Isotopes reveal contrasting water use strategies among coexisting plant species in a Mediterranean ecosystem

Variation in the stable carbon and oxygen isotope composition (d13C, ?18O) of co-occurring plant species may reflect the functional diversity of water use strategies present in natural plant communities. We investigated the patterns of water use among 10 coexisting plant species representing diverse taxonomic groups and life forms in semiarid southeast Spain by measuring their leaf d13C and ?18O, the oxygen isotope ratio of stem water and leaf gas exchange rates. Across species, ?18O was tightly negatively correlated with stomatal conductance (gs), whereas d13C was positively correlated with intrinsic water use efficiency (WUEi). Broad interspecific variation in ?18O, d13C and WUEi was largely determined by differences in gs, as indicated by a strong positive correlation between leaf d13C and ?18O across species The 10 co-occurring species segregated along a continuous ecophysiological gradient defined by their leaf d13C and ?18O, thus revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies ranging from profligate/opportunistic (high gs, low WUEi) to conservative (low gs, high WUEi). Coexisting species maintained their relative isotopic rankings in 2 yr with contrasting rainfall, suggesting the existence of species-specific isotopic niches that reflect ecophysiological niche segregation in dryland plant communities.