Species richness and drought-tolerance traits explain opposing tree growth and δ13C responses to climate extremes

The increasing occurrence and intensity of climate extremes, such as droughts, threaten forests and the many services they provide to society. Mixed-species forests are promoted as a forest management strategy for climate change adaptation, but whether they are generally more resistant to drought than monospecific forests remains unresolved. Particularly, the trait-based mechanisms driving tree resistance to drought remain elusive, making it difficult to predict which functional identities of species best improve tree growth and decrease tree physiological water stress under drought.

Here, 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. For this purpose, we sampled tree cores in a large-scale forest biodiversity experiment (BEF-China experiment). We tested the overarching hypothesis that neighbourhood species richness increases growth and decreases δ¹³C. We further hypothesized that the abiotic (i.e. climatic conditions) and the biotic context modulate these biodiversity-ecosystem functioning relationships. 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.

We found that tree growth increased with neighbourhood species richness. However, we did not observe a universal relief of water stress in species-rich neighbourhoods, nor an increase in the strength of the relationship between richness and growth and between richness and δ¹³C from wet-to-dry years. Instead, these relationships depended on both the traits of the focal trees and their neighbours. At either end of the resistance-acquisition and stomatal control trait gradient, species responded in opposing directions during drought and non-drought years.

We report that the biotic context can determine the nature of biodiversity-ecosystem functioning relationships in experimental tree communities. We derive two key conclusions: (1) drought-tolerance traits of focal trees and their neighbours can explain divergent tree responses to drought and diversity, and (2) contrasting, trait-driven responses of tree species to wet vs dry climatic conditions can promote forest community stability. Mixing tree species with a range of drought-tolerance traits may therefore increase forest productivity and stability but may not universally relieve drought stress.