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North European Heath Communities: A `Network of Variation'

C. H. Gimingham
Journal of Ecology
Vol. 49, No. 3 (Oct., 1961), pp. 655-694
DOI: 10.2307/2257230
Stable URL: http://www.jstor.org/stable/2257230
Page Count: 41
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North European Heath Communities: A `Network of Variation'
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Abstract

1. Floristic and quantitative studies were made on sixty-three stands of lowland heath vegetation ranging through S.W. Norway, through W., S.W., S. and S.E. Sweden to Denmark, N. Germany and N. Holland. The survey was restricted to heaths on the more freely drained soils, in most cases dominated by Calluna vulgaris. 2. The aim was to discover to what extent a phytosociological treatment would result in the recognition of reasonably clear-cut 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. 3. A method of grouping lists from the various stands on the basis of general floristic similarity by `successive approximations' (Poore 1956) was followed. This avoided the use both of `faithful' and `geographical guiding' species, and thus permits comparison of the results with the categories arrived at by workers using these criteria. 4. Four chief groups emerged: Calluna with Erica cinerea; Calluna with Vaccinium spp. (lacking Erica cinerea); Calluna with Empetrum nigrum and Calluna with Genista spp. (The latter proved to be heterogeneous and was provisionally divided into Calluna with Genista anglica with or without G. pilosa, and Calluna with Genista pilosa only.) These showed close agreement with groups established by other workers. Two other types of community were represented: dune heaths with Carex arenaria, etc., and heaths containing much Erica tetralix. 5. Quite a large number of stands had to be regarded as transitional between certain of the above groups, or were unplaced for other reasons. 6. 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, obtained from the floristic analysis, were further examined using the figures for cover contributed by the dwarf shrubs and other species of the same stratum. 7. 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 each of the chief species showed more or less regular increases or decreases, or both (rising to a peak and declining). However, on sub-grouping the stands according to the species coming second in importance it was no longer possible to maintain a simple sequence, and a branching rather than linear pattern of relationships was suggested. 8. Preparation of a histogram for each stand, showing the cover contributed by the chief species, permitted arrangement of stands 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 (`northern' to `southern', and `oceanic' towards `continental'). 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. 9. The stands comprising the chief 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.

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