Vegetation of Suffusion Depressions in the Northern Part of the Subtaiga of Western Siberia

Abstract

Suffusion depressions are closed relief depressions on the flat surfaces of watersheds and terraces. They are formed during water erosion (suffusion) of loesses and loess-like loams. Therefore, the distribution of the depressions is associated with that of the soil-forming rocks - along the south of the taiga zone and further south in the subtaiga, forest-steppe, and steppe. The depressions are islands of preserved natural vegetation, since flat surfaces were plowed over a large area. Previous studies have shown an increase in moisture from the edge to the center of the depressions, and a rather high trophicity of habitats due to the richness in nutrients of loesslike loams. Birch and aspen forests along the edges of the depressions are actively studied. Wet plant communities in the inner parts of the depressions are less studied -mainly in the center of the subtaiga and to the south. The purpose of our work is to study vegetation of the depressions in the northern part of the subtaiga of Western Siberia. The study area is the southeastern part of the West Siberian Plain (55°30'-56°55N, 83°27'-84°45'E) (See Fig. 1). The diameter of the depressions is 20-100 m, the depth usually does not exceed 1.5 m. In spring, the depressions are usually flooded with snow-melt waters until the end of May (sometimes mid-June). The releves of plant communities were made with reference to the relief elements (slope and center). A total of 120 releves were used to describe the vegetation of the depressions, including a list of the species which each layer of the community comprises, and their abundance (projective cover, %). An ecological-floristic classification was carried out using the Braun-Blanquet method. Environmental factors (soil moisture, soil trophicity, soil nitrogen concentration, and soil acidity) were assessed by the phytoindication method using the indicator values of plants developed by Tsyganov. The data were processed and calculated in Excel. We identified five types of depressions with different series of plant communities depending on their shape and drainage properties of soils (See Fig. 3). Based on these five types, a generalized hydrological series of plant communities in the depressions was identified. However, with account to the richness and availability of nutrients in soils, two variants of the hydrological series were considered. Under poorer (meso-trophic) and weakly acidic conditions, the communities of the class Brachypodio-Betuletea on the edge (in the upper part of the slope) of the depressions are further replaced by the swampy communities of the class Alnetea glutinosae on the slope and also in the center (See Fig. 3, a, c, e). In richer (mesoeutrophic) and near-neutral conditions on the slopes of the depressions, the communities of the class Brachypodio- Betuletea on the edge are replaced by the communities of the class Alno-Populetea on the slope and further by the communities of the class Alnetea glutinosae in the center of the depression (See Fig. 3, b, d). The location of the suffusion depressions in the north of the subtaiga and the associated relatively high moistening of habitats determine the structure of the hydrological series where mesophilic communities of the class Brachypodio-Betuletea occupy only the upper part of the slope (See Fig. 4, a). Hygromesophilic and hygrophilic communities of the classes Alno-Populetea and Alnetea glutinosae are formed in the rest of the slope and in the center. Eutrophic species in the plant communities of the depressions are not abundant due to the low mineralization of soil and ground waters. The article contains 4 Figures, 1 Table, and 40 References. The Authors declare no conflict of interest.