Grassland patch dynamics and herbivore grazing preference following urine deposition

Investigated the response of Schizachyrium scoparium (C₄ photosynthetic pathway) and Poa pratensis (C₃) to natural and simulated bison urine deposition in a northern, mixed prairie in South Dakota. Total aboveground biomass and root mass were higher and root:shoot ratios were lower on urine patches. 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 deposition date and plant phenology 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, an increase greater in P. pratensis. The large increase in P. pratensis biomass following urine deposition is probably 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 senescence 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. Aboveground 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 mostly quiescent. -from Authors