Effects of elevated carbon dioxide on British native grassland species and communities

The implications of current climate-change scenarios for British species and communities are not straightforward: short-term effects may be mediated by longer term source-sink interactions; what species do in isolation may not match what they do under competitive conditions; community-level responses may be mediated by herbivory and soil microbiology; ecosystem-level responses may be overridden by direct or indirect human activity. This article reviews UCPE findings in these areas since 1987. In single-species screening of herbaceous species, strong responses to elevated CO₂ occur mainly in robust, fast-growing perennials, though smaller-scale studies do not necessarily support this trend. In woody species screening, increases under elevated CO₂ are higher in deciduous species than in evergreen ones. Time-course experiments show that species which are non-responsive in the long term can respond in the shorter term, and that external mineral nutrient level is also important. Elevating CO₂ at above-ambient temperatures promotes CO₂-responsiveness in lower-yielding herbaceous species, except where defoliation is also present when recovery is better in higher-yielding species. In laboratory microcosms on native soil, fast-growing species do not increase in relative abundance under elevated CO₂ because there appears to be mineral nutrient sequestration by the expanded microflora. In Baby-FACE field microcosms, elevated CO₂ has no significant effect on community composition unless accompanied by elevated temperature and/or by added nutrients. Nutrient additions lead to much bigger gains in non-mycorrhizal species than in mycorrhizal ones. Field microcosms with artificial crevices show a differential effect of natural summer drought on contrasted species. CO₂ screening experiments can be paralleled by similar work on sulphur dioxide and on ozone. In both of these gases, plant response is strongly and inversely related to seedling growth rate. This contrasts with the pattern found under elevated CO₂, which appears to depend upon a combination of plant physiology plus evolutionary history. Responsiveness to CO₂ also helps to identify basic 'functional types' and multivariate methods can show that competitive species depend upon high lateral spread, long life history, high responsiveness to elevated CO₂, and inbreeding reproduction. Expert-system models can predict outline responses of vegetation to climate and management changes, and cellular automata successfully mimic plant morphology and function in relation to resource acquisition and utilisation within component modules. Plant community responses to elevated CO₂ are now being seen less and less as the products of the responsiveness of the component species measured in isolation and more, for example, as the product of habitat richness, especially in respect of mineral nutrients.