Bison grazing patterns on seasonally burned tallgrass prairie.

Abstract

Patterns of bison (Bison bison L.) grazing were examined in a 2-year study on a tallgrass prairie site in Oklahoma subjected to a seasonally and spatially variable burning regime. Mixed groups of bison, composed of cows, yearlings, calves, and young (< 5 years of age) bulls, comprised 90% of the study population and showed selectivity by using burned areas significantly more than expected 23% of the time. Mixed groups avoided unburned areas 63% of the time. In contrast, bull groups of mature bulls > 5 years of age selected unburned areas for grazing 29% of the time and burned areas only 4% of the time. Temporal patterns in bison grazing were evident; selective use of burns persisted for only a short period during the first post-fire growing season, after which burns were grazed in proportion to availability and then selectively avoided as bison shifted grazing efforts to newer burns. Regression analysis verified that bison grazing was negatively related to burn age. Regression also showed that grazing patterns were positively related to burn patch size. Although burn types varied significantly in biomass and overall vegetative composition, bison exhibited only limited preference for any burn type, choosing those with higher relative cover of annual Bromus spp. and sedges. It appears that bison select recently burned areas with relatively low graminoid biomass for grazing, presumably choosing these areas based on forage quality rather than quantity.