Growth dynamics and population development in an alpine grassland under elevated CO2

Leaf expansion, population dynamics and reproduction under elevated CO2 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 mu l l(-1) and 680 mu l l(-1) CO2 in open-top chambers over three growing seasons. Treatments also included natural and moderately improved mineral nutrient supply (40 kg N ha(-1) year(-1) in an NPK fertilizer mix). Seasonal dynamics of leaf expansion, which was studied for the dominant graminoid Carer curvula only, were not affected by elevated CO2 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 CO2 did not cause any further stimulation. Plant and tiller density (studied in all species) increased under elevated CO2 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 CO2 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 CO2, indicating CO2 effects on sexual reproduction in these two dominant species. The results suggest that the effects of elevated CO2 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 CO2 may negatively affect the abundance of the species most characteristic of this alpine plant community.