| Sumario: | Abstract: Southern African savannas are mixed plant communities where C trees co-exist with C grasses. Here foliar δ N and δ C were used as indicators of nitrogen uptake and of water use efficiency to investigate the effect of the rainfall regime on the use of nitrogen and water by herbaceous and woody plants in both dry and wet seasons. Foliar δ N increased as aridity rose for both C and C plants for both seasons, although the magnitude of the increase was different for C and C plants and for two seasons. Soil δ N also significantly increased with aridity. Foliar δ C increased with aridity for C plants in the wet season but not in the dry season, whereas in C plants the relationship was more complex and non-linear. The consistently higher foliar δ N for C plants suggests that C plants may be a superior competitor for nitrogen. The different foliar δ C relationships with rainfall may indicate that the C plants have an advantage when competing for water resources. The differences in water and nitrogen use likely collectively contribute to the tree–grass coexistence in savannas. Such differences facilitate interpretations of palaeo-vegetation composition variations and help predictions of vegetation composition changes under future climatic scenarios.
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