Ungulate grazing drives higher ramet turnover in sodium-adapted Serengeti grasses

Questions Ungulate grazers requiring sodium (Na) will preferentially graze in areas with Na- accumulating forage grasses (i.e, many saline soils with high NaCl). Given that high-Na conditions necessitate specific adaptations in plants, the mechanisms allowing plants to persist under combined herbivory and Na stress are unclear. Has adaptation to soil salinity in East African grasses modified the evolution of grazing tolerance, leading to plants that are susceptible to defoliation? Does grazing differentially impact the survivorship of plants with different tolerances to Na? Finally, can species traits that confer salinity and herbivory tolerances help to explain the presence of mosaic structure in high salinity regions? Location The short-grass plains of Serengeti National Park, Tanzania, East Africa. Methods We tracked the fate of 836 individuals of 26 grass species, over an entire growing season, in the short-grass plains using a network of 41 quadrats (0.25 m2). We investigated the impact of soil properties (e.g., electrical conductivity [EC], bulk density, clay), ungulate herbivory, and grass species identity on patterns of composition and turnover. Finally, we linked plant leaf Na and leaf palatability data to species environmental responses. Results Under comparable levels of grazing pressure, grass species known to accumulate Na have much higher levels of turnover compared to low-Na species. These Na accumulators were generally short, highly palatable and clonal species. Conclusions Herbivory most affects turnover (of “ramets”) of species with the greatest tolerance to Na, suggesting that Na-tolerance ability interacts with defoliation response in Serengeti grasses. As a result, spatial heterogeneity in soil EC produces high-turnover communities that overlay high-EC soil adjacent to more stable communities composed of Na-intolerant species. While Na-intolerant grasses in Serengeti range from short and palatable to taller and less palatable species, grasses that accumulate Na tend to be short and palatable, a likely result of the grazing pressure they experience. This article is protected by copyright. All rights reserved.