The floristic quality index (FQI) is widely used to describe plant communities in terms of their conservation value. Fire and grazing are key drivers influencing plant community composition in tallgrass prairie. However, these drivers have rarely been described in terms of their influence on floristic quality or whether the structure of plant communities in response to fire and grazing reflects belowground properties and processes. We hypothesized that floristic quality would be positively affected by grazers, negatively affected by high fire frequency, and that there would be positive correlations between FQI and belowground structure and function. Twenty-two transects were established in watersheds burned every 1-2, 4, and 20 years, with and without bison grazing. Percent cover of each plant species along each transect was used to determine plant diversity, richness, and FQI. Soil was sampled along each transect to quantify extractable inorganic N, total soil C and N, microbial biomass C and N, potential C and net N mineralization rates, inorganic N availability and bulk density. Diversity, richness, FQI, and belowground responses to fire and grazing were analyzed using PROC MIXED and species composition was analyzed with PERMANOVA. Relationships between FQI and soil quality indicators were examined using Pearson’s correlation procedure.
Plant species composition varied between grazed and ungrazed prairie and in response to fire frequency (PERMANOVA: P < 0.05). There was an interaction between grazing and fire frequency on plant diversity (P < 0.0001) and richness (P = 0.001) resulting from relatively higher diversity and richness in the presence of grazers in watersheds with 1-2 and 20 y fire frequencies that occurred to a less extent under the 4 y burning regime. Floristic quality was higher in grazed than ungrazed watersheds across all fire frequencies (P = 0.02). Inorganic N availability, total N, microbial biomass N, and bulk density were also higher in grazed than ungrazed prairie (P < 0.03). Microbial biomass C (r = 0.51; P = 0.02), and total soil N (r = 0.54; P = 0.01) were positively correlated with FQI. These results indicate that grazing and fire frequency interact to affect species composition, but over all fire frequencies floristic quality is enhanced by the presence of bison, and may be indicative of and N storage soil.