Trade-offs between residual conductance, hydraulic capacitance and water access in Mediterranean species

Dry season droughts may increasingly threaten Mediterranean forests under climate change. While plants employ three desiccation avoidance strategies to avoid or delay dehydration damage, including reduced water loss, enhanced tissue water storage, and improved root water access, resource allocation competition may lead to trade-offs among these strategies that are not yet fully understood. We investigated six Mediterranean woody species by analysing: (1) twig hydraulic capacitance (0.32 - 2.81 mmol m-2 MPa-1) representing tissue water storage capacity; (2) twig residual conductance (gres) at 25 °C (1.23 - 7.73 mmol m-2 s-1) reflecting water loss rate; and predawn water potential (ΨPD) and its difference from midday water potential (∆Ψ) at the end of the dry season as root water access indicators. Significant trade-offs in plant desiccation avoidance strategies were observed as gres positively correlated with ∆Ψ (R2 = 0.78, P = 0.02) and twig hydraulic capacitance negatively correlated with ΨPD (R2 = 0.68, P = 0.04). Consequently, species with greater root water access exhibited lower tissue water storage capacity and higher gres, potentially increasing mortality risk when soil moisture becames limiting. By inverting a plant desiccation model, we also demonstrated that minimum survival-required hydraulic capacitance and a novel risk index were both positively correlated with ΨPD, consistent with historical mortality records. Additionally, despite temperature-dependent gres patterns which revealed species-specific responses, elevated temperatures amplified the risk index for all species.