Fire, grazing by megaherbivores, and extreme climatic fluctuations are historic and present-day determinants of the distribution, structure and function of savanna grasslands. All of these drivers are being altered by human activities, yet our ability to predict whether or not savanna grasslands have convergent or divergent responses to global environmental change is limited. Therefore, one goal of our research is to compare how plant community structure and N availability respond to fire frequency in South African (SA) and North American (NA) savanna grasslands given the differences in evolutionary history, herbivore diversity and soil fertility between NA and SA. To accomplish this goal, we sampled vegetation and N availability in long-term (25-50+yr) experiments with annual, 3-4 yr, and long-term unburned fire frequencies at the Ukulinga Research Farm, SA, and the Konza Prairie Biological Station, NA.

Results/Conclusions

We found that N availability responded similarly to fire frequency in both SA and NA. Using resin bags, we found that seasonal N availability was highest in unburned grassland in both NA and SA, with strongest treatment differences occurring in SA grasslands. In contrast, vegetation responses differed dramatically between NA and SA. In NA, plant species richness and diversity were lowest in annually burned sites and highest in 4-yr burns, whereas in SA, diversity was highest on annually burned sites and lowest on 3-yr burns. These differences reflect differences in the responses by dominant grasses among sites. In NA, annual burning increased dominance of C4 grasses, particularly Andropogon gerardii, and reduced forb richness. In contrast, in SA turnover occurred in dominance among C4 grass species with little effect on forb richness. Specifically, annual burning increased evenness as Heteropogon contortus and Themeda triandra dominated annually burned treatments, Cymbopogon spp. and Eragrostis racemosa dominated 3-yr burn treatments and Aristida junciformis dominated unburned treatments. Thus, our results to date show convergence in an ecosystem level process, N availability, but divergence in community structure. This divergence was caused, in part, by the different evolutionary histories of the dominant grasses in NA and SA. Together these results highlight the challenges we face in extrapolating community responses to changes in common ecological drivers affecting savanna grasslands worldwide.