Thermal optimum and responses to extreme temperature events of forest species in Israel: improving thermotolerance of new plantations to climate change (funded by KKL JNF 2022-2025).

Climate change predictions for the next century include an ongoing warming of air temperatures, on the order of 2–6 °C in the Mediterranean basin. Increasing air temperatures have the potential to alter forest ecosystem function and structure by exceeding the optimal temperatures for photosynthesis. Trees' acclimation to warming depends on the timing of warming during the season, the tree's thermal optimum for growth and photosynthesis, and its physiological ability to respond to extreme temperature events such as heat waves. The proposed research hypothesis is that many of the Mediterranean species operate beyond their thermal optimum (Topt), resulting in a reduced photosynthetic capacity during the warm seasons. On the other hand, a warming climate benefits productivity of thermophilic species that display a significantly higher Topt, including those that originated from Sudanic and Irano-turanian climates (such as Ziziphus spina christi and Acacia species). However, there is an evident gap of knowledge concerning Topt for photosynthesis and growth capacity of the common forest tree species in Israel. The aim of the proposed study is to identify the optimal conditions for photosynthesis and growth of key species in KKL plantations (Ambassadors and HaKdoshim forests), by using combined ecophysiological measurements of gas exchange and thermal sensing. We aim to investigate whether summer temperatures in Israel reach or exceed the threshold of thermal tolerance of key species. To address these questions, under controlled environmental conditions, we will study Topt of the studied species in a greenhouse under the following temperature regimes: 34/28°C, 28/22°C, 22 /16°C and 16/10°C, using state-of-the art physiological tools and modeling. The results of this study will allow educated and optimal selection of species for future afforestation of individual habitats, to achieve the best possible growth and increase carbon sequestration.