The trade-off between drought resilience and instantaneous flux capacity
The strong negative correlation found here between tree growth stability in facing extreme drought events (RT and RS) and physiological traits of water and carbon flux capacity (K l andA m) suggests that an intrinsic fast strategy of instantaneous resource acquisition and processing are disadvantageous in drought-prone habitats, which is underlain by an association between high hydraulic efficiency and small hydraulic safety margin. Species with a fast-growing strategy usually require relatively high xylem hydraulic conductance that allow them to meet higher transpiratiional water demands and the maximization of photosynthetic carbon assimilation when water are readily availability; however, they tend to function with relatively small hydraulic safety margins (Gong et al., 2020; Liu et al., 2015). This renders tree species adopting a fast strategy higher risk of drought-induced xylem embolism, while greater sensitivity to hydraulic dysfunction in such species would lead to greater degrees of reductions in carbon assimilation, radial growth or even tree mortality when facing extreme drought events (Eller et al., 2017; Li et al., 2020b; Pockman and Sperry, 2000). Moreover, species that are more sensitive to drought-induced hydraulic dysfunction would require more carbon input for recovery to pre-drought conditions that would further compromise tree growth in drought-prone environments (Chave et al., 2009; O’Grady et al., 2013).
The result that tree species with lower mean sensitivity in ring growth to inter-annual climate variation have higher radial growth rate (CBA15) further indicates the importance of drought resilience in determining tree growth rate expressed over a long term in water-limited environments. It has been shown that tree species with high climatic sensitivity are more prone to drought-induced mortality (Macalady & Bugmann, 2014; Ogle, Whitham, & Cobb, 2000). Similarly, growths of species with higher climatic sensitivity have been found to be more strongly limited by inter-annual climate swings (Fritts, 1976; Macalady & Bugmann, 2014). Especially in arid and semi-arid regions, even minimal climatic fluctuations may significantly affect tree species with high climate sensitivity (Speer, 2010; Vanderwel, Lyutasarev, & Purves, 2013). Tree species with a fast strategy would be more sensitive in radial growth to variations of environmental water availability due to their intrinsically greater water demand and higher sensitivity to drought stress, which would be less advantageous in environments with persistent water deficiency (Gazol et al., 2017; Martínez-Vilalta et al., 2012). Tree species with high resource acquisition capability can have high rates of carbon assimilation and store more carbohydrates during the wet years but may not be enough to compensate for the adverse impacts of water deficiency in the dry years (Breshears et al., 2009). Therefore, our results indicate that greater resilience to drought stress, rather than high instantaneous rates of resource acquisition and assimilation, is more important in determining long-term tree growth performances in water-limited environments.