Bison grazing in eastern tallgrass prairie does not alter plant diversity after five years

North American tallgrass prairie evolved with regular fire set by Indigenous people and grazing from large herbivores, including the plains bison (Bison bison). In contrast to the uniform effects of prescribed fire, bison graze selectively, creating increased heterogeneity in the landscape. We examined plant community responses to reintroduced bison grazing in high-diversity native and restored tallgrass prairie in Illinois, USA. We evaluated plant community diversity and structure in paired grazed and ungrazed (fenced) plots. After 5 y of grazing at a relatively low target stocking rate of 1 animal unit per 5.2 ha (13 acres), there were no differences in plant community diversity, composition, nonnative: native species ratio, or visual obstruction readings between grazed and ungrazed plots. An interaction exists (or was found) between plant community type and grazing in which grazed savanna plots had greater grass relative abundance compared to ungrazed plots. Ongoing monitoring will evaluate long-term trends.

Methods
Nachusa Grasslands is located near Franklin Grove, Illinois (41.8837068N, 89.3424108W) and is owned and managed by The Nature Conservancy (TNC). Nachusa consists of more than 1600 ha of native prairie remnants, restored prairies, savannas and woodlands, and wetlands. To examine the impact of bison grazing on plant communities, twenty-two 10 m 3 18 m fenced plots (exclosures) and paired unfenced plots were established prior to bison reintroduction (Fig. 1). Fences are electrified metal wire. The plots are stratified to represent the habitat types at Nachusa (Table 1). There are six sampling sites on native prairie (n=3 in the north unit, n=3 in the south unit), six sampling sites on older plantings (n=3 in the north unit, n=3 in the south unit), four sampling sites on newer plantings (n=3 in the north unit, n=1 in the south unit), and three sampling sites on savannas (n=in the south unit). This unbalanced design results from uneven representation of these habitat types across the preserve. We used a stratified random approach to placing exclosures on the landscape within each habitat type. For each exclosure, three 12.5 m permanent transects inside the fence (ungrazed) are paired with three transects of the same length outside the fence (grazed) (Supplemental Fig. S1). Transect lines are located at least 2 m from fence lines and each transect is 3 m from the adjacent transect. Quarter-square-meter quadrats are placed on the east side of each transect line at five regular intervals (0–0.5 m, 3–3.5 m, 6–6.5 m, 9–9.5 m, and 12–12.5 m) for a total of 15 quadrats in each grazed and ungrazed plot.

Within each 0.25 m2 quadrat, we identified all species rooted in the sub-plot and visually estimated their percent cover for a total of 570 surveyed quadrats. Plots were sampled regardless of whether they had been grazed recently. Plant nomenclature follows Wilhelm and Rericha (2017). Plant community composition and cover were determined in the north unit in 2014 (pre-bison), 2017, and 2019. In the south unit, data were collected in 2015 (pre-bison), 2018, and 2020. Because the bison were introduced to the north unit in fall 2014 and the south unit in fall 2015, this corresponds to year zero, three, and five for each grazing unit. In 2014, Daubenmire cover classes (Daubenmire 1959) were used instead of percent cover. Class 1 is 0–5% coverage, class 2 is 5–25%, class 3 is 25–50%, class 4 is 50–75%, class 5 is 75–95%, and class 6 is 95–100%. To include the 2014 data in our analyses, we converted the 2015–2020 data to Daubenmire classes and then transposed all data to percent cover midpoints.

In 2019 and 2020, aboveground plant visual obstruction readings (VOR) were recorded near the time ofpeak biomass (late August) in each plot using a Robel pole (Robel et al. 1970; Benkobi et al. 2000).