Review links syntaxonomic ranks to biogeographic scale to address typological inflation

A conceptual review of Braun-Blanquet phytosociology argues that the rapid description of plant-community units in intensively surveyed regions has produced “typological inflation,” and recommends considering the approximate biogeographical range appropriate to each syntaxonomic rank and vegetation type. The analysis concerns units regulated by the International Code of Phytosociological Nomenclature and draws on European vegetation classifications and earlier recommendations for controlling the growth of syntaxa; it is not a new field survey or quantitative analysis of vegetation data.

The paper traces the problem to the accumulation of formally described syntaxa, particularly in parts of Europe and the Mediterranean where phytosociological research has been intensive. Theurillat summarized the typification of 5,066 new names recorded worldwide between 1987 and 1995, with the largest representation in those intensively studied regions. The author notes that some areas have reached a level of descriptive “saturation,” in which more associations than plant species may have been described. Large vegetation datasets and tools for data management and analysis now permit more efficient comparison, but the review cautions that availability of data does not by itself establish the validity or appropriate scale of a new unit.

Earlier guidance already addressed the problem. The term “typological inflation” was used by Pignatti in 1968, following discussion at a 1963 symposium of what was then the International Association for Vegetation Science. Tüxen proposed in 1967 that formal syntaxa should be proposed only through well-documented general surveys with a wide scope, while local and regional studies should avoid naming new units because they cannot provide a sufficiently broad view of higher syntaxonomic ranks. The review says these recommendations have not been applied through consistently objective and universal criteria, a limitation it attributes to continued practice rather than to a newly observed change in vegetation.

Two possible reasons for the inflationary trend are discussed. The first is the satisfaction authors may derive from having their names attached to newly described syntaxa. Recommendation 46A of the nomenclature code requires the author citation to accompany the name of each syntaxon at least once in every publication. The paper proposes that presenting the original description and typus once per paper would be sufficient, thereby reducing repeated emphasis on authorship. The second factor is “geographic drift”: within an association’s range, some subareas may contain taxa absent elsewhere, creating an opportunity to name a local subassociation or a separate association from a differing species combination.

The geographic-drift example in the paper involves the recently described forest association Glandoro diffusae-Quercetum fagineae, in which Glandora diffusa, characterized as a typical scrub species, was used in the name of a local forest association. The concern is not that a locally occurring taxon can never contribute to diagnosis, but that accompanying species characteristic of another vegetation type may have lower diagnostic value for the unit being defined. A new forest association should instead be diagnosed by a unique combination of typical forest species. The review also notes the use of climatic variability as an argument for creating associations, while citing Guarino and colleagues’ warning that secondary attributes such as climate, biogeography and successional position should not displace floristic composition as the primary basis of diagnosis.

Association is treated as the fundamental unit of the syntaxonomic system, comparable to species in botanical systematics, but it is the association specifically that is defined through a characteristic species combination. This combination need not contain a species that is diagnostic in the strict sense: the recurring combination itself may characterize the community. An association should correspond to a defined habitat type within a clearly delimited territory, with diagnosis based principally on plants that constitute the community rather than companions from adjacent vegetation. A subassociation, comparable to a subspecies, is defined by differential species and may be geographical or transitional, with a range commonly corresponding to a district or sector.

The review’s approximate rank-to-territory scheme is conditional on vegetation type rather than universally fixed. It distinguishes zonal vegetation, determined mainly by climate; azonal coastal and saline vegetation; azonal rocky vegetation; azonal wetland and aquatic vegetation; and highly disturbed anthropogenic vegetation. Main biogeographical levels include kingdom, region, province, sector and district, with subordinate categories also recognized. Associations in zonal vegetation may extend to the sector or province, while rocky vegetation can contain associations with still narrower ranges. Wetland, aquatic, disturbed and some coastal vegetation may also support widely distributed units, but the source does not establish one uniform geographic scale for all such vegetation types.

At broader ranks, alliances group associations with evident floristic-ecological affinities and similar habitats in neighboring territories; they have broader distributions and, according to the paper, at least one characteristic species. Orders gather alliances and generally encompass a province, group of provinces or region, while classes contain orders and have still broader ranges, commonly a region or kingdom. For azonal wetland and disturbed anthropogenic vegetation, some classes extend across several kingdoms and are described as subcosmopolitan. The review associates these differences partly with stenochorous, or geographically restricted, taxa: Mediterranean scrub, cliff and scree vegetation contain many endemics, whereas aquatic habitats and severely disturbed communities rarely do.

The analysis is bounded by its emphasis on the Braun-Blanquet tradition, the International Code, and especially European examples, with limited empirical testing of whether the proposed ranges apply consistently across other floras and vegetation systems. It also presents a qualitative framework rather than a newly validated predictive model. Its central conclusion is that syntaxonomic descriptions require reliable species composition, ecology, sampling and data management alongside explicit biogeographical context. In regions with long histories of intensive survey, that context can help researchers assess whether a proposed unit has a clear diagnostic basis and an appropriate territorial scope, thereby reducing unjustified multiplication of vegetation types.

Subject of Research: Biogeographical criteria for accepting and ranking plant-community syntaxa in Braun-Blanquet phytosociology

Article Title: Syntaxonomic ranks, biogeography and typological inflation

Article References: Loidi, J. (2023). Syntaxonomic ranks, biogeography and typological inflation. Vegetation Classification and Survey, 4, 1285-290. https://doi.org/10.3897/vcs.101648

Image Credits: AI Generated

DOI: 10.3897/VCS.101648

Keywords: phytosociology, syntaxonomy, vegetation classification, Braun-Blanquet approach, typological inflation, plant communities, biogeography, association, syntaxonomic ranks, floristic composition, Syntaxonomic, ranks