[Which forests for which biodiversity under climate change?]

Environmental changes, including climate change, put forest biodiversity under an unprecedented pressure. Yet, biodiversity plays a key role in the functioning of forest ecosystems and the quality of the services they provide to human societies. The spontaneous or "facilitated" adaptation of forests to climate change represents a new challenge for forest science and engineering. One challenge is to mobilize biodiversity to increase the resilience of forest ecosystems and preserve their functional integrity in the face of intensifying stresses and increased disturbance frequency. I first discuss three prerequisites: (i) understanding biodiversity as the legacy of local, regional, and global histories; (ii) considering biodiversity as a functional attribute of forest ecosystems; (iii) considering biodiversity as species assemblages that are patterned by natural and anthropogenic forcings that, in turn, affect the ecosystem's resilience to these forcings. I then analyze the foreseeable consequences for biodiversity of the three strategies currently implemented for forest adaptation in metropolitan France: free evolution of forests; managing forests to increase their resilience; and replacing existing forests with plantations of (often exotic) tree species supposed to be better adapted to forthcoming climate conditions. For each strategy, I outline a benefit/risk assessment. I conclude with the imperative need to revisit forest engineering based on the scientific outputs of functional ecology, where biodiversity-which is not limited to the number of tree species-is not an obstacle to forest management, but a lever for adaptation.