) experienced high mortality (50%, 100%, and 80%, respectively), although they shed leaves when exposed to extreme drought. The C. rufescens saplings that survived the drought had increased lateral root area as opposed to root dieback. This study shows that for deciduousness to be effective at maintaining WP during extreme drought, it must be coupled with belowground responses that slow water loss from roots such as root dieback. Understanding how belowground responses vary among species will improve our predictions of species distributions under climate change.