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

1. There are few long-term experimental studies of plant community response s to changes in grazing intensity. Here we report species' changes in a mes otrophic 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 grassla nd in lowland UK. Treatments comprised sheep grazing vs. no grazing in wint er, 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 differe nt consequences. Species richness was increased by spring grazing, decrease d 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 b etween the surveys, probably as a result of falling soil fertility. The two dominant grasses had declined drastically and most other species had incre ased in abundance. Five new species were found in 1998. 5. Intensive surveys of dicotyledonous species hi 1998 showed five of the 1 2 most common species had responded to grazing treatments. In most cases di cotyledonous species had increased in abundance under heavier grazing in on e or more season, and species richness was increased by spring and winter g razing. Compared with a 1991 survey, the number of species responding to tr eatments had increased by 1998 and seven new species were found. 6. We tested whether species' responses to grazing were linked to life-hist ory traits according to three hypotheses: that heavier grazing would disadv antage (i) species grazed preferentially, (ii) species less able to coloniz e gaps or (iii) more competitive species. Mechanisms differed among seasona l treatments. Responses to heavier summer grazing were linked strongly to g ap colonization ability. Responses to spring and winter grazing were positi vely related to grazer selectivity, a surprising result that might be expla ined if selectivity was positively related to plant regrowth ability. 7. This study shows the need for long-term experimental analyses of communi ty responses to grazing as vegetation responses may develop over a long tim e. The traits analysis suggests it may be possible to predict changes in sp ecies 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 wee d control.