Physiological synergies in tolerance to freezing and drought stress reveal greater exaptation of <i>Quercus</i> to seasonally cold environments compared with <i>Lithocarpus</i>

Abstract Diversification in the genus Quercus, which includes tropical‐temperate transitions, is hypothesized to have been enabled by rapid colonization of new niches. To evaluate the role of ecophysiological adaptation to climate, we draw on close relatives in Lithocarpus and assess patterns within evolutionary context. We test two hypotheses: (i) drought and frost tolerance show greater evidence of adaptation to climate in evergreen species of Quercus than Lithocarpus ; and (ii) drought and frost tolerance traits show evidence of correlated evolution, indicating shared mechanisms of stress tolerance across environmental gradients. Focusing on 24 Asian evergreen oaks and stone oaks, we evaluate cold and drought tolerance of leaves sampled from living collections in common conditions. We pair this with natural history records, to derive species' climates of origin. We find frost tolerance is predicted by minimum temperatures within native distributions, but drought tolerance is not predicted by water availability. We detect a correlation between drought and frost tolerance in Quercus but not Lithocarpus , suggesting these trait correlations are not a result of biophysical constraints and find no evidence of correlations on evolutionary time‐scales. Instead, we highlight the role of tolerance synergies in Quercus as providing exaptations for novel climates, facilitating their global contemporary distribution. Read the free Plain Language Summary for this article on the Journal blog.