A plant trait analysis of responses to grazing in a long-term experiment

1. There are few long-term experimental studies of plant community responses to changes in grazing intensity. Here we report species' changes in a mesotrophic grassland after 12 years of a grazing experiment and relate these changes to species' life-history traits. 2. The experiment was set up in 1986 on an extensified species-poor grassland in lowland UK. Treatments comprised sheep grazing vs. no grazing in winter, grazing vs. no grazing in spring, and two grazing intensities in summer, in a 2 x 2 x 2 factorial design with two replicate blocks. 3. Point quadrat surveys in 1998 showed responses to grazing treatments by 17 of the 22 most common species. Species showed different responses, many of which were specific to a grazing season. Community changes were similar under spring and winter grazing, but the heavier summer grazing had different consequences. Species richness was increased by spring grazing, decreased by heavier summer grazing and unaffected by winter grazing. 4. More species responded to treatments in the 1998 survey compared with a survey in 1990. Furthermore, the whole experimental grassland had changed between the surveys, probably as a result of falling soil fertility. The two dominant grasses had declined drastically and most other species had increased in abundance. Five new species were found in 1998. 5. Intensive surveys of dicotyledonous species hi 1998 showed five of the 12 most common species had responded to grazing treatments. In most cases dicotyledonous species had increased in abundance under heavier grazing in one or more season, and species richness was increased by spring and winter grazing. Compared with a 1991 survey, the number of species responding to treatments had increased by 1998 and seven new species were found. 6. We tested whether species' responses to grazing were linked to life-history traits according to three hypotheses: that heavier grazing would disadvantage (i) species grazed preferentially, (ii) species less able to colonize gaps or (iii) more competitive species. Mechanisms differed among seasonal treatments. Responses to heavier summer grazing were linked strongly to gap colonization ability. Responses to spring and winter grazing were positively related to grazer selectivity, a surprising result that might be explained if selectivity was positively related to plant regrowth ability. 7. This study shows the need for long-term experimental analyses of community responses to grazing as vegetation responses may develop over a long time. The traits analysis suggests it may be possible to predict changes in species composition under grazing through an understanding of the mechanisms of plant responses. Grassland managers require such information in order to manipulate grazing regimes to achieve, for example, diversification or weed control.