BISON, BOULDERS, AND WILDFIRES – MONITORING BISON-HILLSLOPE INTERACTIONS AFTER PRESCRIBED BURNS

From the beginning of the Pleistocene up until the arrival of European settlers into the region, the North American Bison (Bison Bison) were once a ubiquitous presence on Americas Great Plains. Like other larger grazers, Bison act as a keystone species in grassland landscapes due to their influence on ecosystem heterogeneity, and the impact their grazing and wallowing behaviors have on sediment flux into streams. Likewise, grassland wildfires were once, and still are an important mechanism in controlling the spatial distribution of plant species. The removal of the American bison from the Great Plain and reduction of natural fires has led to numerous ecological and geomorphic changes. Yet it is often difficult to directly capture animals acting as geomorphic agents after a grassland fire without altering their natural behavior.

Bison are known to prefer to graze newly burned areas due to the abundance in plant species that thrive after a fire. Furthermore, it well understood that Bison behavior, including wallowing and grazing, directly affects the flux of fine sediment downslope, it has not yet been determined what role they may have on the transport of larger rock fragments downslope. Much of the Great Plains is underlain by alternating softer lithologies that weather into regolith and harder lithologies that weather into course boulders and blocks that mantle the soil covered hillslope. Therefore, establishing if Bison can disturb, loosen, and trigger the transport of course rock armor after a fire will increase our understanding of post fire erosion in the past before the elimination of Bison from the Great Plains.

Using a combination of trail cameras and marked rocks, we have established Bison-rock fragment interactions after a prescribed spring burn in a watershed of the Konza Prairie Biological Station. We capture block movement as result of Bison naturally moving back and forth across the landscape while grazing the post-fire hillslope. Over half of our marked rock fragments moved over the initial 4-week monitoring period by some amount. Bison can cause block movement by both sliding along the hillslope as well as flipping and rolling. Perhaps most surprising, we found evidence of block movement in upslope direction. Further work will examine Bison effects in relation to the seasonality of the prescribed burn.