Mixed-species forests are promoted as a forest management strategy for climate change adaptation, but whether they are more resistant to drought than monospecific forests remains contested. Particularly, the trait-based mechanisms driving the role of tree diversity under drought remain elusive. Using tree cores from a large-scale biodiversity experiment, we investigated tree growth and physiological stress responses (i.e. increase in wood carbon isotopic ratio; δ13C) to changes in climate-induced water availability (wet to dry years) along gradients in neighbourhood tree species richness and drought-tolerance traits. We hypothesized that neighbourhood species richness increases growth and decreases δ13C and that these relationships are modulated by the abiotic (i.e. climatic conditions) and the biotic context. We characterized the biotic context using drought-tolerance traits of focal trees and their neighbours. These traits are related to cavitation resistance vs resource acquisition and stomatal control. Tree growth increased with neighbourhood species richness. However, we did not observe a universal relief of water stress in species-rich neighbourhoods. The effects of neighbourhood species richness and climate on growth and δ13C were modulated by the traits of focal trees and the traits of their neighbours. At either end of each drought-tolerance gradient, species responded in opposing directions during dry and wet years. We show that species’ drought-tolerance traits can explain the strength and nature of biodiversity-ecosystem functioning relationships in experimental tree communities experiencing drought. Mixing tree species can increase growth but may not universally relieve drought stress.

We used data from a large-scale forest biodiversity experiment with a gradient of species richness ranging from monocultures up to mixtures of 24 tree species planted at two sites (A and B). We used this experimental set-up to select 15 focal tree species along two principal trait gradients related to species’ drought tolerance: a resistance-acquisition and a stomatal control trait gradient. We further used focal trees and their neighbours to create a realised neighbourhood species richness (NSR) gradient of 1-, 2- and 4-neighbour species. We sampled 7 tree species at site A and 7 species at site B and one species (Schima superba) at both sites to test for potential differences between sites. From all focal trees, we extracted one increment core in 2019 at diameter at breast height (dbh) using a 3-threaded Haglöf increment borer with 3.5 mm core diameter. We measured tree-ring width and stable carbon isotopes on these cores and used them as indicators of focal tree responses to the interactive effects of climate, NSR and drought-tolerance traits. The final, completely balanced dataset comprised 336 trees, with 7 trees per species and NSR-level from 114 plots. See methods for more details on the experimental design.

Main experiment of the Biodiversity–Ecosystem Functioning Experiment China (BEF-China, www.bef-china.com) at Xingangshan, Dexing, Jiangxi (29° 08′–29° 11′N, 117° 90′–117°93′E).

This study used tree cores from 15 native tree species, planted with seedlings raised from seeds collected from nearby forests. Taxonomic information is available in Supplementary Material.