Effects of fire, grazing and topographic variation on vegetation structure in tallgrass prairie

Questions: How do fire and grazing by bison affect the composition and structure of tallgrass prairie plant communities and their temporal stability? Are these responsesmodulated by topographic location? Location: Konza Prairie Biological Station, Kansas, USA. Methods: Plant community composition was monitored in permanent plots in native grassland sites receiving different long-term prescribed burning and bison grazing regimes across a topographic gradient (lowlands, slopes, uplands) following 20 yr of variable burning treatments and 13 yr of grazing by bison. A combination of multivariate analyses was used to determine how community structure, life forms and individual species responded to the accumulated effects of long-termfire and grazing by native bison in upland, slope and lowland soils. Results: Species diversity wasmaximized in sites that were infrequently burned and grazed by bison with the strongest response on infrequently burned slope sites, while diversity was lowest on frequently burned ungrazed sites and on frequently burned slope sites. In general, grass cover was highest in infrequently burned ungrazed sites and lowest on frequently burned grazed sites, while forb richness was highest in infrequently burned and grazed sites. Community response to fire and grazing differed across the topographic gradient. In general, frequent burning favoured C4 grasses, which reduced the abundance of C3 forbs, especially in lowland sites. Responses of dominant grasses and forbs to fire and grazing varied depending on topographic position. Community stability was positively correlated with species richness. Conclusions: Overall, we found that fire and grazing are themain determinants of plant community composition and structure in this grassland system, but that topography mediates these effects. A combination of bison grazing and periodic fire is necessary to maximize diversity and community stability across this native grassland landscape.