COS 180-3 - Heatwaves do not limit recovery following defoliation but alter leaf drought tolerance traits in a deciduous oak

Abstract: Many forest stressors—including biotic attacks and heatwaves (HW)—are expected to increase in frequency due to climate change. Therefore, a better understanding of how multiple stressors interact is needed in order to predict forest responses. Heatwaves are one stressor expected to increase in frequency, making it more likely that they will coincide with disturbances like defoliation. However, 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) pin oak (Quercus palustris) saplings to a simulated spring heatwave of +10°C for 25 days and measured gas exchange, percent leaf area recovery, and carbohydrate storage, turgor loss point (Ψ_TLP), and minimum leaf conductance (g_min).

During the heatwave, stem respiration exhibited stronger thermal acclimation in DEF than UNDEF saplings, while stomatal conductance and net photosynthesis both increased. As a result, the heatwave had very little impact on tree carbon balance, with no impact on leaf area recovery or carbohydrate storage of DEF saplings. The reflush leaves produced by DEF saplings, however, had higher g_min than UNDEF leaves, and this effect was amplified by the heatwave. Across all treatments, higher g_min was associated with higher daytime stomatal conductance and a lower Ψ_TLP. The results suggest defoliation stress may not be exacerbated by higher temperatures. However, reflush leaves appear to utilize a less conservative leaf water use strategy; this may increase C gain and help refill reserves but limit their ability to minimize water loss. While lower Ψ_TLP could help DEF trees maintain gas exchange under mild drought, they may be more vulnerable to dehydration under severe drought.