Effects of free-air CO2 Enrichment (FACE) on experimental grassland communities

1. Experimental grassland communities (turves) were exposed to elevated (60 Pa) and ambient (35 Pa) CO2 partial pressures (pCO(2)) in a Free-air Carbon Dioxide Enrichment (FACE) experiment between 30 March 1995 and 4 July 1996. The vegetation was cut once during the experiment prior to the final harvest (harvest 2). 2, No significant treatment effects on total plant biomass at the whole turf level were detected, although biomass was typically about 25% higher under fumigation in year 1 and about 15% higher in year 2, 3, Biomass for two of the six sown species was significantly higher at harvest 2 than at harvest 1. There were no significant differences between individual species biomass under the two CO2 treatments at either harvest I or 2 or in terms of overall cumulative biomass. However, in four of the five sown species in both years biomass tended to be higher in the fumigated than in the control rings (Cerastium holosteiodes, Phleum pratense, Plantago lanceolata and Poa trivialis). In contrast, Lolium perenne showed increased biomass under the control treatment relative to the fumigated treatment in both years. Owing to the high variance both within and between rings for each of the two treatments the statistical power of most, but not all, of the analyses carried out was poor. 4, The relative proportions of each species in the turves under fumigated and control treatments was broadly similar after the first summer, with differences in die second year being mainly owing to the negative response of L. perenne to CO2 fumigation.