Stand density modulates drought sensitivity via physiological trait plasticity in co-occurring <i>Eucalyptus</i> species

Abstract Climate change is intensifying drought events globally, yet the extent to which stand density modulates drought sensitivity across co-occurring tree species remains poorly resolved. Here, we investigated how interplant competition influences physiological responses to episodic drought in four Eucalyptus species (Eucalyptus grandis, E. urophylla, E. cladocalyx, and E. cloeziana) representing contrasting climatic origins. Using ACi curves, photosynthetic limitation partitioning, and multivariate trait analysis, we examined trait responses across four planting densities (10 000, 2500, 1111, and 400 trees ha−1), during a natural drought event (pre-rain) and following a subsequent rainfall event (post-rain) in a field experiment. Under pre-rain conditions, trees in denser stands exhibited more negative midday xylem water potentials (Ψx ≈ −4.58 MPa) and greater reductions in net photosynthesis (up to 50%), stomatal conductance (up to 60%), and carboxylation capacity (up to 35%), particularly in E. grandis and E. cladocalyx. Although hydraulic recovery occurred rapidly post-rain, photosynthetic responses lagged behind hydraulic recovery under high-density conditions. Stomatal limitation was the dominant constraint pre-rain, and its persistence post-rain was influenced by both species identity and spacing, especially for E. cladocalyx and E. cloeziana. Principal component analysis revealed that drought sensitivity was driven by coordinated changes in photosynthetic, hydraulic, and water-use traits. Our results demonstrate that stand density mediates drought responses through both resource competition and trait plasticity, with wider spacing enhancing physiological resilience in young Eucalyptus trees. These findings highlight the importance of integrating structural and physiological insights to inform forest management strategies, particularly in plantation systems, under future climate variability.