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

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). 2. 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. 3. 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. 4. 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.