Leaf carbon and oxygen isotopes are coordinated with the leaf economics spectrum in Mediterranean rangeland species

1. The leaf economics spectrum (LES) describes covariation in traits relevant to carbon and nutrient economics across plant species, but much less is known about the relationship between the LES and leaf water economy. We propose an approach combining the measurement of two leaf traits related to water-use economy, leaf carbon (C-13) and oxygen (O-18) isotopic composition, and the measurement of leaf morphological and nutrient traits to investigate the link between leaf carbon and nutrient economics and water use. 2. We tested the relationships between leaf traits linked to carbon and nutrient use within the LES and water-use traits using leaf O-18 as a proxy of stomatal conductance (g(s)) and C-13 as a proxy of intrinsic water-use efficiency (WUEi) across 15 Mediterranean rangeland species grown in an irrigated common garden and in a natural rangeland in Southern France. 3. The target species spanned a wide range of variation in leaf morphological and nutrient trait values and a wide range of leaf O-18 and C-13 values. Principal component analysis revealed multiple associations among leaf morphology, nutrients and isotopic composition, with the first axis alone explaining 56.0% of the total variation across species. Leaf O-18 and C-13 covaried with leaf morphology and leaf nutrient concentrations along a single resource-use axis. Species with high leaf O-18 and C-13 (low g(s) and high WUEi) exhibited a resource-conservative strategy (high leaf dry matter content, low leaf N, P and K), whereas species with low leaf O-18 and C-13 (high g(s) and low WUEi) showed a more resource-acquisitive strategy (high specific leaf area and leaf N, P and K). These leaf trait syndromes and resource-use strategies were strongly conserved across sites with contrasting environmental conditions, indicating that foliar O-18 and C-13 can be included as an integral part of the LES for this set of rangeland species. 4. Overall, the data suggest a tight coupling and coordination between water, carbon and nutrient-use strategies across herbaceous plant species. A dual O-18 and C-13 isotope approach combined with LES trait measurements is a promising tool to more comprehensively assess the diversity of resource-use strategies among coexisting plant species.