Leaf isotopes reveal tree diversity effects on the functional responses to the pan-European 2018 summer drought

Recent droughts have strongly impacted forest ecosystems and are projected to increase in frequency, intensity, and duration in the future together with continued warming. While evidence suggests that tree diversity can regulate drought impacts in natural forests, few studies examine whether mixed tree plantations are more resistant to the impacts of severe droughts. Using natural variations in leaf carbon (C) and nitrogen (N) isotopic ratios, that is δ¹³C and δ¹⁵N, as proxies for drought response, we analyzed the effects of tree species richness on the functional responses of tree plantations to the pan-European 2018 summer drought in seven European tree diversity experiments. We found that leaf δ¹³C decreased with increasing tree species richness, indicating less drought stress. This effect was not related to drought intensity, nor desiccation tolerance of the tree species. Leaf δ¹⁵N increased with drought intensity, indicating a shift toward more open N cycling as water availability diminishes. Additionally, drought intensity was observed to alter the influence of tree species richness on leaf δ¹⁵N from weakly negative under low drought intensity to weakly positive under high drought intensity. Overall, our findings suggest that dual leaf isotope analysis helps understand the interaction between drought, nutrients, and species richness.