The 'Braun-Blanquet approach' provides a methodological framework for vegetation classification that seeks an optimal combination of the above criteria and that reconciles conflicting requirements of different scales and purposes. However, it is not an unambiguous and uniform set of recipes, and it has been subject to diverse modifications. Despite the variety of different versions, practitioners agree on certain fundamentals, which distinguish the Braun-Blanquet approach from most other ways of vegetation classification: (1) The classification is based on the (total) species composition of the sample plots (floristic-sociological method), whereas structural or environmental criteria play a subordinate role. (2) The classification units called syntaxa (singular: syntaxon) are arranged into a hierarchical system according to their floristic similarity. The principal ranks of this system are, from bottom up, association, alliance, order, and class. (3) There are generally accepted rules for the scientific naming ofsyntaxa (see the section entitled 'Phytosociological ranks and nomenclature').
Within the Braun-Blanquet approach, the concept of character and differential species is important for the recognition of previously defined syntaxa. Differential species are those that positively differentiate, by their occurrence, the target syntaxon from other syntaxa. Character species are a special case ofdifferential species: they positively differentiate the target syntaxon from all other syntaxa. The differential and character species combined are called diagnostic species. The validity of diagnostic species may be restricted to comparisons within the syntaxon of the next higher rank or within a physiognomic vegetation type. Diagnostic species are based on the concept of fidelity, that is, concentration of their occurrence or abundance within the given syntaxon. Traditionally, arbitrary measures of fidelity were used, such as constancy in the target syntaxon had to be at least twice as high as in any other syntaxon. Nowadays, statistical fidelity measures are increasingly used (see the section entitled 'Numerical approaches'). However, in spite of several attempts at a formal definition of differential and character species, no widely accepted agreement in this respect has been reached so far.
Phytosociology faces difficulties in the classification of vegetation types that lack species of narrow ecological amplitude which could be used as character species ofthe respective syntaxa. This problem led early practitioners to avoid stands without specialist species as 'atypical' and 'fragmentary' and oversample those containing presumed character species. Even when sampled and recognized, such poorly characterized vegetation types were often excluded from the syntaxonomic system. Vegetation types poor in diagnostic species may be incorporated into the system in several ways, for example: (1) Deductive classification affiliates such units as so-called basal or derivative communities to higher syntaxa ofthe system, from which their formal names are derived (e.g., Elymus repens [Artemisietea vulgaris] derivative community). (2) According to the concept of central syntaxon, there can be one negatively differentiated syntaxon within the next superior syntaxon of the hierarchy; central syntaxa have the same ranks (e.g., association) and nomenclature as normal syntaxa.
This diversity of approaches within the Braun-Blanquet system must be unified where all vegetation types ofa large area are to be placed in a single coherent system, such as in modern projects of national vegetation classifications. These projects have usually developed consistent systems ofstandardized and operational methodology ofvegetation classification based on the Braun-Blanquet approach.
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