The interpretation of variation in north-European dwarf-shrub heath communities

Abstract

Dwarf-shrub heaths occur throughout a wide habitat-range in W. Europe, in both montane and lowland areas, through portions of the zones of two major forest types. Correspondingly, marked variations in the floristic composition of heath communities are evident. Identification of numerous “directions of variations” contributes to an assessment of the influence of the environmental factors concerned. 1. One approach to the study of variation is by means of classification. There is general agreement regarding the basic phytosociological grouping of N.W. European heath stands, but different treatments have been proposed for the arrangement of these units in hierarchies. 2. Stand data classified by the “method of successive approximation” resulted in groups conforming to the associations recognised by theBraun-Blanquet system. However, statistical classification (normal association analysis) produced comparable groups only when dwarf-shrub species alone were considered. An analysis using the whole flora first separated (a) herb-rich stands, (b) lichen rich stands, (c) dune heaths. Additional information derived from a classification using “geographical guiding species” is discussed. 3. For further information on variation in composition, techniques of ordination of stands are considered. An ordination of histograms showing cover contribution by dwarf-shrubs resulted in a network of relationships, the strands of which represent variation along important climatic gradients. Principal components analysis of the same data and ordination of the results, byKershaw (1967), confirmed the lack of discrete association of stands, and identified three axes of variation: (i) a north-south trend, (ii) an oceanic-continental trend, and (iii) an edaphic trend, from relatively wet to dry. Ordination of stand data from restricted sampling areas singled out edaphic gradients for special attention. 4. Consideration of the relationships between British health communities and those of the Continent shows that environmental gradients identified on the Continent are repeated under much more oceanic conditions in the British Isles. Different combinations of species result, and some British communities are not directly comparable with Continental types. Intensive management of British heaths by burning and grazing has superimposed a further direction of variation towards floristic impoverishment and the monoculture ofCalluna.