Grassland Patch Dynamics and Herbivore Grazing Preference Following Urine Deposition

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Abstract
Field experiments were performed over two growing seasons to investigate the response of Schizachyrium scoparium (C4 photosynthetic pathway) and Poa partensis (C3) to natural and simulated bison urine deposition in a northern, mixed prairie in South Dakota. We also assessed potential feedbacks of urine deposition on herbivore grazing by monitoring grass response to defoliation and herbivore grazing preference for vegetation occupying urine patches. Total aboveground biomass and root mass were higher and root: shoot ratios were lower on urine patches than in the surrounding plant community. Higher total aboveground biomass on urine patches resulted primarily from increased aboveground P. pratensis production. Urine deposition in May had little effect on aboveground production of S. scoparium except during July when S. scoparium was most active. Urine disposition date and plant penology appear important in determining changes in species composition. Following urine deposition, aboveground N concentrations of P. pratensis and S. scoparium were higher on patches relative to conspecifics off patches. This increase in N concentration following urine deposition was greater in P. pratensis. We suggest the largest increase in P. pratensis biomass following urine deposition is related to its relatively large response to increased soil N availability and its rhizomatous habit. Root N concentrations were higher on urine patches. Poa pratensis on urine patches initiated growth earlier in the season and postponed senescene relative to plants off patches. Aboveground production following clipping was greater on urine patches and N concentrations in regrowth of both species were higher than concentrations in plants not previously clipped. Above ground herbivore utilization was greater on urine patches than on adjacent vegetation. Although urine patches covered only 2% of the study site, they provided 7% of the biomass and 14% of the N consumed by aboveground herbivores from June through August. Urine patches probably provided an even greater source of forage and N for herbivores earlier and later in the growing season when surrounding vegetation was most quiescent.