title: Herbivory tolerance of Agropyron smithii populations with different grazing histories.
abstract: The effects of defoliation on growth and nitrogen (N) nutrition were examined in populations of Agropyron smithii (western wheatgrass) collected from a heavily grazed black-tailed prairie dog (Cynomys ludovicianus) colony (ON-colony) and a nearby lightly grazed, uncolonized area (OFF-colony). Defoliated and nondefoliated plants were grown at low soil N availability with similar sized defoliated individuals of A. smithii from a grazing-exclosure population as a common competitor. Sequential harvests were made over 24 days following defoliation. Growth analysis plus biomass and N yield and distribution data were used to identify features which may contribute to plant defoliation tolerance. Defoliation reduced total production 34% across populations. Defoliated plants produced as much new blade tissue, but only 67% as much new root biomass as did nondefoliated controls. Plants from prairie dog colonies accumulated biomass at a faster relative rate than did plants from uncolonized sites, in part, because of a 250% greater mean relative growth rate of blades and more than 200% greater rate of biomass production per unit blade biomass. Total N accumulation was significantly greater in defoliated ON- than OFF-colony individuals. The mean relative accumulation rate of N was increased by defoliation in ON-colony plants, but reduced by defoliation in OFF-colony plants. The mean rate of N accumulation per unit root biomass was more than 300% greater in the ON- than OFF-colony population. Colony plants initially had a greater proportion of biomass and N remaining after defoliation in roots. Initial differences between populations in the distribution of biomass and N were eliminated as colony plants concentrated 24-day accumulation of biomass and N in aboveground structures. The data suggest that the combination of growth, N nutrition, and biomass and N distribution characteristics of the colony population likely confer a high rate of resource capture on heavily grazed prairie dog colonies.
authors.affiliation: Natural Resource Ecology Laboratory, Colorado State University, 80523, Fort Collins, CO, USA.|Department of Range Science, Colorado State University, 80523, Fort Collins, CO, USA.
authors.forename: H W
authors.initials: HW
authors.lastname: Polley
authors.affiliation: Natural Resource Ecology Laboratory, Colorado State University, 80523, Fort Collins, CO, USA.|Department of Range Science, Colorado State University, 80523, Fort Collins, CO, USA.
authors.forename: J K
authors.initials: JK
authors.lastname: Detling
keywords: Biomass production; Defoliation; Grasslands; Nitrogen accumulation; Prairie dog colony