Long-term responsiveness to free air CO2 enrichment of functional types, species and genotypes of plants from fertile permanent grassland

To test inter- and intraspecific variability in the responsiveness to elevated CO₂, 9–14 different genotypes of each of 12 perennial species from fertile permanent grassland were grown in Lolium perenne swards under ambient (35 Pa) and elevated (60 Pa) atmospheric partial pressure of CO₂ (pCO₂) for 3 years in a free air carbon dioxide enrichment (FACE) experiment. The plant species were grouped according to their functional types: grasses (L. perenne, L. multiflorum, Arrhenatherum elatius, Dactylis glomerata, Festuca pratensis, Holcus lanatus, Trisetum flavescens), non-legume dicots (Rumex obtusifolius, R. acetosa, Ranunculus friesianus), and legumes (Trifolium repens, T. pratense). Yield (above a cutting height of 4.5 cm) was measured three times per year. The results were as follow. (1) There were highly significant differences in the responsiveness to elevated pCO₂ between the three functional types; legumes showed the strongest and grasses the weakest yield increase at elevated pCO₂. (2) There were differences in the temporal development of responsiveness to elevated pCO₂ among the functional types. The responsiveness of the legumes declined from the first to the second year, while the responsiveness of the non-legume dicots increased over the 3 years. During the growing season, the grasses and the non-legume dicots showed the strongest response to elevated pCO₂ during reproductive growth in the spring. (3) There were no significant genotypic differences in responsiveness to elevated pCO₂. Our results suggest that, due to interspecific differences in the responsiveness to elevated pCO₂, the species proportion within fertile temperate grassland may change if the increase in pCO₂ continues. Due to the temporal differences in the responsiveness to elevated pCO₂ among species, complex effects of elevated pCO₂ on competitive interactions in mixed swards must be expected. The existence of genotypic variability in the responsiveness to elevated pCO₂, on which selection could act, was not found under our experimental conditions. Received: 11 May 1997 / Accepted: 11 August 1997