Dominant species are expected to have a large impact on their communities, including potentially reducing the abundance and diversity of other species present. Throughout North America, tallgrass prairies are characterized by the abundance of two dominant C4 grass species, Andropogon gerardii and Sorghastrum nutans. To investigate the role of these dominants in structuring the plant community, we seeded 46 experimental plots (28 m2) with a diverse mix of prairie species, either with or without the two dominant grasses. In the fourth growing season after planting, we estimated cover for each species present, as well as biomass of the experimental dominants and several other functional groups.
Results/Conclusions
A diverse community of 80 plant species established in the experimental plots. Most of the species were from our seeding mix, but there were also ruderal species that were not purposely seeded. One self-introduced species, Solidago altissima, was ubiquitous and became dominant (~70% average cover) in areas that were slightly lower in elevation (18 plots). In lowland plots where S. altissima was dominant, overall species richness was greatly reduced and the two experimental grass species (A. gerardii and S. nutans) were relatively uncommon (<30% average cover) in plots into which they were planted. In the remaining 28 upland plots, the experimental grasses were dominant in plots into which they were seeded (>50% average cover). S. altissima also was present in all upland plots, but with only half the average cover compared to lowland plots.
Within the upland plots, we examined how the experimental dominant grasses affected species richness of the rest of the community. Within plots where experimental dominants were planted, there was five-fold variation in the biomass of the dominant grasses. Total species richness of 1 m2 plots did not vary across this range of variation. A different pattern emerged when we examined the same data by dividing the 1 m2 plots into four 0.25 m2subplots. Subplots with greater than plot-wide average biomass of dominant grasses had significantly lower than plot-wide average species richness. At this smaller scale, the two dominant species seemed to reduce species richness of the other plant species.
Overall, our results demonstrate that the ability of dominant grasses to establish quickly is dependent on microhabitat. When dominant grasses do establish successfully, they can reduce species richness of the rest of the community on a local spatial scale.