The study of species-environment and communityenvironment relations is the key to the functional interpretation of plant communities and to applications of phytosociology in bioindication and predictive modeling. Since the time of Braun-Blanquet, many releves have been made in conjunction with measurements of soil, topographic, and climate variables. If releve coordinates are known, environmental variables can also be post hoc read from maps or modeled from geodata. Environmental data are of multivariate nature, which requires condensing their information content and choosing the most meaningful variables. As in species-by-releve matrices, the dimensions of environmental variation can be reduced by extracting continuous gradients (ecological factors) or by forming clusters (site types). Relationships with the environment can be established for community types or species.
While often restricted to verbal descriptions and simple outlines of schematic correspondences (e.g., vegetation type-soil type) in early phytosociology, vegetation type-environment relationships are nowadays studied based on measured variables. These data enable to establish environmental envelopes, which define the possible occurrence of each vegetation type in ecological space. The overall significance of environmental differentiation between types can be tested, for example, by nonparametric permutation procedures (MRPPs).
There is a long tradition in phytosociology of defining ecological groups of species that exhibit similar behavior along gradients and represent species of similar realized niche ('ecological amplitude') rather than fundamental niche ('physiological amplitude'). Such groups are mainly based on expert knowledge and are only partly calibrated on independent measurements of environmental variables. Also, the derivation of ecological indicator values of plant species strongly relies on phytosociological descriptions of vegetation patterns, from which the principal ecological gradients are extracted. While separate species group systems and indicator value systems have been devised for vegetation of arable fields, grasslands, and forests, Heinz Ellenberg created a general, semiquantitative system of indicator values for the central European flora, in which most species of vascular plants, bryophytes, and lichens are assigned a value on an ordinal scale, ranging from 1 to 9 and representing the estimated ecological optimum with respect to the principal factors light, temperature, continentality, moisture, soil reaction, nutrient availability, and salinity. Being unique in summarizing the niches of an entire flora, Ellenberg values are widely used for calibrating ecological conditions based on plant communities. The concept of plant indicator values has been recently adapted for use beyond central Europe.
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