Dominant plant species may affect ecosystem processes through their effects on abiotic conditions and litter composition. However, experimental manipulations of the presence of dominant species are rare and the relative influence of dominant species on ecosystem processes are largely unknown. 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. We investigated whether litter decomposition rates are affected by dominant grasses by placing litterbags in plots with and without the dominant species present. Litterbags contained one of six litter treatments: grass (primarily S. nutans), legume (Baptisia lactea), or forb (Solidago rigida) litter alone, or in combination (grass+legume, grass+forb, and grass+legume+forb).
Dominant grass biomass made up more than 50% of total aboveground biomass when S. nutans and A. gerardii were present. Dominant grass presence also increased total biomass by 25% and decreased light availability compared to plots where these species were subtracted from the seed mix. After 18 months of litter decay, there was a significant effect of litter type and community type on decomposition rates. In single-species litterbags, grass litter decomposed slower than forb litter, with the most rapid mass loss occurring in legume litter. Mixed-species litterbags showed non-additive effects, with most litter mixtures decomposing faster than expected based on the mass loss observed in single-species litterbags. Furthermore, litterbags placed within communities containing the dominant grass species decomposed more slowly, but only for the legume and grass+legume treatments. Other species types or mixtures showed no difference in decomposition rates when placed in communities with or without the dominant grasses present. Thus, dominant grasses influenced decomposition rates via changes in litter composition and environmental conditions, but the effect of dominant grass presence on decomposition was unique to different litter mixtures.