Statistical Measures of Fidelity Applied to Diagnostic Species in Plant Sociology

The idea of a diagnostic species is an important concept in plant sociology. However, since over a century ago, when the term “association†was introduced, the identification of diagnostic species has been among the most controversial topics in phytosociological practice. With the aim of promoting methodological standardization in plant sociology, this paper addresses- 1) the need to distinguish between the concepts and methods involved in the definition of syntaxa (analysing relevés, characterization, diagnosis, naming and typification), and 2) the need to support and improve existing syntaxonomical classification schemes using statistical measures of fidelity to identify diagnostic species. The phytosociological literature describes numerous different approaches to the designation of diagnostic species. Here, we examine two such approaches to determine diagnostic species using as an example the class Atriplici julaceae-Frankenietea palmeri within the context of a data set of 5092 relevés taken of coastal plant communities distributed along the Pacific rim of North America. Diagnostic species were determined using both the phi-coefficient of association to detect differential species and the Ochiai index to designate character species. Our findings support the results obtained by combining classic phytosociological methods (expert knowledge, rearrangement of relevé tables, presence tables, etc.) with clustering methods.