Grazing History, Defoliation, and Competition: Effects on Shortgrass Production and Nitrogen Accumulation

Abstract

Plants of Bouteloua gracilis were collected from heavily grazed prairie dog (Cynomys ludovicianus) colonies (ON—colony) and lightly grazed uncolonized sites (OFF—colony) in Wind Cave National Park, South Dakota. Our objectives were to determine (1) how grazing history affected production, nitrogen uptake, and biomass and nitrogen allocation following defoliation and (2) how such responses were affected by competing neighbors of the same population. The growth chamber experiment was factorial with three main treatments in 12 possible combinations. Main treatments were: population (plants from lightly and heavily grazed sites), defoliation (three intensities), and competition (with or without neighbors of the same population). There were no significant interactions among treatments. OFF—colony plants produced 121% as much biomass and their N. yield was 203% as great as ON—colony plants. ON—colony plants allocated a higher percentage of biomass and N to the roots from OFF—colony plants, while the latter allocated a greater percentage of resources to leaf sheaths and reproductive structures. In response to defoliation, plants from the two populations had an average increase of 122% in N uptake per unit of root biomass and a 141% increase in total leaf—blade N yield. Heavily defoliated plants from both populations increased biomass and N allocation to blades, but total biomass production was substantially reduced. Intra—population competition reduced biomass production and N yield in individuals of both populations, but it did not affect total and aboveground N concentrations (per unit biomass) or N uptake per unit of root biomass. Our results support the hypothesis that genetically based morphological and physiological differentiation has occurred in B. gracilis as a result of strong selection pressures from grazing mammals on prairie dog colonies. We suggest the ON—colony population displays a strategy that reduces grazing severity rather than a "grazing tolerance" response to aboveground herbivory.