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Indices for dyke vegetation. 1982. Nature in Cambridgeshire 25, 34—40.

INTRODUCTION

Wetlands are perhaps the most threatened British habitat, and one part of this habitat is the dykes and drains of the Fenland and similar alluvial plains. Although there are many reports of deterioration in fen dyke vegetation, there is a lack of firm evidence. Studies on hedgerows received a great impetus with the introduction of Dr Max Hooper's (1970) hedge age index, which was simple to apply and yielded fascinating information. Unfortunately dyke indices cannot provide evidence of Anglo-Saxon origin, but it is hoped that by their use Trust members will be able both to monitor changes (by recording the same dykes over a period of years) and to assess the botanical value of various parts of the Fens (by intensive surveys in single seasons).

The dykes and drains in the Fenland have been managed since their creation, as their drainage efficiency is maintained only by regular dredging to remove accumulating silt and by weed control measures to prevent vegetation hindering the easy movement of water through them. The traditional method of weed control was cutting, and this is still used, though to a lesser extent. The first chemical control was by copper sulphate, which is no longer permitted. Over the past few decades the use of aquatic herbicides has greatly increased, and so has the farming requirement for lowered water levels. High-quality plant communities are now rare outside nature reserves, and dykes with little or even no macrophyte (large-plant) vegetation are common.

It is simple to recognise and classify the extremes - first-class vegetation and empty channels. However, there is a need for a quantitative method of assessing vegetation, particularly in the intermediate stages of damage. 'That dyke is not so good' is imprecise and can convey different ideas to different readers. A complete community analysis, though the best method of description, is too cumbersome - and too complicated for non-specialists - for the assessment of an area as being of, say, moderate quality. The convenient solution is the use of a vegetation index. This leads to each dyke community being given a number or letter and to all dykes with the same number or letter being considered as having vegetation of equivalent quality. This enables the vegetation to be classified, and comprehended, easily and quickly. The danger of using an index is that the observer may become obsessed by the number or letter and forget what this means in terms of plants, and so be too concerned when the classification alters by a small amount, without considering whether this small change is ecologically significant in terms of the total dyke environment.

Two attempts have been made to devise dyke indices, by de Lange and van Zon (in press, but earlier versions have been published in 1973 and 1977) and by Haslam & Wolseley (1981). Both assess the quality of the vegetation of man-made channels in alluvial plains such as the Fenland. They are constructed differently, and this paper compares their function and use.