Vascular plant responses to sheep grazing in a 12-year-long experiment on mesotrophic grassland at Little Wittenham Nature Reserve, Oxfordshire, England
There are few long-term experimental studies of plant community responses to changes in grazing intensity. In a mesotrophic grassland in southern England, after 12 years of a grazing experiment, individual plant species responses were recorded and these related to changes in life-history traits.
Study site: The experiment was set up at Little Wittenham Nature Reserve in Oxfordshire, UK (National Gird reference SU568924; 15°37' N, 1°10' W), on a mesotrophic grassland underlain by a calcareous clay loam. The grassland was species-poor and dominated by grasses typical of agriculturally improved sites, such as perennial rye-grassLolium perenne, creeping bent Agrostis stolonifera and rough meadow grass Poa trivialis. It was classified as transitional between British National Vegetation Classification vegetation types MG7 Lolium perenne ley and MG6c Lolium perenne–Cynosurus cristatus grassland, Trisetum flavescens subcommunity.
The site had been resown in the 1940s and was intensively managed using sheep grazing and inorganic fertilizers until 1984 when fertilizer application ceased.
Responses to grazing: Vegetation responses to season and intensity of sheep grazing were investigated, contrasting, three grazing seasons and within each season two grazing intensities in a 2 × 2 × 2 factorial design. Seasons were: winter (2 November–21 March); spring (22 March–21 May); and summer (22 May–1 November). The two intensities in winter and spring comprised no grazing or grazing by two Suffolk × Mule ewes per paddock. All paddocks were grazed in the summer, but two intensities were applied: light, to a sward height of 9 cm, or heavy, to a height of 3 cm. Summer sward heights were achieved and maintained by adjusting stocking rates following weekly or fortnightly height measurements with a sward stick. The eight treatments were applied in a randomized block design with two blocks, in 16 paddocks of 50 × 50 m. The experiment was set up in 1986 and there were no initial differences in species composition between treatments
Vegetation surveys: A point quadrat survey of the vegetation in each paddock was carried out between 6 and 10 August 1990, and using identical techniques between 4 and 13 August 1998.
Species traits: Data were obtained for the 17 most common species in the experiment (those present in at least eight of the 16 paddocks.
Surveys in 1998 showed responses to grazing treatments by 17 of the 22 most common vascular plant species. Species showed different responses, many of which were specific to a grazing season. Community changes were similar under spring and winter grazing, but 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 to 1990. Changes in the whole experimental grassland between the surveys, probably as a result of falling soil fertility, were recorded. The two dominant grasses (perennial rye-grassLolium perenne and creeping bent Agrostis stolonifera) declined drastically and most other species had increased in abundance. Five new species were found in 1998.
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.
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.
Conclusions: This study shows the value of long-term experiments to identify plant community responses to grazing. Traits analysis suggests it may be possible to predict changes in species composition under grazing through an understanding of the mechanisms of plant responses. Such information will assist grassland managers to manipulate grazing regimes to achieve specified aims, for example, sward diversification or weed control.
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