Phytosociology

Overview

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.

Association model

The basic unit of syntaxonomy, the organisation and nomenclature of phytosociological relationships, is the "association", defined by its characteristic combination of plant taxa. Sometimes other habitat features such as the management by humans (mowing regime, for example), physiognomy and/or the stage in ecological succession may also be considered. Such an association is usually viewed as a discrete phytocoenose. Similar and neighbouring associations can be grouped in larger ecological conceptual units, with a group of plant associations called an "alliance". Similar alliances may be grouped in "orders" and orders in vegetation "classes". The setting of syntaxa in such a hierarchy makes up the syntaxonomical system.

The most important workers to define the modern system were initially Charles Flahault, with the work of his student Josias Braun-Blanquet being the what is generally considered the final version of syntaxonomical nomenclature. Braun-Blanquet further refined and standardised the work of Flahault and many others when he worked on the phytocoenosis of the southern Cévennes. He established the modern system of classifying vegetation. 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

Phytosociological data contain information collected in relevés (or plots) listing each species cover-abundance values and the measured environmental variables. This data is conveniently databanked in a program like TURBOVEG allowing for editing, storage and export to other applications.

Data is usually classified and sorted using TWINSPAN in host programs like JUICE to create realistic species-relevé associations. Further patterns are investigated using clustering and resemblance methods, and ordination techniques available in software packages like CANOCO or the R-package vegan.