Biodiversity Insurance of Forest Productivity Has Strengthened Under Recent Climate Change

Many studies show that tree diversity promotes forest productivity, and few recent ones suggest that this diversity effect may strengthen under climate warming. Yet this pattern has rarely been tested with observations. Here, we investigated how the species mixture affects forest productivity in response to increasing aridity, relying on 25,838 French forest surveys between 2005 and 2016. We showed that 79.8% of the variation in forest productivity was explained by interactions among tree species richness, baseline and temporal changes in water supply and stand density. Although forest productivity decreased with water deficit (for both baseline conditions and temporal changes), species richness mitigated the magnitude of this productivity reduction. These findings indicate that species mixture stabilizes productivity along a water supply gradient and enhances resistance to increasing aridity. In addition, we found that this species diversity insurance of forest productivity in the face of water supply variation is also dependent on stand density. Our modeling approach evidenced that the positive biodiversity effect, mainly observed in forests where recent climate change has decreased water supply, diminished as tree density increased, and even becoming negative in forests having highest tree density under favorable hydric conditions. Covering a large spectrum of climate conditions, this study reveals how tree species diversity insure forest productivity against climate change over time. Regarding the anticipated acceleration of global warming, forest management should prioritize tree diversity to sustain wood productivity and carbon storage, particularly in water-limited conditions. Additionally, foresters should consider tree density effects in their planning to preserve the beneficial effects of tree diversity on forest productivity.