Stomatal and Hydraulic Redundancy Allows Woody Species to Adapt to Arid Environments

Functional redundancy is considered a pivotal mechanism for maintaining the adaptability of species by preventing the loss of key functions in response to dehydration. However, we still lack a comprehensive understanding of the redundancy of leaf hydraulic systems along aridity gradients. Here, photosynthesis (A_n), stomatal conductance (g_s) and leaf hydraulic conductance (K_leaf) during dehydration were measured in 20 woody species from a range of aridity index (AI) conditions and growing in a common garden to quantify stomatal redundancy (SR), the extent of stomatal opening beyond the optimum required for maximum photosynthesis (A_max), leaf hydraulic redundancy (HR), and the extent of leaf hydraulic conductance (K_leaf) beyond the optimum required for maximum g_s (g_s-max). The findings revealed that species from arid habitats tended to have higher SRs but lower HRs than did species from humid habitats. The relatively high SR in arid species arose from relatively high g_s-max values. The relatively low HR arose from the relatively high K_leaf value at a 5% reduction in g_s-max (K_leaf-gs). Our results suggest that greater stomatal redundancy and lower hydraulic redundancy prevent the loss of photosynthesis and water transportation, respectively, and thus might be the key adaptive mechanisms for plants to adapt to drought conditions.