Growth-defense trade-offs in Quercus variabilis and Quercus mongolica seedlings under drought stress

Intensified drought induced by climate change is threatening the natural regeneration of forests, yet coexisting tree species can maintain their survival and regeneration by adopting divergent carbon allocation strategies. This study compared leaf-scale drought responses of Quercus variabilis and Q. mongolica seedlings originating from the ecotone of the Zhongtiao Mountains.

Multiple indicators were measured under four water regimes: control (CK), light drought (LD), moderate drought (MD), and severe drought (SD), including leaf morphology, photosynthetic and gas exchange parameters, hydraulic traits, carbon storage traits, stress defense traits, and nutrient stoichiometry. Principal component analysis (PCA) and structural equation modeling (SEM) were used to evaluate interspecific differences in drought adaptation strategies.

Drought suppressed leaf growth and gas exchange, lowered leaf water potential and relative water content, aggravated damage to hydraulic function, and boosted biochemical responses associated with stress defense in both species. For Q. variabilis, leaf non-structural carbohydrate (NSC) concentrations were tightly correlated with growth-related traits, indicating that NSCs play a vital role in sustaining growth and accumulating carbon after growth limitation, reflecting a growth-prioritized resource allocation strategy. In contrast, leaf NSCs of Q. mongolica seedlings exhibited stronger coordinated covariation with stomatal behavior and antioxidant defenses, suggesting that this species prioritizes defense maintenance and physiological stability under drought stress, representing a defense-prioritized resource allocation strategy.

Seedlings of Q. variabilis and Q. mongolica adopt growth-prioritized and defense-prioritized adaptation strategies under drought, respectively. Such physiological divergence serves as a physiological foundation for niche differentiation, divergent regeneration patterns, and coexistence of seedlings of the two Quercus species within the ecotone.