Though geographically distant, mesic savanna grasslands in central North America (NA) and in South Africa (SA) are both dominated by C4 grasses that co-occur with C3 forbs and woody plants. They also have similar growing season temperatures and precipitation and share the same historic and present-day drivers of ecosystem function and structure: fire and herbivory by large ungulates. Based on these common features, patterns and controls on aboveground net primary productivity (ANPP) are expected to be convergent. However, an alternative perspective is that differences in evolutionary history, soil fertility, and diversity of the flora and extant megaherbivores may result in SA systems responding in fundamentally different ways to fire, grazing and resource manipulations when compared to NA systems. To address this issue, we used identical experimental designs, where fire (annually, every 3-4 years, and unburned), herbivory (grazed and ungrazed via herbivores exclosures) and nitrogen (10 g m-2) were experimentally manipulated at sites in NA (Konza Prairie Biological Station, KS) and SA (Kruger National Park and the Ukulinga Research Farm) to assess aboveground net productivity (ANPP) responses to fire, grazing and N additions over a three year period.
Results/Conclusions
Fire affected ANPP similarly in NA and SA, with ANPP stimulated by 30-100% in burned sites with deep soils and abundant soil moisture and unaffected at other sites. Although total ANPP did not decline with fire, forb biomass was strongly decreased by fire (80%) in NA but was not significantly decreased in SA. The legacy of long-term grazing also affected ANPP similarly, however previous grazing history resulted in no fire induced decrease in forb ANPP on either continent. Finally, the well-documented N and fire interaction responses documented at Konza Prairie were also evident in South Africa, with ANPP significantly enhanced (35-55%) with N additions in N-limited annually burned sites, but not in infrequently burned or unburned sites. We concluded that ANPP responses to fire, grazing and N are generally convergent across these two continents despite the potential contingent factors of differences in soil fertility and plant and large herbivore diversity. The primary exception was in responses of the forb component of the community, which is consistent with results from concurrent plant community studies at these sites.
