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

Abstract Droughts during dry seasons may pose a greater threat to Mediterranean forests under climate change. Effective desiccation avoidance strategies include reduced water loss rates, enhanced tissue water storage capacity, and greater root water access, which can avoid or delay dehydration damage. However, resource allocation competition may lead to trade-offs among these strategies that are not yet fully understood. In this study, six woody species from a Mediterranean forest were selected. We measured their twig hydraulic capacitance, ranging from 0.32–2.81 mmol m−2 MPa−1, to assess tissue water storage capacity, and twig residual conductance (gres) at 25 °C, ranging from 1.23–7.73 mmol m−2 s−1, to evaluate water loss rate. We found that the leaves of all four evergreen Mediterranean species featured sunken or hidden stomata, which may contribute to their low gres. The gres was also measured across a 25 °C–30 °C-40 °C temperature gradient, revealing species-specific response patterns. Predawn water potential (ΨPD) and the difference between predawn and midday water potential (∆Ψ) at the end of the dry summer season were used to estimate root water access. 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 their mortality risk during extreme droughts when soil moisture is unavailable. By inverting a plant desiccation model, we demonstrated that both the calculated minimum hydraulic capacitance required for daily plant survival and a novel risk index positively correlated with ΨPD, supporting this conclusion. Notably, these findings align with historical statistics of tree mortality. Additionally, the risk index increased under scenarios of elevated temperature.