Drought can cause major damage to plant communities, but species damage thresholds and post-drought recovery of forest productivity are not yet predictable. We asked the question how should forest net primary productivity recover following exposure to severe drought? We used an El Niño drought event as a natural experiment to test whether post-drought recovery of gas exchange could be predicted by properties of the water transport system, or if metabolism, primarily high abscisic acid level, might delay recovery. We aimed to test the hypothesis that recovery of gas exchange would be limited by xylem embolism, carbon limitation or high foliar abscisic acid levels. To do this we monitored sapflow of a single cohort of leaves, as well as leaf level gas exchange, water status and foliar abscisic acid for eight evergreen species through a severe drought event and following subsequent rehydration.
Severe drought caused major declines in leaf water potential, elevated foliar abscisic acid levels and reduced stomatal conductance and assimilation rates in our eight canopy and understory species. Leaf water potential surpassed levels associated with incipient damage to the vascular system in two dominant woody tree species. Following heavy rainfall foliar abscisic acid in all species recovered to pre-stressed rates within one day. Gas exchange of single cohorts of leaves in all sample species, except those trees predicted to have suffered xylem damage, recovered to pre-stressed rates within one day. Thus, recovery of plant gas exchange was predicted by xylem safety margin providing strong support for cavitation vulnerability as an index of damage under natural drought conditions.