Trait coordination and trade-offs constrain the diversity of water use strategies in Mediterranean woody plants

Exploring the diversity of above and belowground water-use traits among coexisting plant species can improve understanding of soil water partitioning in species-rich plant communities and provide insights into how dryland vegetation copes with drought stress. We assessed the diversity of plant water use strategies among coexisting woody species at 10 Mediterranean ecosystems with contrasting climatic conditions (62 species in total). Soil and stem water isotopic composition (ẟ¹⁸O, δ²H) were measured to estimate plant water uptake depth using Bayesian mixing models, while leaf carbon and oxygen isotopes (ẟ¹³C, Δ¹⁸O) and the ratio of leaf area to sapwood area of terminal shoots were used as time-integrative proxies of intrinsic water use efficiency, stomatal conductance, and plant investment in foliage relative to woody stems, respectively. We found that distinct vertical ecohydrological niche segregation, driven by differences in water uptake depth among coexisting species, is widespread and ubiquitous across all sites. Leaf-level stomatal regulation stringency and water use efficiency also differ widely among coexisting woody species and are both coordinated with water uptake depth across species and life forms. Within Mediterranean plant communities, larger woody species use deeper soil water sources and exhibit more conservative, water-saver strategies at leaf level coupled to greater investment in foliage relative to sapwood. In contrast, smaller woody species within the same plant communities rely on shallower soil water sources and exhibit a more profligate, water-spender strategy at leaf-level coupled to lower investment in foliage relative to sapwood. We conclude that drought and heat stress in Mediterranean ecosystems favour tight coupling and coordination between multiple above and belowground water use traits, resulting in possibly unavoidable trade-offs that constrain the diversity of whole-plant water-use strategies that are ecologically and physiologically feasible in native plant communities.