Sympatric Oaks Exhibit Local Divergence and Convergence in Adaptive Strategies Along Climate Gradients

The coexistence mechanisms of wide-ranging species across climate gradients are crucial for predicting their geographic range shift and community assembly. Two coexisting oaks (Quercus acutissima and Quercus variabilis) were sampled from broad-leaved forests across China. Fifteen plant functional traits related to morphology, physiology and stoichiometry were determined to assess the interspecific and intraspecific variability, trait integration and ecological strategies. Nine among the 15 studied functional traits differed significantly between the two oaks. Given all the studied traits, the two oaks exhibited similar intraspecific variability (29.0 vs. 27.9), but diverged trait-level variability, with Q. acutissima showing 13.1% lower morphological but 11.4% higher physiological variability than Q. variabilis. The connectivity and complexity within trait correlation networks of the two oaks covaried across favourable temperature ranges (ca. 15°C-20°C), but diverged below 15°C and above 20°C and along precipitation gradients. Both the oaks were clustered around competitive and stress-tolerant ecological strategies within their sampling ranges; however, Q. variabilis were more competitive but less stress-tolerant than Q. acutissima when growing at their distribution edges. Our results imply local divergent and convergent adaptation of the sympatric oaks via altering trait-level plasticity and trait integration along climate gradients, potentially leading to greater niche differentiation and complex community assembly.