North European Heath Communities: A `Network of Variation'

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Abstract
Floristic and quantitative studies were made on 63 stands of lowland heath ranging through S. W. Norway, through W., S. W., S. and S. E. Sweden to Denmark, N. Germany and N. Holland. The aim was to discover to what extent a phytosocio-logical treatment would result in the recognition of community types which could be compared with those described by other workers, and to what extent evidence of continuous variation in community composition could be obtained. Lists from the various stands were grouped on the basis of general floristic similarity, resulting in the recognition of 4 chief groups: Calluna vulgaris with Erica cinerea Calluna with Vaccinium spp. (lacking E. cinerea); Calluna with Empetrum nigrum Calluna with Genista spp. (the latter proved to be heterogeneous and was subdivided.). These showed agreement with groups established by other workers. However, quite a large number of stands had to be regarded as transitional between certain groups, or were unplaced for other reasons. A. "line intercept" method of measuring the cover contribution of the chief species is described. Indications of more or less continuous variation in the composition of the stands were further examined using the figures for cover contributed by the dwarf shrubs and other species of the same stratum. Stands were first grouped according to the species next in the lead after Calluna. The groups could be arranged in a sequence throughout which the mean cover of the chief species showed regular increases or decreases. However, a branching rather than a linear pattern of relationships was suggested. Preparation of a histogram for each stand, showing the cover contributed by the chief species, permitted ordination by inspection. This was consistent with the idea of a network of relationships, each showing more or less continuous variation. Lines of variation were tentatively associated with climatic trends. Where local microclimates are more severe than those typical for a given region, the resulting communities fall on lines of variation composed of stands drawn mainly from farther north. The stands comprising the main groups based on floristic analysis are shown to belong to one or a few closely related lines of variation. The two types of treatment are thus complementary rather than exclusive, each contributing important information in the study of directions of variation in community composition.