The right releve in the right vegetation unit: a new typicality index to reproduce expert judgement with an automatic classification programme

Question: How to classify forest vegetation releves automatically in the traditional phytosociological system and thus distinguish typical releves - easier to classify - fromreleves with non- diagnostic species and intermediate releves? Material: A data set of 11 324 forest vegetation releves, including 4880 releves classified a priori by experts in phytosociology down to the association level ( 100 associations in 30 alliances and ten classes, covering all forest vegetation and environmental conditions encountered in France). Methods: A new typicality index was formalized to quantify the probability of automatically classifying a given relevein the same vegetation unit as would do an expert in phytosociology. Computation of the typicality index is based on two parameters: a level of affinity linked to the number of diagnostic species of the vegetation unit present in the releve ', and a level of differentiation that is greater when the risk of confusion in classification of the relevebetween different units is smaller. Results: The automatic classification was identical to expert judgement for 60% of the 4080 calibration releves. The typicality index isolated atypical releves that are more difficult to classify. The model was successfully transferred to classify an independent data set of 6444 releves from the French National Forest Inventory performed in 2008 and to distinguish 1114 ( 17%) typical releves of phytosociological associations. Conclusions: Interest in this new typicality index is manifold. It is: ( 1) operational for the current phytosociological system that forms the basis of the Natura 2000 system; ( 2) easy to implement from characteristics of species and communities; and ( 3) based on criteria of uncertainty used by phytosociological experts. This study establishes a clear bridge between recent works on collecting, storing and analysing vegetation releves, on one hand, and the traditional phytosociological approach, on the other. It should trigger further studies on the spatial and temporal distribution of European habitats.