Singer et al. present in this paper an evaluation of the abundance of elk (Cervus elaphus) and bison (Bison bison) and their grazing on ungulate winter ranges in two national parks (Yellowstone National Park and Rocky Mountain National Park), by documenting grazer effects on the size of N pools, plant production biomass, N yields in plants, soil fertility, and sustainability of an ecosystem. Grazer effects on the grasslands of Yellowstone National Park (YNP) classically fit all the criteria of the accelerating nutrient scenario. In response to grazing, grassland plant species abundance was largely unaltered, net annual aboveground primary production (NAPP) was stimulated, consumption of key N-rich forages by ungulates was moderate and their abundance was sustained, and soil N mineralization rates doubled. The authors concluded that there was no major December loss or gain of N from Rocky Mountain National Park (RMNP) grasslands due to ungulate grazing. N yield, N concentrations were not reduced and inputs and outputs of N due to elk were roughly equivalent. There was substantial losses of N due to ungulates in the aspen and short willow types of RMNP. The authors believe that the higher ungulate densities and rates of plant consumption in RMNP, large declines in N-rich forage plants, and possibly a tendency of ungulates to move N from willow and aspen vegetation types to other types in RMNP, contributed to deceleration of nutrient cycling in two vegetation types of RMNP compared to acceleration in grasslands in YNP.
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