Green, Ronald A. 1998. Nitrogen Distribution in a Perennial Grassland: The Role of American Bison. Colorado State University.
I examined the effect of a large native grazing herbivore, Bison bison, on Nitrogen distribution in a northern mixed-grass prairie at Wind Cave National Park, South Dakota. I conducted a field experiment to evaluate the effect of defoliation frequency on aboveground net primary production (ANPP), shoot Nitrogen concentration, and above ground Nitrogen yield of graminoids. ANPP was significantly reduced at weekly and biweekly defoliation frequencies, but unaffected relative to unclipped controls at monthly and bimonthly frequencies. Clipping at all frequencies increased shoot Nitrogen concentration above that of controls, and this increase was greatest at monthly or more frequent defoliations. Total aboveground Nitrogen yield and potential Nitrogen yield to grazers were greatest at intermediate (bimonthly to biweekly) frequencies. The results suggest that grazers may maximize their nutritional status in this system by periodically regrazing areas at approximate monthly intervals.
In a second study, I evaluated the patterns of landscape and habitat use by bison cow-calf herds to help estimate the amount of Nitrogen that bison transport within landscape regions and among habitat types. Cow-calf herds used landscape regions by periodically moving between landscapes. Frequency of use and length of stays varied by landscape and time of year. Cow-calf herds preferentially used cool-season grassland habitats during the 1985 growing season and in the winter, but less than expected based on availability during the 1986 growing season. Prairie dog colonies dominated by graminoids were used in proportion to availability in the 1985 growing season, preferentially in the 1986 growing season, and avoided during the winter. Landscape use and habitat use varied by year and season within year and appeared to be related to changes in available resources.
In a third study, I estimated the amount of Nitrogen that bison transported within two landscapes and among habitats via Nitrogen consumption and deposition by urine and dung. In Research Reserve, where prairie dog colonies comprised 33% of the area, cumulative Nitrogen deposition by bison was twice as much on graminoid dominated portions of prairie dog colonies as any other habitat. However, cumulative Nitrogen intake by bison was nearly equal to deposition in all habitats resulting in small net gains or losses. In Red Valley, where prairie dog colonies comprised only 8% of the area, Nitrogen deposition and intake was greatest in cool-season grassland habitats and lowest on prairie dog colonies. Within landscape regions, 75-90% of the areas had Nitrogen depositions from bison of less than 0.3 g/m^2. Bison mediated Nitrogen flux was substantially higher on prairie dog colonies dominated by graminoids than on uncolonized sites. Bison consumed and deposited Nitrogen in similar habitats thus not causing a net flow of Nitrogen between habitats.