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Alpine plant species have been shown to exhibit a more pronounced increase in leaf photosynthesis under elevated CO than lowland plants. In order to test whether this higher carbon fixation efficiency will translate into increased biomass production under CO enrichment we exposed plots of narrow alpine grassland (Swiss Central Alps, 2470 m) to ambient (355 μl l) and elevated (680 μl l) CO concentration using open top chambers. Part of the plost received moderate mineral nutrient additions (40 kg ha year of nitrogen in a complete fertilizer mix). Under natural nutrient supply CO enrichment had no effect on biomass production per unit land area during any of the three seasons studied so far. Correspondingly, the dominant species Carex curvula and Leontodon helveticus as well as Trifolium alpinum did not show a growth response either at the population level or at the shoot level. However, the subdominant generalistic species Poa alpina strongly increased shoot growth (+47%). Annual root production (in ingrowth cores) was significantly enhanced in C. curvula in the 2nd and 3rd year of investigation (+43%) but was not altered in the bulk samples for all species. Fertilizer addition generally stimulated above-ground (+48%) and below-ground (+26%) biomass production right from the beginning. Annual variations in weather conditions during summer also strongly influenced above-ground biomass production (19-27% more biomass in warm seasons compared to cool seasons). However, neither nutrient availability nor climate had a significant effect on the CO response of the plants. Our results do not support the hypothesis that alpine plants, due to their higher carbon uptake efficiency, will increase biomass production under future atmospheric CO enrichment, at least not in such late successional communities. However, as indicated by the response of P. alpina, species-specific responses occur which may lead to altered community structure and perhaps ecosystem functioning in the long-term. Our findings further suggest that possible climatic changes are likely to have a greater impact on plant growth in alpine environments than the direct stimulation of photosynthesis by CO. Counter-intuitively, our results suggest that even under moderate climate warming or enhanced atmospheric nitrogen deposition positive biomass responses to CO enrichment of the currently dominating species are unlikely.

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Summary1. The effect of elevated CO 2 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 CO 2 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 CO 2 ) greatly stimulated growth but did not reduce the CO 2 -induced TNC accumulation. 4. Under elevated CO 2 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 CO 2 effect on nitrogen concentration in Trifolium but not in the other species. 5. In below-ground tissue neither TNC nor nitrogen concentration responded to CO 2 fertilization. 6. The nitrogen pool per unit land area at peak season biomass remained unaffected by the CO 2 treatment. 7. Overall our results suggest that the late successional dominant sedge Carex curvula remains unaffected by elevated CO 2 , 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 CO 2 . 8. None of these changes in active plant tissue translate into compositional changes in naturally senesced litter suggesting caution with predictions of CO 2 effects on decomposition based on data from green plant material.

Leaf expansion, population dynamics and reproduction under elevated CO were studied for two dominant and four subdominant species in a high alpine grassland (2500 above sea level, Swiss Central Alps). Plots of alpine heath were exposed to 335 μl l and 680 μl l CO in open-top chambers over three growing seasons. Treatments also included natural and moderately improved mineral nutrient supply (40 kg N ha year in an NPK fertilizer mix). Seasonal dynamics of leaf expansion, which was studied for the dominant graminoid Carex curvula only, were not affected by elevated CO during two warm seasons or during a cool season. Improved nutrient supply increased both the expansion rate and the duration of leaf growth but elevated CO did not cause any further stimulation. Plant and tiller density (studied in all species) increased under elevated CO in the codominant Leontodon helveticus and the subdominant Trifolium alpinum, remained unchanged in two other minor species Poa alpina and Phyteuma globulariifolium, and decreased in Carex curvula. In Potentilla aurea elevated CO compensated for a natural decline in shoot number. By year 3 the number of fertile shoots in Leontodon and individual seed weight in Carex were slightly increased under elevated CO, indicating CO effects on sexual reproduction in these two dominant species. The results suggest that the effects of elevated CO on the population dynamics of the species studied were not general, but species-specific and rather moderate effects. However, the reduction of tiller density in Carex curvula, in contrast to the increases observed in Leontodon helveticus and Trifolium alpinum, indicates that elevated CO may negatively affect the abundance of the species most characteristic of this alpine plant community.

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