In situ effects of elevated CO2 on the carbon and nitrogen status of alpine plants

Abstract

1. The effect of elevated CO2 on tissue composition in an alpine grassland (Swiss Central Alps, 2500 m) under both natural and increased nutrient supply (NPK) is summarized. 2. During 3 years of CO2 enrichment the concentration of total non-structural carbohydrates (TNC) in leaves increased by 32% in Leontodon helveticus (largely sugar) and by 56% in Trifolium alpinum (largely starch) but did not change significantly in the dominant sedge Carex curvula and in Poa alpina, currently a rare species at this site. 3. Enhanced mineral nutrient supply (unlike elevated CO2) greatly stimulated growth but did not reduce the CO2-induced TNC accumulation. 4. Under elevated CO2 nitrogen concentrations (per g TNC-free dry matter) of green leaves decreased in Leontodon (-21%) and in Trifolium (-24%) but not or only slightly in Carex and in Poa. NPK addition compensated this CO2 effect on nitrogen concentration in Trifolium but not in the other species. 5. In below-ground tissue neither 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 the late successional dominant sedge Carex curvula remains unaffected by 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 elevated CO2. 8. None of these changes in active plant tissue translate into compositional changes in naturally senesced litter suggesting caution with predictions of CO2 effects on decomposition based on data from green plant material.