IN-SITU EFFECTS OF ELEVATED CO2 ON THE CARBON AND NITROGEN STATUS OF ALPINE PLANTS

1. The effect of elevated CO2 on tissue composition in an alpine grass land (Swiss Central Alps, 2500m) under both natural and increased nutr ient supply (NPK) is summarized. 2. During 3 years of CO2 enrichment t he concentration of total non-structural carbohydrates (TNC) in leaves increased by 32% in Leontodon helveticus (largely sugar) and by 56% i n Trifolium alpinum (largely starch) but did not change significantly in the dominant sedge Carex curvula and in Poa alpina, currently a rar e species at this site. 3. Enhanced mineral nutrient supply (unlike el evated CO2) greatly stimulated growth but did not reduce the CO2-induc ed TNC accumulation, 4. Under elevated CO2 nitrogen concentrations (pe r g TNC-free dry matter) of green leaves decreased in Leontodon (-21%) and in Trifolium (-24%) but not or only slightly in Carer and in Pea. NPK addition compensated this CO2 effect an nitrogen concentration in Trifolium bur not in the other species. 5. In below-ground tissue nei ther TNC nor nitrogen concentration responded to CO2 fertilization. 6. The nitrogen pool per unit land area at peak season biomass remained unaffected by the CO2 treatment. 7. Overall our results suggest that t he late successional dominant sedge Carex curvula remains unaffected b y elevated CO2, independently of mineral nutrient supply, whereas the co-dominant and sub-dominant forbs Leontodon helveticus and Trifolium alpinum show both an increase of TNC as well as N depletion under elev ated CO2. 8. None of these changes in active plant tissue translate in to compositional changes in naturally senesced litter suggesting cauti on with predictions of CO2 effects on decomposition based on data from green plant material.