Plant water use strategy is strongly constrained by species size in Mediterranean ecosystems

Global warming and changes in precipitation regimes resulting from climate change are altering plant water availability and water use efficiency, which compromises key ecosystem services such as vegetation productivity, ecosystem carbon storage, and ecohydrological regulation. The study of plant water sources and leaf-level water use strategies is particularly important for the management and conservation of Mediterranean ecosystems, which are threatened by increasing climatic aridity. We investigated the water use strategies of 66 woody plant species distributed across 10 sites along a steep 600 km aridity gradient in  south-eastern and central Iberian Peninsula. We used soil and plant water stable isotopes (δ²H and δ¹⁸O) to quantify the proportion of water extracted from different soil layers, as well as leaf stable isotopes (δ¹³C, δ¹⁸O and Δ¹⁸O_enrichment above source water) as proxies for water use efficiency and stomatal regulation. Our results indicate that water extraction depth along the soil profile is strongly constrained by plant species size and height. Bayesian models revealed that small and medium size shrubs use a much greater proportion of shallow soil water than large shrubs and trees, a pattern that is remarkably consistently across study sites and species. Mixed models revealed strong associations between leaf δ¹³C, Δ¹⁸O_enrichment and xylem water δ¹⁸O across sites, indicating that woody species with tighter stomatal regulation achieve higher water use efficiency and generally extract water from deeper soil layers. This conservative water use strategy is much more common in larger woody species than in smaller ones. Our findings demonstrate the coexistence of sharply contrasting water use strategies in Mediterranean plant communities.  Small and mid-size woody species are heavily reliant on shallow soil water and exhibit a highly acquisitive and profligate water use strategy that enables them to take rapid advantage of an ephemeral resource (topsoil water) after rainfall pulses, although they are less efficient in the use of water at leaf level. On the other hand, large shrubs and trees that use more stable deeper water sources exhibit a much more conservative water use strategy.