Divergent responses to fire in South African and North American grassland communities

Background/Question/Methods

Does fire frequency affect plant community structure and composition similarly in grasslands in North America and South Africa? Plant community structure and composition were compared among annual, intermediate and unburned treatments within two long-term fire manipulative experiments in native grasslands at Konza Prairie Biological Station (KNZ), Kansas, USA and Ukulinga Research Farm (URF), KwaZulu-Natal, South Africa, using identical methodology over a five year period. Because fire may reduce soil nitrogen (N) availability and thus affect community structure, N additions were superimposed on the fire treatments as a means of assessing direct vs. indirect mechanisms driving responses to fire.

Results/Conclusions

Although richness was higher at URF (157) than at KNZ (83), there was divergence in plant community response to fire frequency despite convergent responses to nutrient additions. At KNZ, frequent fire resulted in dominance by few, tall, deep-rooted rhizomatous grasses (e.g. Schizachyrium scoparium and Andropogon gerardii). On unburned sites, shorter, shade tolerant species such as Poa pratensis increased, although A. gerardii remained dominant. Richness increased with decreasing fire frequency at KNZ. At URF, frequent fire resulted in short, diverse grassland weakly dominated by a range of species, typified by Themeda triandra. Decreasing fire frequency reduced richness and resulted in dominance by relatively tall caespitose grasses such as Aristida junciformis. N addition reduced species richness in both sites. The divergent response to fire frequency is likely linked to the dominant species and their characteristic traits, including height and method of clonal reproduction, with the rhizomatous growth form of A. gerardii dominating at KNZ. URF does not have an equivalent grass species; instead a range of tufted, non-rhizomatous species dominate across the fire frequency treatments at URF. Reductions in soil N due to frequent fire did not appear to be a common mechanism driving responses in community composition in these two grasslands.