The present study involved measurements of the stable carbon isotope composition (δ13C) and intrinsic water-use efficiency (iWUE) of three C3 leguminous Prosopis spp. (P. juliflora, P. cineraria, and P. pallida) foliage at different canopy positions (east and west) from saline (SLH) and non-saline habitats (NSH). Integrated measurements of the stable carbon isotope composition (δ13C) of plant tissue were broadly used to study iWUE, taking into consideration the effect of leaf age and canopy position on C isotope discrimination. Mature foliage of P. pallida from an SLH with a west canopy position had significantly higher δ13C (less negative) than that from NSH. On the west side, Δ13C values ranged from 17.8‱ (P. pallida) to 22.31‱ (P. juliflora) for a west canopy position, while they varied from 18.05‱ (P. pallida) to 22.4‱ (P. cineraria) on the east canopy side. Because the patterns are similar for the three Prosopis species, the difference in carbon isotope discrimination (Δ13C) between the canopy position (west and east) is relatively consistent among species and sites, ranging between 17.8 ± 4.43‱ for the young foliage in the west and 18.05 ± 4.35‱ for the east canopy position. The iWUE of P. pallida was twice that of P. cineraria. The iWUE of P. juliflora was higher from NSH than SLH. Mature leaves possessed a higher iWUE than the young leaves. We concluded that exotic P. juliflora and P. pallida have higher iWUE values than the native P. cineraria, which might be due to the rapid below-ground development of plant roots in the Arabian deserts of the United Arab Emirates (UAE). This could enable the alien species access to deeper humid soil layers or water resources.
Keywords: canopy position; carbon isotope composition; intrinsic water-use efficiency; leaf C; leaf N; leaf age.