The ability of large herbivores to increase primary production in many grassland ecosystems has been linked to their stimulation of plant available soil nitrogen (N) and enrichment in shoot N content. However, herbivores have additional effects on the structure of grassland vegetation that may also impact the efficiency that plants use shoot N to produce biomass per unit time (i. e., nitrogen production, NP). Knowledge of any effect that herbivores have on NP will provide a better understanding of how herbivores can mechanistically feedback on grassland processes. The influence of herbivores on NP was examined by comparing plant production and shoot N content inside and outside exclosures at ten grasslands in Yellowstone National Park (YNP), an N–limited ecosystem, where animals previously have been shown to increase soil N availability, shoot N content, and plant production. NP was measured aboveground (NPA, g shoot growth g shoot N–1 day–1), belowground (NPB, g belowground growth g shoot N–1 day–1), and at the whole grassland level (NPW, g whole plant growth g shoot N–1 day–1). The study was conducted over a three-year period (1999–2001) during which precipitation varied markedly. Grazers increased annual NPA, NPB and NPW by 32%, 55%, and 48%, respectively, in 1999, an average to wetter than average year across the widely distributed sites. Herbivores had no effect on NP measures during 2000 and 2001, two drought years. These findings revealed that grazer facilitation of grassland production in YNP was at least partially due to a previously unrecognized enhancement of NP by herbivores, which was likely dependent on moisture conditions.
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Vol. 73 • No. 4