A lichen community is a group of lichen species that consistently grow together in response to specific environmental conditions. In general ecology, the term refers to any recurring assemblage occupying similar substrates, while in vegetation science (phytosociology), these assemblages are classified into formal, named types called syntaxa, such as associations and alliances.
Because lichens occupy very small areas, such as a single patch of bark or a stone, their study has played a prominent role in debates over whether vegetation units should represent whole plant communities or smaller, substrate-bound partial communities (synusiae). These assemblages are not static; they change over time through succession as new species colonize a surface and compete for space and light.
The formal syntaxonomic tradition is strongest in Europe, especially in the Braun–Blanquet school, where bryophyte- and lichen-dominated vegetation has been incorporated into continent-wide classification systems. North American lichen ecology has more often relied on gradient analysis and ordination rather than formal naming, though community composition is widely used in biomonitoring on both continents.
Lichen communities are important in conservation and environmental assessment because their composition responds strongly to substrate chemistry, air quality, moisture regime and habitat continuity. Named assemblages such as the epiphytic Lobarion have also been used in conservation evaluation and habitat interpretation. Beyond their species composition, modern research also focuses on the functional traits of these communities, such as growth forms and chemical properties, which influence ecosystem processes like nutrient cycling.
Definition and scope
Ecologists commonly use the term lichen community for recognizable assemblages of lichens that recur on a given substrate or under a given set of environmental conditions. Formal vegetation science uses narrower terms. Lichen vegetation treats the lichen layer as vegetation in its own right, while lichen phytosociology applies relevé-based classification to that vegetation and names the resulting formal community types, or syntaxa, under the International Code of Phytosociological Nomenclature (ICPN). A syntaxon is an abstract type, not the individual stand or patch of lichen growth recorded on a particular tree trunk, rock face or soil surface. Barkman also argued that the term "synusia" is best reserved for the abstract unit rather than the concrete patch seen in the field. In that usage, the actual lichen patch on a trunk, rock or soil surface is a concrete society or stand, while the synusia is the generalized type abstracted from comparable societies. This divergence was indicative of a broader contrast in scientific tradition. While continental Europe favoured a typological approach (grouping vegetation into fixed types), North American lichenology relied more on an individualistic approach, viewing communities as shifting along environmental gradients.
Scale is unusually important in lichen studies. Micro-communities are very small, substrate-bound units, and in formal literature they are often interpreted as synusiae or, in more structural terminology, merocoenoses—partial communities within a wider phytocoenosis—rather than as complete site-level vegetation. More exact synusial definitions make a further distinction that is worth stating explicitly. In Barkman's formulation, a synusia is not simply any substrate-bound patch of vegetation. It is a structural part of a larger plant community that occupies a particular microhabitat but also shares the same layer, broadly similar seasonality, and a similar way of using that habitat. For example, a single tree trunk may count as one epiphytic micro-community, but within it one can distinguish several synusiae. These might include a crustose-lichen society on bark plateaux, a different epiphytic society in bark fissures, and a fruticose lichen layer overlying a foliose or crustose one. Set against a whole phytocoenosis, the scale difference can be illustrated by an example: an entire forest stand is a phytocoenosis, the trunk-dwelling lichen vegetation within it forms one or more epiphytic synusiae, and the actual patch on a particular trunk is a concrete stand or society from which a synusia may be abstracted. Barkman's use of these terms was mainly structural, concerned with how partial communities are delimited within a vegetation stand. Later critiques, especially by Berg and co-authors, focused more on a classificatory problem: whether such partial communities should occupy the same formal hierarchy as whole phytocoenoses.
History
Modern lichen phytosociology developed as Braun-Blanquet methods for classifying vascular-plant vegetation were extended to lichen assemblages on bark and rock, a typological approach that contrasted with the North American tendency to treat lichen communities as shifting along environmental gradients rather than as fixed types. Pier Luigi Nimis traced its beginnings to work in the 1920s by Eduard Frey, Antonín Hilitzer and Fritz Ochsner, who applied phytosociological ideas to lichen assemblages on bark and rock. Ochsner's 1928 treatment of epiphytic (bark-dwelling) vegetation, which included units such as the Lobarion and Xanthorion, became especially influential in later bark-community studies. Another milestone was Gustaf Einar Du Rietz's 1945 contrast between acidophytic communities of nutrient-poor bark and richer-bark epiphytic communities, a framework that shaped later discussion of bark chemistry and host trees.
The first comprehensive classification of Central European lichen communities was Oscar Klement's (, 1955). Jan J. Barkman's Phytosociology and Ecology of Cryptogamic Epiphytes (1958) then became the foundational monograph of cryptogamic epiphyte vegetation in Europe. British lichenologists adopted the continental framework more pragmatically. James, Hawksworth and Rose's 1977 conspectus accepted the usefulness of named communities but cautioned against forcing lichen variation into a rigid proliferation of narrowly delimited associations. Nimis's 1991 review argued that numerical methods should play a larger part in lichen community studies, while also warning against inflation of community names. The main modern continental synthesis is the EuroVegChecklist of 2016, which placed bryophyte- and lichen-dominated vegetation into a pan-European framework.
Concepts and methods
Phytosociological studies usually begin with a relevé, a plot record of the species present and their abundance in a homogeneous stand. Lichen relevés are typically much smaller than plots used for vascular plant vegetation, because many communities are tied to a patch of bark, a rock face or a small soil crust. A survey of more than 11,000 bryophyte and lichen relevés found median plot sizes around 0.04–0.09 m2 (roughly the size of a sheet of paper) for bark-, rock-, soil- and wood-dwelling communities. This scale difference is one reason many lichen syntaxa are treated as micro-communities or synusiae rather than as whole-site communities. Barkman distinguished between two legitimate sampling aims in epiphyte studies. A plot meant to characterize a forest stand by its epiphytes should take all trees into account, including those lacking epiphytes, whereas a study aimed at describing epiphytic communities themselves should use smaller and more homogeneous samples from comparable parts of the bark surface. Some authors have argued more strongly that plots below 1 m2 usually represent synusiae or incomplete phytocoenoses rather than full stand-level communities, though that is a methodological position rather than a rule of the nomenclatural code.
Classical Braun–Blanquet practice distinguishes communities by their overall floristic composition and by diagnostic or characteristic species. Constancy is an internal frequency measure, recording how often a species appears within the samples of one community type, whereas fidelity is a comparative measure, recording how strongly that species is concentrated in one type rather than others. Modern vegetation science often estimates fidelity with statistics such as the phi coefficient (a correlation measure ranging from −1 to +1) and uses the broader term diagnostic species for taxa that help separate one unit from another.
Traditional lichen phytosociology often used preferential sampling, in which experts selected stands judged to be typical and homogeneous, rather than random or systematic plots. Supporters saw this as a practical way to capture recognizable community nodes, but critics argued that it made later comparison and synthesis more difficult. Numerical classification and ordination have consequently become more important, especially in large monitoring programmes and in traditions outside Europe. Large survey reports have also used TWINSPAN and related multivariate methods to analyse lichen and bryophyte–lichen plot data. A numerical study of epiphytic lichen vegetation in Tasmanian cool temperate rainforest similarly showed that unconstrained ordination could recover several partly independent gradients in community composition. The main gradients related to host-tree age and bark texture, substrate wetness, and microclimatic constancy.
Syntaxonomy and nomenclature
The ICPN governs the formal naming of lichen syntaxa in the same way that it governs other vegetation syntaxa. The standard ranks are association, alliance, order and class, conventionally marked by the endings -etum, -ion, -etalia and -etea.
Rank Standard ending Example Typical substrate / scaleAssociation-etumParmelietum carporrhizantisBark (corticolous); an association-level unitAlliance-ionLobarion pulmonariaeBark of old broadleaves; a group of related epiphytic associationsOrder-etaliaPeltigeretaliaGround vegetation; a broader terricolous groupingClass-eteaRhizocarpetea geographiciSiliceous rock; a broad substrate-defined class
Under modern ICPN rules, valid publication also requires a nomenclatural type, usually a designated type relevé (a designated reference sample plot). As a result, some older lichen and bryophyte–lichen names have later been validated or typified under current standards. Older cryptogam literature sometimes used alternative names such as union, federatio, ordulus and classicula for these ranks, but those terms are now obsolete.
The main conceptual tension appears in the code itself. Its definition of syntaxa is centred on phytocoenoses, yet it also allows cryptogamic microcoenoses and synusiae to be treated as syntaxa even when they do not correspond to full phytocoenoses. Berg and co-authors argued that vegetation classification becomes logically inconsistent when whole plant communities and their parts are placed in the same hierarchy; epiphytic lichen assemblages on bark, for example, are not equivalent in scale to the forest communities that contain them. European practice has nevertheless remained pragmatic, and the EuroVegChecklist placed bryophyte- and lichen-dominated vegetation in a separate continental system of 27 classes, 53 orders and 137 alliances. The result is a working system that is widely used, even while its theoretical basis remains debated.
Major types