Reduced soil moisture drives leaf anatomical shifts more than chronically elevated temperatures in European temperate trees

Chronic reductions in soil moisture combined with high air temperatures can modify tree carbon and water relations. However, little is known about how trees acclimate their foliar structure to the individual and combined effects of these two climatic drivers. We used open-top chambers to determine the multi-year effects of chronic air warming (+5 °C) and soil moisture reduction (-50 %) alone and in combination on the foliar anatomy of two European tree species. We further investigated how these climatic drivers affected the relationship between foliar anatomy and physiology/chemistry in young downy oak and European beech trees. After four years, reduced soil moisture led to the development of thinner leaves with narrower epidermis and lower gas exchange for oak and beech but to a lesser extent for the latter. In contrast, prolonged warming did not affect the anatomical and physiological/chemical traits in either species. Warming also did not exacerbate the impacts of dry soils, highlighting soil moisture as the key driver in leaf anatomical shifts. While soil moisture altered oak foliar anatomy, and physiology and chemistry of both species, our work revealed a limited acclimation potential towards more drought- and heat-tolerant leaves as conditions get drier and warmer, suggesting potentially high vulnerability of both species to future climate.