Plant responses to species removal and experimental warming in Alaskan tussock tundra

We manipulated air temperature and the presence of the seven dominant plant species in Alaskan tussock tundra and measured shoot growth, branching, aboveground biomass, and reproduction of the remaining plant species. Warming stimulated shoot growth of the dominant sedges and shrubs after one and two years of manipulation and total leaf biomass of the dominant shrubs after three years. Warming decreased aboveground biomass of Eriophorum vaginatum, Cassiope tetragona and most non-vascular species. Warming also reduced total reproductive output of two of three species measured. Removal of single species had no effect on shoot growth of the remaining species. However, total aboveground biomass and reproduction of Ledum palustre increased with removal of other shrub species, suggesting that competition limits biomass accumulation in L. palustre. Sphagnum removal increased the aboveground biomass of Betula nana. The higher frequency of significant warming versus species removal effects on plant growth and biomass suggests that direct limitation by environmental conditions is more important than limitation by species interactions in tussock tundra. Furthermore, we found no significant interactions between warming and species removal, suggesting that increased temperature per se will not alter the intensity of species interactions. When combined with knowledge of dispersal abilities and controls over establishment, extrapolation of species responses to environmental manipulation may thus allow us to predict effects of climate change on community composition.