NATURAL PASTURE COMMUNITY RESPONSE TO ENRICHED CARBON-DIOXIDE ATMOSPHERE

We examined the response of a pasture community in southern Quebec (Ca nada) to long-term exposure of enriched atmospheric CO2 conditions. Th e study was conducted using open-top growth chambers directly placed o n top of the natural pasture community. To investigate the change in t he overall species composition in time and space, we used canonical co rrespondence analysis, a direct ordination method. Over the three year s, the overall community responded significantly to enriched CO2. The analyses show that, after three years, CO2 was the most important envi ronmental variable affecting the species composition. Initially the pr esence of the wall of the chambers influenced the composition but CO2 became more important by the third year. Soil and air temperatures onl y slightly influenced the community composition. The first two axes of the canonical correspondence analysis explained a large proportion of the variation in the three years and these trends appeared to increas e with time. Species such as Agropyron repens appeared to be positivel y influenced by the presence of the wall (slightly warmer conditions). However, the analyses suggest that Phleum pratense and Trifolium repe ns, for example, were favored by the increase in atmospheric CO2. The variation in species composition in enriched versus ambient CO2 chambe rs suggests that the effect of the environmental factors, particularly CO2, were important in affecting the rate and pattern of succession. Furthermore, the temporal increase in importance of the variable CO2 i n the present analyses indicates that there might be a time-lag in res ponse to atmospheric enrichment.