GROWTH-RESPONSES OF AN ALPINE GRASSLAND TO ELEVATED CO2

Alpine plant species have been shown to exhibit a more pronounced incr ease in leaf photosynthesis under elevated CO2 than lowland plants. In order to test whether this higher carbon fixation efficiency will tra nslate into increased biomass production under CO2 enrichment we expos ed plots of narrow alpine grassland (Swiss Central Alps, 2470 m) to am bient (355 mu l l(-1)) and elevated (680 mu l l(-1)) CO2 concentration using open top chambers. Part of the plots received moderate mineral nutrient additions (40 kg ha(-1) year(-1) of nitrogen in a complete fe rtilizer mix). Under natural nutrient supply CO2 enrichment had no eff ect on biomass production per unit land area during any of the three s easons studied so far. Correspondingly, the dominant species Carex cur vula and Leontodon helveticus as well as Trifolium alpinum did not sho w a growth response either at the population level or at the shoot lev el. However, the subdominant generalistic species Poa alpina strongly increased shoot growth (+47%). Annual root production (in ingrowth cor es) was significantly enhanced in C. curvula in the 2nd and 3rd year o f 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 in fluenced above-ground biomass production (19-27% more biomass in warm seasons compared to cool seasons). However, neither nutrient availabil ity nor climate had a significant effect on the CO2 response of the pl ants. Our results do not support the hypothesis that alpine plants, du e to their higher carbon uptake efficiency, will increase biomass prod uction under future atmospheric CO2 enrichment, at least not in such l ate 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-ter m. Our findings further suggest that possible climatic changes are lik ely to have a greater impact on plant growth in alpine environments th an the direct stimulation of photosynthesis by CO2. Counter-intuitivel y, our results suggest that even under moderate climate warming or enh anced atmospheric nitrogen deposition positive biomass responses to CO 2 enrichment of the currently dominating species are unlikely.