Effects of Bison Wallows on Soils and Plants in the Northern Mixed-Grass Prairie

Bison (Bison bison) are a keystone species in the North American grasslands, acting as ecosystem engineers that contribute to prairie ecological restoration across the Great Plains. A key and notable behavior by bison is the creation of large, bare-soil, semi-circular excavations known as wallows. These land and soil disturbances expose soil seedbank, potentially influencing native and medicinal plant species' richness. However, the specific ecological effects of wallows (W) compared to adjacent non-wallow areas (NW) remain unknown. To investigate this, we sampled 33 wallows across three bison-managed localities in South Dakota: Custer State Park (CSP), Sicˇhánggˇu-Oyánke (Rosebud) Wólakota Buffalo Range (WBR), and Wakpá Ipáksˇaŋ (Flandreau) Santee Sioux Buffalo Program (FSS). At each wallow and adjacent non-wallow site, we measured (1) soil geochemistry, (2) plant biomass, and (3) germinable seedbank, assessed under controlled greenhouse conditions. Using Kruskal-Wallis tests, we found (1) significant differences among localities (CSP vs. FSS vs. WBR; P ≤ 0.05) in soil nutrients including Ca, Cu, Mg, and N; compared to sites (W vs. NW; P ≤ 0.05), which were significant for Zn and Organic Matter; (2) wallows were sandier than non-wallows by 3.3 ± 1.2% (P = 0.007); (3) plant wet and dry biomass did not differ significantly between W and NW sites; and (4) germinable seedbank density was higher in non-wallows, with the highest richness observed in CSP (P ≤ 0.05). Despite lower seedbank density in wallows, greenhouse germination revealed four native and/or medicinal plant species uniquely present in wallow soils, highlighting their potential ecological and cultural significance.