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, ab oveground biomass, and reproduction of the remaining plant species. Warming stimulated shoot growth of the dominant sedges and shrubs after one and tw o 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 to tal reproductive output of two of three species measured. Removal of single species had no effect on shoot growth of the remaining species. However. t otal 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 bio mass of Betula nana. The higher frequency of significant warming versus spe cies removal effects on plant growth and biomass suggests that direct limit ation by environmental conditions is more important than limitation by spec ies interactions in tussock tundra. Furthermore, we found no significant in teractions between warming and species removal, suggesting that increased t emperature per se will not alter the intensity of species interactions. Whe n combined with knowledge of dispersal abilities and controls over establis hment, extrapolation of species responses to environmental manipulation may thus allow us to predict effects of climate change on community compositio n.