Formalized Hierarchically Nested Expert System for Classification of Mesic and Wet Grasslands in Poland

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The goal of this study was to propose a hierarchically nested classification system comprising four principal levels of the Braun-Blanquet system for Polish grasslands of the class <em>Molinio-Arrhenatheretea</em>. Using the Cocktail method, we defined consistent criteria for delimitation of the class, three orders, nine alliances, and 45 associations. Formal definitions were prepared using the summed cover and presence/absence information of species groups and individual dominant species. We created an expert system with a set of assignment rules that unambiguously classify relevés to a single unit at the given abstraction level of the Braun-Blanquet system in such a way that a relevé matched by the definition of a focal vegetation unit must be matched by definitions of all superior units. Of 11,535 relevés classified to <em>Molinio-Arrhenatheretea</em>, 36% were recognized at the association level, and 57% and 85% at the alliance and order level, respectively. All relevés were assigned unambiguously, meaning that a single relevé could not be assigned to more than one unit within the same hierarchical level (no overlap between vegetation units). This study is the first proposal of a hierarchically nested classification system that classifies grassland vegetation at different syntaxonomical levels unequivocally. It is important to create definitions for different syntaxonomical levels because the majority of vegetation patches do not fit to the associations, but can only be assigned to high-rank units such as alliance, order, or class.