Grazing and vegetation change: deflected or modified succession?
Abstract
Vegetation change during the first 6 years of secondary succession in a calcareous former arable field and the effect of controlled grazing by sheep was studied in 1984-89 at the University of Oxford's Wytham estate, UK. The vegetation was grazed in 30 × 30 m paddocks from 13 to 23 Apr. or 9 to 24 Sep. by 3 sheep or from Apr. to Nov. or Aug. to Nov. by 6 sheep/paddock, or was not grazed. The results were compared with similar data from 40- to 100-years-old grassland nearby. The relative effects of time, place, sheep grazing and season of grazing on community change were assessed by canonical community analysis: detailed responses of the commoner species were investigated by analysis of variance. The two approaches complemented each other; multivariate analysis clearly separated effects on the community scale but was weak at detecting complex interactions. Grazing deflected succession but was subordinate to an intrinsic rate of vegetation change and to local variations in its direction. Succession only moved immediately towards the composition of the local ancient grasslands under the heaviest grazing treatment. Irrespective of grazing treatment and local conditions, secondary succession towards species-rich calcicolous grassland appeared to take at least a century to run its course. It was suggested that grazed grassland and forest successions could operate over similar time-scales. The implications for countryside management and 'creative conservation' are discussed.