Leaf drought and heat tolerance are integrated across three temperate biome types.

Leaf-scale heat and drought tolerance provide direct measures of the ability to withstand environmental stress and can be used to evaluate plant susceptibility to emerging climatic extremes. However, recent droughts increasingly occur with heatwaves, causing plants to withstand two simultaneous environmental stresses. Tolerance of leaf-level processes to heat and drought stress have mostly been studied independently, preventing an understanding of whether tolerance co-occurs for these two environmental stresses. To address this, we measured leaf photosynthetic heat tolerance as the critical temperatures at which photosystem II efficiency starts to decrease (T) and shows a decrease of 50% (T) or 95% (T) in three temperate biomes (desert, oak-pine forest, and mediterranean-type shrubland). We also characterized drought tolerance as the water potential at leaf turgor loss point (π) and cellular membrane stability in response to simulated drought. We found coordination of heat and drought tolerance through a significant relationship of π with T and T that varied with season, whereas T showed no relation to π. Species with greater drought tolerance also showed greater membrane stability, implicating membrane leakiness as a potential mechanism of physiological decline during stress. Despite local variation in temperature and precipitation extremes, leaf heat and drought tolerance converged to common cross-biome relationships, providing evidence of interdependence that spanned distinct climates.