Delimitation of vegetation units in phytosociology is traditionally based on expert knowledge. Applications of expert-based classifications are often inconsistent because criteria for assigning relevés to vegetation units are seldom given explicitly. Still, there is, e.g. in nature conservation, an increasing need for a consistent application of vegetation classification using computer expert systems for unit identification.
We propose a procedure for formalized reproduction of an expert-based vegetation classification, which is applicable to large phytosociological data sets. This procedure combines Bruelheide's Cocktail method with a similarity-based assignment of relevés to constancy columns of a vegetation table. As a test of this method we attempt to reproduce the expert-based phytosociological classification of subalpine tall-forb vegetation of the Czech Republic which has been made by combination of expert judgement and stepwise numerical classification of 718 relevés by TWINSPAN. Applying the Cocktail method to a geographically stratified data set of 21 794 relevés of all Czech vegetation types, we defined groups of species with the statistical tendency of joint occurrences in vegetation. Combinations of 12 of these species groups by logical operators AND, OR and AND NOT yielded formal definitions of 14 of 16 associations which had been accepted in the expert-based classification. Application of these formal definitions to the original data set of 718 relevés resulted in an assignment of 376 relevés to the associations. This assignment agreed well with the original expert-based classification. Relevés that remained unassigned because they had not met the requirements of any of the formal definitions, were subsequently assigned to the associations by calculating similarity to relevé groups that had already been assigned to the associations. A new index, based on frequency and fidelity, was proposed for calculating similarity. The agreement with the expert-based classification achieved by the formal definitions was still improved after applying the similarity-based assignment. Results indicate that the expert-based classification can be successfully formalized and converted into a computer expert system.
Nomenclature: Kubát et al. (2002).
Abbreviations: FDI = Fidelity Index; FFI = Frequency-Fidelity Index; FQI = Frequency Index.