Patterns and implications of plant-soil delta C-13 and delta N-15 values in African savanna ecosystems

Southern African savannas are mixed plant communities where C-3 trees co-exist with C-4 grasses. Here foliar delta N-15 and delta C-13 were used as indicators of nitrogen uptake and of water use efficiency to investigate the effect of the rainfall regime oil the use of nitrogen and water by herbaceous and woody plants in both dry and wet seasons. Foliar delta N-15 increased as aridity rose for both C-3 and C-4 plants for both seasons, although the magnitude of the increase was different for C-3 and C-4 plants and for two seasons. Soil delta N-15 also significantly increased with aridity. Foliar delta C-13 increased with aridity for C-3 plants in the wet season but not in the dry season, whereas in C-4 plants the relationship was more complex and non-linear. The consistently higher foliar delta N-15 for C-3 plants Suggests that C-4 plants may be a Superior competitor for nitrogen. The different foliar delta C-13 relationships with rainfall may indicate that the C-3 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. (C) 2008 University of Washington. Published by Elsevier Inc. All rights reserved.