The aim of phytosociology is to achieve a sufficient empirical model of vegetation using combinations of plant species (or subspecies, i.e.
taxa) that characterize discrete vegetation units. Vegetation units as understood by phytosociologists may express largely abstract vegetation concepts (e.g. the set of all hard-leaved
evergreen forests of western
Mediterranean area) or actual readily recognizable vegetation types (e.g. cork-oak oceanic forests on
Pleistocene dunes with dense canopy in
Iberian Peninsula). Such conceptual units are called
syntaxa (singular "syntaxon") and can be set in a hierarchy system called "synsystem" or syntaxonomic system. Creating new syntaxa or adjusting the synsystem is called
syntaxonomy. Before the rules were agreed upon, a number of slightly different systems of classification existed. These were known as "schools" or "traditions", and there were two main systems: the older Scandinavian school and the Zürich-Montpellier school, also sometimes called the Braun-Blanquet approach.
Relevé The first step in phytosociology is gathering data. This is done with what is known as a
relevé, a plot in which all the species are identified, and their abundance both vertically and in area are calculated. Other data are also recorded for a relevé: the geographic location, environmental factors and vegetation structure.
Boolean operators and (formerly) tables are used to sort the data. As the calculations needed are difficult and tedious to do manually, modern ecologists feed the relevé data into software programs that use algorithms to crunch the numbers. Braun-Blanquet's method uses the
scientific name of its most characteristic species as namesake, changing the ending of the
generic epithet to "-
etum" and treating the
specific epithet as an
adjective. Thus, a particular type of
mesotrophic grassland widespread in western Europe and dominated only by the grass
Arrhenatherum elatius becomes "
Arrhenatheretum elatioris Br.-Bl.". To distinguish between similar plant communities dominated by the same species, other important species are included in the name, but the name is otherwise is formed according to the same rules. Another type of mesotrophic
pasture dominated by black knapweed (
Centaurea nigra) and the grass
Cynosurus cristatus, which is also widespread in western Europe, is consequently named
Centaureo-Cynosuretum cristati Br.-Bl. & Tx.. If the second species is characteristic but notably less dominant than the first one, its genus name may be used as the adjective, for example in
Pterocarpetum rhizophorosus, a type of
tropical scrubland near water which has abundant
Pterocarpus officinalis and significant (though not overwhelmingly prominent) red mangrove (
Rhizophora mangle). Today an
International Code of Phytosociological Nomenclature exists, in which the rules for naming syntaxa are given. Its use has increased among botanists. In Anglo-American ecology, the association concept is mostly linked to the work of the mid-twentieth century botanist
Henry Gleason, who set it up as an alternative to
Frederic Clement's views on the
superorganismic framework. The philosophical parameters of the association concept have also come under study by environmental philosophers as to how it values and defends the natural environment.
Vegetation complexes Modern phytosociologists try to include higher levels of complexity in the perception of vegetation, namely by describing whole
successional units (vegetation series) or, in general, vegetation complexes. Other developments include the use of
multivariate statistics for the definition of syntaxa and their interpretation. ==Data collections==