Growth responses of an alpine grassland to elevated CO2.

Alpine plant species have been shown to exhibit a more pronounced increase in leaf photosynthesis under elevated CO2 than lowland plants. In order to test whether this higher carbon fixation efficiency will translate into increased biomass production under CO2 enrichment we exposed plots of narrow alpine grassland (Swiss Central Alps, 2470 m) to ambient (355 μl l-1) and elevated (680 μl l-1) CO2 concentration using open top chambers. Part of the plost received moderate mineral nutrient additions (40 kg ha-1 year-1 of nitrogen in a complete fertilizer mix). Under natural nutrient supply CO2 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 CO2 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 CO2 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 CO2. Counter-intuitively, our results suggest that even under moderate climate warming or enhanced atmospheric nitrogen deposition positive biomass responses to CO2 enrichment of the currently dominating species are unlikely.