Hydraulic diversity stabilizes forest productivity in a large-scale subtropical tree biodiversity experiment

Extreme climatic events such as droughts threaten forests and their climate mitigation potential globally. Stability, the ability of forests to maintain functioning in periods of stress, is therefore expected to be a primary focus of forest management in the 21st century. A key management strategy suggested for enhancing stability may be to increase tree species richness in secondary and plantation forests. Here, we aim to understand the drivers that may promote forest stability in mixed-species tree communities. We use structural equation models to explain how tree species richness, asynchronous species dynamics and diversity in hydraulic traits affect the stability of yearly forest productivity along an experimentally manipulated biodiversity gradient ranging from monocultures up to mixtures of 24 tree species. Tree species richness improved stability by increasing species 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 mediated by diversity in species’ hydraulic traits in relation to drought tolerance and stomatal control within the community, but not by the community-weighted means of these hydraulic traits. The examined hydraulic traits may be used to select suitable tree species and design mixtures that stabilize productivity in an increasingly variable climate through diverse response strategies, while excluding those that would succumb to drought or competition. The identified mechanisms by which tree species richness stabilizes forest productivity emphasize the importance of hydraulically diverse, mixed-species forests to adapt to climate change.