Selective grazing modifies previously anticipated responses of plant community composition to elevated CO 2 in a temperate grassland

Abstract

Our limited understanding of terrestrial ecosystem responses to elevated CO 2 is a major constraint on predicting the impacts of climate change. A change in botanical composition has been identified as a key factor in the CO 2 response with profound implications for ecosystem services such as plant production and soil carbon storage. In temperate grasslands, there is a strong consensus that elevated CO 2 will result in a greater physiological stimulus to growth in legumes and to a lesser extent forbs, compared with C3 grasses, and the presumption this will lead in turn to a greater proportion of these functional groups in the plant community. However, this view is based on data mainly collected in experiments of three or less years in duration and not in experiments where defoliation has been by grazing animals. Grazing is, however, the most common management of grasslands and known in itself to influence botanical composition. In a long‐term Free Air Carbon Dioxide Enrichment ( FACE ) experiment in a temperate grassland managed with grazing animals (sheep), we found the response to elevated CO 2 in plant community composition in the first 5 years was consistent with the expectation of increased proportions of legumes and forbs. However, in the longer term, these differences diminished so that the proportions of grasses, legumes and forbs were the same under both ambient and elevated CO 2 . Analysis of vegetation before and after each grazing event showed there was a sustained disproportionately greater removal (‘apparent selection’) of legumes and forbs by the grazing animals. This bias in removal was greater under elevated CO 2 than ambient CO 2 . This is consistent with sustained faster growth rates of legumes and forbs under elevated CO 2 being countered by selective defoliation, and so leading to little difference in community composition.

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