Interspecific Variation in Potential Water-Use Efficiency and Its Relation to Plant Species Abundance in a Water-Limited Grassland

1. Interspecific and temporal variations in presumed water-use efficiency, as measured by carbon isotope discrimination (Δ), were examined in a Greek semiarid herbaceous grassland, and the hypothesis tested that competition for water is a factor in determining vegetation structure. 2. Experiments were conducted for 3 years in an upland grassland consisting of exclusively C3perennial species. The most abundant species were the grasses Poa pratensis L., Lolium perenne L. and Festuca valida (Uechtr.) Penzes; the forbs Achillea millefolium L., Plantago lanceolata L. and Taraxacum officinale Weber ex Wigg; and the legume Trifolium repens L. 3. Rooting depth did not explain plant Δ in this shallow-soil site, where soil moisture was concentrated in the top of the profile. 4. Although temporal variation for Δ was significant within most species, changes in ranking of species were slight throughout three growing seasons. 5. Δ Was negatively related to species biomass production, which means that species' abundances were positively related to species' water-use efficiencies. Δ Was positively related to leaf N content, which we interpret as an autocorrelation with water availability. 6. The results of this study are consistent with the notion that, in an environment where water is scarce, plants may compete effectively by increasing their potential water-use efficiency as measured by Δ, and that species abundance is regulated by competition-driven water depletion.