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.