Resilience and multi-faceted diversity of grazed and ungrazed great plains grassland plant communities to severe drought

The simultaneous agricultural conversion of North American prairies and removal of native megagrazers—primarily bison (Bison bison)—has limited our understanding of how bison impact prairie plant communities. Further obscuring the role of native megagrazers is that, in what little prairie remains, either grazers are absent or domestic cattle are dominant. We used a three-decade experiment to assess how fire frequency and reintroducing bison affected upland tallgrass prairie plant communities and their phylogenetic diversity (PD). Grazing and fire shifted plant communities significantly over time. The ungrazed treatment was a largely static community dominated by four grass species, with occasional forbs and subshrubs. In contrast, community composition in the bison treatment shifted dramatically, with increasing abundance of native forbs, subdominant grasses, and small annual plant species. Compared to ungrazed areas, bison treatments had 225 % more unique species, 125 % more unique plant families, and higher richness within common plant families such as Asteraceae, Fabaceae, and Poaceae. Cattle-grazed communities fell between these two treatments with higher abundance and richness of Carex spp. Most treatments had similar, unchanging PD values over time, except annually burned, ungrazed prairie, where PD steadily declined. All treatments were resistant and/or resilient to an extreme drought in 2011 and 2012, with no lasting changes in composition or PD before, during or after the drought. This resistance to change emphasizes the importance of the core grassland plant species to maintain functional group composition. Grazing and fire drove shifts in tallgrass prairie plant communities, which were resilient to extreme drought.