A community and population perspective of the possible role of grazing in determining the ground flora of ancient woodlands
2001
Watkinson, A.R. | Riding, A.E. | Cowie, N.R.
The ground flora in ancient woodlands is affected by a wide range of factors at a range of scales. In an analysis of 495 sites of ancient woodland throughout Great Britain, regional trends in ground flora species composition were explained primarily by climatic and soil variables, while the most important variable determining species richness was soil pH. In this analysis, approximately 85 per cent of the variation in ground flora composition could not be accounted for and was probably a result of local variation in a range of ecological processes including grazing by vertebrates. Within sites, the use of exclosures indicates that large herbivores may have a dramatic impact on vegetation structure and composition, but it is a very crude tool. The exclosures are often too small, and measurements do not allow for variations in the density or seasonality of grazing and ignore the fact that plant responses are typically non-linear. Manipulation of grazing levels is critical if we are to understand the impact of grazing on vegetation structure and composition. Individual species vary in their responses to grazing. A population approach is used to illustrate how it is possible to explore how individual species respond to grazing. This is illustrated with reference to Anemone nemorosa and how intermediate levels of grazing are expected to favour this species. Complex, and as yet poorly understood, feedback mechanisms between the plant and the grazing animal will contribute to the spatial structuring of grazing impacts on the woodland ground flora. A major challenge for predicting the impact of grazing on plant populations and communities is the quantification of the spatial grazing pattern.
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