Abstract
The relationship between plant species diversity and ecosystem CO 2 and water vapour fluxes was investigated for planted calcareous grassland communities composed of 5, 12, or 32 species assembled from the native plant species pool. These diversity manipulations were done in factorial combination with a CO 2 enrichment experiment in order to investigate the degree to which ecosystem responses to elevated CO 2 are altered by a loss of plant diversity. Ecosystem CO 2 and H 2 O fluxes were measured over several 24‐h periods during the 1994 and 1995 growing seasons. Ecosystem CO 2 assimilation on a ground area basis decreased with decreasing plant diversity in the first year and this was related to a decline in above‐ground plant biomass. In the second year, however, CO 2 assimilation was not affected by diversity, and this corresponded to the disappearance of a diversity effect on above‐ground biomass. Irrespective of diversity treatment, CO 2 assimilation on a ground area basis was linearly related to peak above‐ground biomass in both years. Elevated CO 2 significantly increased ecosystem CO 2 assimilation in both years with no interaction between diversity and CO 2 treatment, and no corresponding increase in above‐ground biomass. There were no significant effects of diversity on water vapour flux, which was measured only in the second year. There were indications of a small CO 2 effect on water vapour flux (3–9% lower at elevated CO 2 depending on the light level). Our findings suggest that decreasing plant species diversity may substantially decrease ecosystem CO 2 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 ecosystem CO 2 assimilation to elevated CO 2 .