Coordination between water uptake depth and the leaf economic spectrum in a Mediterranean shrubland

Abstract

Abstract Water is the most limiting resource for plant survival and growth in arid environments, but the diversity of water‐use strategies among coexisting species in dryland communities is not well understood. There is also growing interest in assessing whether a whole‐plant coordination exists between traits related to water‐use and the leaf economic spectrum (LES). We used water stable isotopes (δ2H, δ18O) to quantify water uptake proportions from different soil depths by 24 species in a Mediterranean shrubland. Leaf traits associated with water‐use efficiency, stomatal regulation (δ13C, δ18O) and the LES (SLA, N, P, K concentrations) were also measured. We assessed potential trade‐offs between the above‐mentioned leaf traits, water uptake depth and their relationship with species abundance. We found distinct ecohydrological niche segregation among coexisting species. Bayesian models showed that our shrubland species used a median of 37% of shallow soil water (0–30 cm) and 63% of deep water (30–100 cm). Still, water source proportions varied considerably among species, as shallow soil water‐use ranged from a minimum of 6.4% to a maximum of 68%. Interspecific variability in foliar carbon investment (SLA) and nutrient concentrations was remarkably high, indicating diverse nutrient‐use strategies along the LES. Leaf δ18O, δ13C and δ15N values also differed widely among species, revealing differences in stomatal regulation, water‐use efficiency and nitrogen acquisition mechanisms. After accounting for evolutionary history effects, water uptake depth was coordinated with the LES: species using shallower soil water from fertile topsoil layers exhibited a more acquisitive carbon‐ and nutrient‐use strategy, whereas water uptake from deeper but less fertile soil layers was linked to a more conservative nutrient‐use strategy. Leaf‐level water‐use traits significantly influenced species abundance, as water‐savers with tight stomatal regulation and high water‐use efficiency were dominant. Synthesis. Greater utilisation of water stored in nutrient‐rich topsoil layers favoured a more acquisitive nutrient‐use strategy, whereas a deeper water uptake pattern appeared to constrain access to nutrients. Our findings thus suggest a largely inescapable trade‐off and coordination between soil water uptake depth and carbon‐ and nutrient‐use strategies in low‐fertility drylands.