Extreme climatic events threaten forests and their climate mitigation potential globally. Understanding the drivers promoting ecosystem stability is therefore considered crucial for mitigating adverse climate change effects on forests. Here, we use structural equation models to explain how tree species richness, asynchronous species dynamics, species-level population stability and drought-tolerance traits relate to the stability of forest productivity along an experimentally manipulated species richness gradient ranging from 1 to 24 tree species. Tree species richness improved community stability by increasing asynchrony. That is, at higher species richness, inter-annual variation in productivity among tree species buffered the community against stress-related productivity declines. This effect was positively related to variation in stomatal control and resistance-acquisition strategies among species, but not to the community-weighted means of these trait syndromes. The identified mechanisms by which tree species richness stabilizes forest productivity emphasize the importance of diverse, mixed-species forests to adapt to climate change.

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 with distinct species pools. We sampled six random extinction scenarios allocated to two sites (A and B; three at each site) with a total of 396 plots and 158,400 planted trees. 21 plots were excluded prior to our analysis due to failed establishment success of the planted trees, which left 375 plots (n=218 mixtures and n=157 monocultures) for our analysis. Individual tree basal diameters, tree height and species identity were measured annually from 2010 (site A) and 2011 (site B) onwards until 2019 at the end of the growing season. To avoid edge effects, the central 12×12 trees were measured for each plot in the 4-, 8-, 16- and 24-species mixtures, while a smaller group of the central 6×6 trees was measured for monocultures and 2-species mixtures.

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).

Individual tree basal diameters, tree height and species identity were measured annually from 2010 (site A) and 2011 (site B) onwards at the end of the growing season until end 2019. Our productivity estimates cover a complete series of forest growth over the course of 9 and 8 years for site A and B, respectively.

This study used 40 native tree species, planted with seedlings raised from seeds collected from nearby forests. Taxonomic information is available in Supplementary Material.

In this study, we analyzed the direct and indirect effects of tree species richness, asynchrony, average species-level population stability, drought-tolerance diversity and the community-weighted means of drought-tolerance traits on the stability of forest community productivity.