A field study of the effects of elevated CO2 and plant species diversity on ecosystem-level gas exchange in a planted calcareous grassland

The relationship between plant species diversity and ecosystem CO2 and wate r vapour fluxes was investigated for planted calcareous grassland communiti es composed of 5, 12, or 32 species assembled from the native plant species pool. These diversity manipulations were done in factorial combination wit h a CO2 enrichment experiment in order to investigate the degree to which e cosystem responses to elevated CO2 are altered by a loss of plant diversity . Ecosystem CO2 and H2O fluxes were measured over several 24-h periods duri ng the 1994 and 1995 growing seasons. Ecosystem CO2 assimilation on a groun d area basis decreased with decreasing plant diversity in the first year an d this was related to a decline in above-ground plant biomass. In the secon d year, however, CO2 assimilation was not affected by diversity, and this c orresponded to the disappearance of a diversity effect on above-ground biom ass. Irrespective of diversity treatment, CO2 assimilation on a ground area basis was linearly related to peak aboveground biomass in both years. Elev ated CO2 significantly increased ecosystem CO2 assimilation in both years w ith no interaction between diversity and CO2 treatment, and no correspondin g increase in above-ground biomass. There were no significant effects of di versity on water vapour nux, which was measured only in the second year. Th ere were indications of a small CO2 effect on water vapour flux (3-9% lower at elevated CO2 depending on the light level). Our findings suggest that d ecreasing plant species diversity may substantially decrease ecosystem CO2 assimilation during the establishment of such planted calcareous grassland communities, but also suggest that this effect may not persist. In addition , we find no evidence that plant species diversity alters the response of e cosystem CO2 assimilation to elevated CO2.