On the determination of optimal levels in phytosociological classification

A method is introduced to compare results of a clustering technique at different levels of abstraction, or of different clustering techniques. The method emphasizes within cluster homogeneity as well as discontinuities between clusters. It has been derived from Hogeweg's method with some important changes. First each cluster is handled separately to determine the ratio between homogeneity and similarity to the nearest neighbour cluster. For a given clustering a weighted average value is computed over all clusters. This average value is standardized using an expected average value for a cluster configuration with the same number of clusters having the same sizes. A low level of the ratio between expected and observed values is supposed to indicate an optimal clustering. A derivation of the criterion is given and results from three sets of data with different properties are evaluated.     

Syntaxonomy of the Onopordum acanthium communities in temperate and continental Europe

Ruderal communities dominated by Onopordum acanthium are encountered in those regions of Europe with a dry and continental climate. Variability in the climate and local phytogeography within these regions is reflected in the floristic variability of the communities studied. A floristic cline within the communities was summarised by means of multivariate classification and ordination methods. Two associations distributed in temperate Europe and belonging to the Onopordetalia, the Onopordetum acanthii s.str. and Carduo acanthoidis-Onopordetum acanthii, were distinguished; The Carduo acanthoidis-Onopordetum is varied and occurs in several geographic races and sub-races. The Onopordetum acanthii s.str. is confined to deep inner-montane valleys with a continental climate. The other Onopordum acanthium communities described from Spain belong to the Scolymo hispanici-Onopordetalia nervosi.Abbreviations CA = correspondence analysis - CLC = complete linkage clustering