Typology and syntaxonomic composition of vegetation territorial units: novel approach suggested with the case study of Arctic marshes
Authors: Lavrinenko I. A. 
Published: December 2020

General characteristic of approaches of Russian and Western European phytosociologists to the typology of territorial units of vegetation is presented in the paper. Two-dimensional classification of vegetation proposed by V. Sochava (1968) is the basis of Russian approaches, in which 2 classification categories are identified: phytocenomers and phytocenochores. Homogeneous territorial vegetation units are represented by phytocenoses of a certain syntaxa (phytocenomer), then 3 main types are distinguished within heterogeneous units: complexes, ecological series, and ecological-genetic series (Rachkovskaya, 1963; Guricheva, 1965; Isachenko, 1967; Gribova, Isachenko, 1972). In Western European phytocoenology, the approach to the analysis of territorial vegetation units was introduced by R. Tüxen, who in the 1970s created the foundations of the research topic called symphytosociology (Tüxen, 1973, 1978). He developed the approach to include different levels of syntaxa in the classification of territorial units of vegetation. In 1973, R. Tüxen proposed a methodology for the study of vegetation complexes and the method for converting syntaxa into sigma-syntaxa. In 1981, J.-M. Géhu and S. Rivas-Martínez suggested sigmetum as the main unit of symphytosociology. In 1982, S. Rivas-Martínez proposed the tesela as an elementary territorial unit of chorology and phytogeography. Tesela corresponds to the environmentally homogeneous area and includes communities of one sigmeta. The next level of plant cover organization is the catena — territorial vegetation unit determined by geomorphological features of the area. A catena includes several teselas spatially arranged in a mosaic or arranged in belts following to one or more environmental gradients. Geosigmetum or geoseries is the main typological unit of vegetation of the catena.

We took into account the disadvantages of these approaches in the development of a proprietary typological scheme. The lack of a rigorous methodology for allocation and identifying territorial units of vegetation, absence of generally recognized nomenclature, and only intuitive level of allocation of the units themselves are disadvantages of the Russian school. The symphytosociological approach is formal in the scientific terms since it does not take into account the important structural features of the territorial units of vegetation. This approach simplifies their characterization to a list of syntaxa and their combinations and is limited to only two levels of the hierarchy (sigmetum and geosigmetum). The approaches to the typology of territorial units of vegetation, currently used in symphytosociology, are a copy of phytosociological ones. Despite this, the logic of map legend formation differs from the principles of vegetation classification.

We propose the types of structures (ecological-genetic series, ecological series, complexes) of the existing communities’ combinations to take as a basis of the typological scheme. They reflect the quintessence of the “ecological basis” of phytocenochore habitats, namely the distribution features, intensity, direction, and the result of the interaction between environmental factors.

The unification of territorial units of vegetation is carried out based on real and regularly repeated combinations of higher rank (from micro- to meso- level and so on). This allows us to reflect levels of a hierarchical organization of vegetation in the typology and nomenclature.

The vegetation of the Barents sea coastal marches was chosen as a model object due to the well-studied syntaxonomic composition and pronounced spatial patterning. This patterning appears mainly due to the discrete gradation of the prevailing environmental factors: the degree of salinity and moisture of the substrate. Within the framework of the typological scheme, the most general categories of territorial units are identified. They correspond to the basic levels of the hierarchical organization of vegetation cover — division, class, and type, which are highly likely to be common to any mapped territory, regardless of its zonal disposition. For a more complete display of the structure and complexity of the spatial organization of phytocenochores, 3 auxiliary categories were introduced — subclass, group, and subtype.

Division — is the highest unit of a typological scheme that combines the territorial units of vegetation of a particular natural area confined to such geomorphologic structures as following: 1) watersheds covering the area between the edge of the slopes of the bed-rock terraces in river valleys; 2) river valleys exposed to the flooding regime, the boundaries of this geomorphologic structures are drawn on the edge of the slope, which separates the valleys from the watershed areas; 3) low marine terraces variably flooded by tidal waters, this geomorphologic structure is limited by the bed-rock terrace slope edge.

Class is the subordinated unit within the division. They combine topographically well differed territorial units of vegetation, in which the composition of syntaxa and syntaxon combinations reflect the ecological specificity of simple relief forms of the same origin (i. e. hill, ridge, runoff hollow, lake depression, floodplain, etc.) or their elements (slopes, terraces). Phytocenochores of the class rank are formed by combinations of communities, different in ecology (palsa-bogs, coastal marshes, etc.). This is due to the specific distribution and influence of environmental factors and their combinations within the class. The syntaxonomic composition of communities and their combinations within the class reflects the position of vegetation territorial units in the landscape and their habitats’ ecological distinctiveness.

The territorial units of vegetation of the same class within different subzones and geobotanical districts can differ significantly in composition (diagnostic syntaxa and the main elements of their combinations) due to the zonal and regional originality. It is the basis for their allocation to the rank of subclass. Phytocenochores of subclasses are usually not territorially conjugate and belong to different units of geobotanical zoning (subzones, geobotanical districts). The class of territorial units of Barents Sea coastal marshes is divided into salt and brackish marsh subclasses. They belong to different geobotanic districts and are considered as ecological variants of coastal marsh class with a uniqueness of composition of syntaxa and their combinations.

Within the class/subclass of vegetation territorial units, phytocenochores of meso- or macro-level are often clearly distinguished, discretely, or continually replacing each other along the gradient of leading environmental factors. They were allocated to the rank of group, each is physiognomically distinct and represented by a continuum or relatively homogeneous mosaic of syntaxa and their combinations. The composition of diagnostic syntaxa and their combinations, reflecting the ecological peculiarity of the habitats of this typological unit and the class (subclass) as a whole, is preserved within the group. Groups are territorially conjugate and represent elements within a uniform ecological or ecological-dynamic series of communities and their combinations. The definition of groups, as well as subclasses, can be problematic because of the complexity of the geomorphological features. This is the reason why this typological unit has an auxiliary character.

Type — the basic elementary unit of the typological scheme, displayed mainly on maps of large and medium scales. To distinguish the type two main criteria are being applied as follows: 1) the type of spatial structure, and 2) syntaxonomic composition of the elements of the vegetation territorial unit. As a type the following phytocenochores can be recognized:

1) phytocenoses;

2) simple combinations of the first supraphytocenotic level (ecological-genetic series, ecological series, complexes);

3) complex combinations that include a combination of phytocenochores of the first 2 types.

The types represented by heterogeneous territorial units of vegetation include complete combinations of all possible elements of phytocenochores (series, ecological series, etc.) that can occur in the conditions of type habitats. Usually, within the phytocenochores, combinations of communities are represented by the selective set of syntaxa represented in the complete combinations. For a comprehensive inventory of the regional territorial units, diversity, and its reflection on the maps, auxiliary typological unit of subtype rank is introduced, which manifests the internal variability of a certain type and reflects the completeness of the type representation in a particular territory.

For the names of typological units of different ranks, it is proposed to use the names of diagnostic syntaxa of association and lower rank, since they most reflect the local and regional originality of the mapping area. The Brown-Blanquet approach to the classification of vegetation, as the methodically most developed among others nowadays, is the basis for the nomenclature of typological units.

For the name of typological units, we propose to use the ending “-chorietum”, derived from phytocenochora (coenochora). It reflects the chorological aspect of the unit and intended to be used in the nomenclature for all of the territorial vegetation units categories — of any rank of complexity and to rankless territorial units. The term -chorietum is added to the name of the diagnostic syntaxon.

For homogeneous territorial units, we propose to use syntaxon name, since any phytocoenosis is an elementary territorial unit of vegetation of the rank of association and below. For simple combinations (ecological-genetic series, ecological series, complexes), regardless of the number of their forming syntaxa, the names of two syntaxa are used. The ending -chorietum is added to the genus name of the taxon of diagnostic syntaxon. This reflects the first supraphytocoenotic level of the structural organization of the territorial unit of vegetation. If the diagnostic syntaxon has the rank of subassociation or variant, then name of the typological unit uses name of the association with the ending -chorietum, followed by the full name of the subassociation or variant. If the type of territorial units is represented by an incomplete series of syntaxa and is assigned to a subtype, then after the diagnostic syntaxon the ending -subchorietum is used. If the type is represented by a complex combination of phytocoenoses and their combinations, the name of the diagnostic syntaxon with the ending -synchorietum is used.

When forming the names of typological units of class and lower rank, add the ending corresponding to their rank to the basis of the genus name of the taxon of the diagnostic association, that is, instead of the -etum ending, we use the ending corresponding to the rank of the territorial unit. If the basis of the genus name ends in a consonant, then we use the connecting vowel (“i” or “o” following to Appendix I of the ICPN). If the name of the rank of the phytocenochore consists of the names of two syntaxa, then the first diagnostic syntaxon is unchanged, and the ending corresponding to the rank is added to the last syntaxon. Examples: for class —-chorietea (Luzulo confusae–Salicichorietea nummulariae), subclass — -subchorietea(Puccinellisubchorietea phryganodis), group — -chorieteum(Caricichorieteum glareosae), type — -chorietum (Callitricho–Ranunculetum trichophylli—Carici rariflorae–Salicichorietum glaucae) and -synchorietum (Caricetum subspathaceae arctanthemetosum hulteni—Potamogetonosynchorietum filiformis), subtype — -subchorietum (Parnassio palustris–Salicetum reptantis—Scirpo–Hippuridosubchorietum tetraphyllae).

To name division, which represents the highest rank, the term choriophyta is proposed. The term has been added to the Latin names of the divisions of the tundra zone, for example, watersheds — Tundra divortium choriophyta, river valleys — Tundra vallis choriophyta, low sea terraces — Tundra maritimes choriophyta. To describe and display the spatial structure of phytocenochore types represented by simple and complex combinations, it is proposed to use the set of symbols reflecting the structural features.

Using the example of coastal marshes of the Haypudyrskaya Bay of the Barents Sea (Puccinellichorietea phryganodis), a typological scheme of territorial vegetation units was developed. The subclass of salt marshes (Puccinellisubchorietea phryganodis) was taken for a case study. Within the subclass 4 groups of 27 categories were selected to create the geobotanical map (1 : 25 000). Those categories are of the rank of types and subtypes of different complexity levels and 13 homogeneous and 14 heterogeneous are among them. In addition to them, 5 types of phytocenochores that intrude marshes from adjacent divisions have been identified.

The application of the typological scheme allows taking into account not only the syntaxonomic composition and predominant combinations but also their patterning and complexity. This will largely reflect the originality of the vegetation of the given area, as well as habitas specificity, geomorphological and ecological features of the territory.