Heatwaves do not limit recovery following defoliation but alter leaf drought tolerance traits

As heatwave frequency increases, they are more likely to coincide with other disturbances like insect defoliation. But it is unclear if high temperatures after defoliation impact canopy recovery or leaf traits which may affect response to further stressors like drought. To examine these stressor interactions, we subjected defoliated (DEF) and undefoliated (UNDEF) oak saplings to a simulated spring heatwave of +10 degrees C for 25 days. We measured gas exchange, leaf area recovery, carbohydrate storage, turgor loss point (psi TLP), and minimum leaf conductance (gmin). During the heatwave, stem respiration exhibited stronger thermal acclimation in DEF than UNDEF saplings, while stomatal conductance and net photosynthesis increased. The heatwave did not affect leaf area recovery or carbohydrate storage of DEF saplings, but reflush leaves had higher gmin than UNDEF leaves, and this was amplified by the heatwave. Across all treatments, higher gmin was associated with higher daytime stomatal conductance and a lower psi TLP. The results suggest defoliation stress may not be exacerbated by higher temperatures. However, reflush leaves are less conservative in their water use, limiting their ability to minimise water loss. While lower psi TLP could help DEF trees maintain gas exchange under mild drought, they may be more vulnerable to dehydration under severe drought. Leaf area recovery and carbohydrate storage after spring defoliation were not impacted by a coinciding heatwave. However, reflush leaves had higher minimum rates of water loss-and this effect was exacerbated by the heatwave, suggesting reduced drought tolerance.