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Abstract. I examined the role of species interactions on the population dynamics of the herb Thalictrum alpinum and the sedge Carex vaginata by removing the dwarf shrub Dryas octopetala over four growing seasons at Finse, in the alpine region of south Norway, Furthermore, by increasing temperature (open top chambers) and nutrient availability (nutrient addition), I assessed the effects of climate warming on biotic interactions.

Dryas removal increased the number of Thalictrum and Carex leaves, and flowering frequency of Carex. Flower stems and leaf stalks of Thalictrum, and the leaves of Carex, became significantly shorter following Dryas removal. Warming and nutrient addition increased vegetative plant growth, whereas warming alone had positive effects on sexual reproductive effort of the target species. My results suggest that there is both competition for nutrients, and a facilitative shelter effect of Dryas on its neighbours.

Species interactions affected population dynamics of Thalictrum and Carex. Interactions between the impacts of Dryas removal and abiotic factors on leaf production, suggested that interactions between Dryas and neighbouring species might be modified if temperature and, in particular, nutrient availability increase under global warming.

This study shows that both biotic interactions and abiotic environmental conditions may affect alpine plant population dynamics. Furthermore, it shows that climate change may modify species interactions.

Species interaction effects should be included in climate change experiments and in future models predicting plant community changes under global warming.