Dominant species are thought to play a significant role in community dynamics and ecosystem processes. However, experimental manipulations of the presence of dominant species are rare. In tallgrass prairie ecosystems, dominance by perennial C4 grasses is well documented, and restored prairies often show increased dominance of these species compared to native prairie. To examine the ecological role of dominance, we established a high-diversity tallgrass prairie restoration (>60 species planted) where we experimentally assembled communities that included or subtracted the two dominant grass species, Andropogon gerardii and Sorghastrum nutans, from the planted seed mix. After four growing seasons, we examined the impact of the presence or absence of the dominant species on plant community composition, aboveground and belowground biomass, and the pools and fluxes of soil carbon and nitrogen.
Results/Conclusions
Upland plots containing the dominant grasses had approximately 25% greater aboveground and belowground biomass when compared to those where the dominant species were subtracted from the seed mix. On average, dominant grass biomass made up more than 50% of total aboveground biomass when the species were present. In the absence of the dominant grasses, we observed partial compensation in the biomass of all plant functional groups. Surprisingly, one non-planted, self-recruiting forb species, Solidago altissima, showed the greatest increases in biomass and made up more than 50% of aboveground biomass in plots where dominant grasses were excluded. Dominant grasses also significantly decreased the richness of the rest of the community (approximately 3.5 fewer species per m2 in plots containing the dominant grasses), with ruderal forb species and several subdominant grasses showing the greatest declines in abundance when the dominant species were present. Soil carbon and nitrogen pools and fluxes were less responsive to the presence or absence of the dominant species. We observed no significant differences between our treatments in potential net nitrogen mineralization rates, soil carbon and nitrogen content, or bulk density.
The presence of dominant species significantly increased plant biomass and decreased plant species richness in tallgrass prairie, even early in ecosystem establishment. Pools and fluxes of soil carbon and nitrogen were less responsive to dominant species presence in the short term; however the potential long term impacts are currently unknown.