Sigma-syntaxa of the Wrangel Island
Authors: Kholod S. S. 
Published: October 2019

The vegetation cover of Wrangel Island has been investigated according to the symphytosociological approach, with sigma-syntaxon as a base category. This approach was laid by European phytosociologists (Tüxen, 1973; Géhu, 1986; Schwabe, 1989), and later was developed in our country (Gogoleva et al., 1987; Naumova et al., 1987; Golub, Chorbadze, 1991) and in North America (Ansseau, Grandtner, 1990). The classification of territorial units of vegetation cover has become possible after the concept of complex vegetation was justified. European phytosociologists (Krause, 1952) have proposed the concept of geographical vicariants of complex or mosaic vegetation. In our country the equivalent of widely understood European complex vegetation have become the concept of phytocoenochora (Sochava, 1972). The main unit of the study is a sigma-association which consists of a set of closely related phytocoenochoras. Last were identified in the field based on landscape ecological method and profiles one. Phytocoenochora consists of communities or elements, each of which is assigned to a specific syntaxon according to a floristic classification of vegetation (Kholod, 2007). Sigma-syntaxa have been obtained in the course the tabular data processing: the rows of the table are syntaxa, columns – concrete phytocoenochoras (sigma-relevé). The basis of the sigma-syntaxon is a diagnostic group consisting of a series of differential syntaxa and constant one. To identify the sigma-associations a threshold of 10 specific sigma-relevés was adopted. The typological unit was named a sigma-type if the number of relevés is smaller. In total 53 sigma-syntaxa of which 13 – sigma-associations and 40 — sigma-types were determined for Wrangel Island. These typological categories were combined in units of a higher hierarchical rank — sigma-unions, some are preliminary. An algorithm of sigma-syntaxa record consists of following categories: Composition, Structure, Environment, Location. This algorithm repeats in general the scheme of syntaxa record, but has a number of features that distinguish it from the latter. Syntaxonomic composition of phytocoenochoras, size of their area, coefficient of classification differentiation are considered in the section Composition. The last coefficient reflects the degree of typological proximity of syntaxa included in the diagnostic group of sigma-association. This section also discusses some indicators of species diversity: mean species richness of communities and the total number of species in the sigma-association (levels of α- and γ-diversity, respectively). Index of Whittaker describes a differentiating (-β) diversity. Section Structure provides severalparameters: synusia which is common to all elements of phytocoenochora, index of fractionality or complexity, which is the inverse of the average area of phytocoenochora,s element, uniformity of differentiation measure which is required to display the degree of equality of element areas. In addition, this section lists the types of vegetation structures, previously developed for the territory in question. The parameters of the shape of polygons like the coefficient of dissection and the index of elongation-roundness are used. The Location section presents position of sigma-syntaxa elements in relief or in drain-geochemical series of landscapes as well as their place in zonal category.