Restoring Frequent Fire Results in Habitat Improvement for Bison but Minimal Early Reduction of Woody Encroachment

Abstract

Fire suppression in mesic grasslands has led to widespread woody plant encroachment (WPE)—the expansion of shrubs and trees into grasslands, often with negative effects on biodiversity and ecosystem services. Increasing woody cover reduces grass abundance and fine fuels, lowering fire intensity and facilitating further shrub expansion, making WPE difficult to reverse with prescribed fires. From 2021 to 2023, we applied annual prescribed burns to heavily encroached areas with and without bison access (paired exclosures) to test whether frequent fire and grazing by bison (Bison bison) begin to reverse WPE and restore grassland biodiversity and structure. Bison activity, measured by dung counts, nearly tripled during the first year of reintroducing fire. In the second and third year, bison usage was 50–70% higher than pre-fire values, though not statistically significant and slightly lower than areas burned annually for 30 years. Shrub area declined in both grazed and ungrazed plots, with the greatest reductions after the first year of burning and in grazed areas. Tree mortality was marginally higher without bison, and plant community composition shifted slightly, with grass cover increasing more in ungrazed plots. Shrub stem mortality was higher in exclosures but resprouting offset this effect. These results suggest that fire combined with bison grazing can help reduce WPE, but effects are modest in the short term. One immediate benefit is improved bison habitat. Restoration practitioners should recognize that even aggressive fire management and megafauna reintroduction may require decades to achieve meaningful woody reduction and should be paired with adaptive management.