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

Abstract

Summary 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. 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 × 2 × 2 factorial design with two replicate blocks. 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. 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. Intensive surveys of dicotyledonous species in 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. 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. 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.

Factors and processes affecting plant biodiversity in permanent grasslands. A review

“…Most other studies have found no effect of grazing seasonality on species richness (Gibson and Brown 1991;Smith et al 1996). However, some authors (Bullock et al 2001;Gibson and Brown 1991;Watt et al 1996) have found that a combination of spring and winter grazing increases species richness in grasslands, while Bullock et al (2001) have shown that heavy summer grazing decreases species richness in some grassland. Moreover, spring and winter grazing primarily increases the proportion of dicotyledonous species, such as legumes (Watt et al 1996).…”

“…The summer grazing mechanism was clearly through the opening of vegetation gaps, and superior gap colonizers plant species showed a more positive response to higher stocking rate (Bullock et al 2001). For example, Silvertown and Smith (1989) observed that heavy grazing by sheep in Akinola et al 1998;Dutoit et al 2003;Zimmergren 1980 Low similarity in species composition between soil seed bank and established vegetation Amiaud and Touzard 2004;Edwards and Crawley 1999;Martinkova et al 2009a;Smith et al 2002 Seed predation…”

“…Conversely, low-stocking rate favors stress-tolerant grasses (sensu Grime et al 1988) and forbs but depending on the starting floristic composition and site conditions ). The richness of dicotyledonous species increases with increased stocking rate by sheep (Bullock et al 2001). However, this relationship is valid only above a poorly defined threshold of stocking rate; a comparison of three low sheep grazing intensities (one, two, and four sheep per hectare) does not show any differences in the plant species composition of grassland vegetation (Calladine et al 2002).…”

“…The influence of grazing seasonality by sheep seems to be study-dependent, ranging from beneficial for spring grazing (Bullock et al 2001) to neutral, whatever the season (Bullock et al 1994). Grasslands grazed by cattle throughout the year have greater plant species richness than the same grasslands grazed occasionally (Sternberg et al 2000).…”

“…Grazing seasonality ↑ Ruderal and competitive species Bullock et al 2001;Gibson and Brown 1991;Sternberg et al 2000;Watt et al 1996 Trampling induces gaps, which are favorable for seed recruitment and maintenance of species Amiaud and Touzard 2004;Bullock et al 1995;Chambers 1993;Edwards and Crawley 1999;Milberg 1993;Rapp and Rabinowitz 1985;Watt and Gibson 1988 Establishment capacities…”