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Carbon isotope discrimination (DELTA) was compared between populations of dominant perennial plant species, differing in life expectancy, in two deserts with contrasting vegetation types. In both deserts, plants of the shorter-lived species showed significantly higher DELTA and greater intrapopulation variance in this character compared to the long-lived species. These results indicate underlying differences in gas-exchange physiology, and suggest a positive correlation between water-use efficiency and lifespan in desert plants. Differences in variance for this character may reflect greater microenvironmental variation experienced by shorter-lived plants and/or different forms of selection acting on water-use traits. Spatial distributions were significantly clustered for the shorter-lived species and significantly uniform for the long-lived species, indicating that competition has been important in the development of the long-lived populations. The long-lived Larrea tridentata showed a significant, negative correlation between DELTA and Thiessen polygon area, suggesting a positive relationship between water-use efficiency and longevity within this species. This relationship was weakly supported in the other warm desert species, Encelia farinosa, but was not observed within populations of the cold desert species, Gutierrezia microcephala and Coleogyne ramosissima. These results suggest that DELTA reflects key aspects of plant metabolism related to lifespan; these differences may ultimately influence interactions among desert plants and the structure of desert plant communities.