The association concept revisited
Although there is no consense among phytosociologists about how to define an association, little attention has been given to this question in the recent literature. Braun-Blanquet 's concept was based on character species with restricted geographical validity. However, all efforts to clarify the geographical limits of character species in a methodologically sound way have failed so far. The re-examination of the original definition of the association established by the International Botanical Congress at Brussels in 1910 reveals a possible solution: (1) The first and most important criterion for an association is its "definite floristic composition". It is shown that this criterion is only fulfilled if the syntaxon has a sufficient number of good diagnostic species. It is recommended to use the "total cover value ratio" to measure the diagnostic value of a species and to consider those species as good diagnostic species which have a total cover value at least ten times higher than in the compared syntaxon. (2) Associations belonging to the same formation should clearly differ from each other either in their site conditions or in their distribution. Floristical difference contains no useful information as long as it can't be interpreted as reflection of a different habitat or a different vegetation history. Population processes, intraspecific variation, autocorrelation, microhabitat mosaics and stochastic events set an absolute limit to all classification efforts.