Acclimation to drought and warming alters stress response to heatwaves in tree seedlings

Increasing frequency and intensity of droughts and heatwaves worldwide cause reductions in forest productivity, threatening the ecosystem services that forests provide and their carbon sequestration potential. A better understanding of the trait variability in space and time would improve the predictability of models on individual and ecosystem scale. We looked at the acclimation potential of drought and heat tolerance in three important tree species in Sweden: Picea abies, Betula pendula and Quercus robur. We hypothesized that i) tree species can acclimate to warming and drought leading to better resistance to heatwaves, and ii) heat and drought tolerance are coupled and that acclimation to drought would lead to increased heat tolerance and vice versa. In a climate chamber experiment in southern Sweden, seedlings of the three species were planted in pots and exposed to two different temperatures, and a well-watered and drought treatment. After a three-month acclimation phase, the trees were exposed to two consecutive heat waves with a two-week recovery period. Before and after acclimation and heatwaves, drought and heat tolerance indicators were measured, such as turgor loss point (ψ_TLP) and leaf thermal tolerance (T50). Leaf drought and heat tolerance acclimated to respective treatments, with different magnitudes depending on species. The three species also had different survival strategies to heatwaves. The results reveal the acclimation potential of tree species to drought, warming and the combination. They underline the importance to consider trait acclimation and the combined effects of drought and heat on tree physiology in plant and ecosystem models.