Grazers may increase grassland plant species diversity through mechanisms such as selective consumption of graminoids resulting in release from competition in subordinate forb species, or the enhancement of small-scale habitat heterogeneity. This study tested the hypothesis bison on tallgrass prairie reduce local plant competition and increase the growth, reproduction, abundance, and diversity of forbs. In addition, because grazers, fire, and other drivers result in high spatio-temporal variation in limiting plant resources in tallgrass prairie, we tested the hypothesis that prairie forbs show high phenotypic plasticity in life history traits in response to large grazers.
The growth, reproduction, biomass allocation, and abundances of six common perennial forb species, and estimates of local neighborhood and physical environmental factors were compared in replicate tallgrass prairie sites with and without bison. Greater light availability and percent bare ground; and lower grass canopy density, height, and cover in habitats with bison indicated reduced aboveground plant competition in sites with large grazers. The activities of bison resulted in higher growth and reproduction for all six forb species studied and higher forb species richness. Habitats with bison were also characterized by much higher seed reproduction but no differences in vegetative reproduction compared to habitats without large grazers. Patterns of biomass allocation also showed high plasticity in these species, with reduced allocation to stem and increased allocation to reproduction in habitats with bison. The results of this study provide evidence the activities of bison reduce grass-forb competition, and that release from aboveground competition with grasses increases the growth, seed reproduction, abundance, and ultimately the diversity of perennial forb species in tallgrass prairie. The results further indicate perennial forbs in tallgrass prairie show high phenotypic plasticity in life history traits such as growth, reproduction, and resource allocation patterns.