Beyond proxies: towards ecophysiological indicators of drought resistance for forest management

As drought-induced mortality increases globally in forest biomes, it becomes necessary for foresters to have access to reliable predictors of species vulnerability to drought and mortality risk under different climatic scenarios. On one hand, there exist several 'operational' indicators of drought resistance, which are based on technical literature, observations, expert knowledge and species bioclimate. However, they are not available for all species, reduce a species to a single value and have the same limitations as species distribution models. On the other hand, different traits can be measured to estimate mechanistically species' vulnerability to drought and, in particular, to hydraulic failure, a key process of tree mortality under drought. These traits typically include xylem vulnerability to cavitation, stomatal regulation, minimum leaf conductance and water storage capacity. However, the mechanistic approach, based on functional traits, has never been compared with the operational approach. In this study, we review if indicators commonly used by foresters provide information on Abies species' vulnerability to hydraulic failure. We measured a set of traits in a common garden experiment of closely related Mediterranean Abies species. These traits were used to configure and parametrize SurEau, a plant hydraulic model dedicated to simulating plant mortality risk due to hydraulic failure under extreme drought conditions. SurEau was then used to compute a single indicator of vulnerability (time to hydraulic failure - THF) and to assess mortality risk in future climate. We found that among circum-Mediterranean firs, a high THF was largely driven by differences in minimum leaf conductance. Some operational indicators are good proxies of THF but fail to distinguish between closely related Mediterranean Abies species. We argue that the mechanistic approach could help foresters in species selection and in estimating the risk faced by forest tree species in a changing climate. While accounting for the variability of traits, hydraulic models can be forced with different climatic scenarios, thereby allowing hydraulic failure risk assessment by the end of the century.