The study of peat macrofossil composition of peat is an important element of paleoecological research of mire. This makes it possible to identify the dynamics and features of the mire formation process. Despite their complexity and the small number of appropriate specialists, such studies do not lose their relevance, as confirmed by many publications in recent years [Kalnina et al., 2015; Baisheva et al., 2019; Vincze et al., 2019; Sinyutkina, 2020; Razjigaeva, 2021; Logvinova et al., 2022; Kutenkov et al., 2022; Maslov, 2023].

In this paper we present the results of studies of the mire formation process in the Ob River valley, based on the construction of successional series of paleoplant communities. We chose the Ishtan mire as the object of study. It is located in the southern part of the Krivosheinsky and northern part of the Shegarsky districts of the Tomsk region (Fig. 1). Drilling of the peat deposit with core sampling was carried out in different parts of the mire, taking into account environmental conditions and vegetation cover. The cores were named "I1", "I2" and "I3". In order to characterize the peat deposit, we prepared samples and analyzed the botanical composition of peat, the degree of decomposition of plant macrofossils and the level of mineral pollution of all three cores (120 samples in total): "I1" - 45 samples, "I2" - 40, "I3" - 35. Sampling was carried out at intervals of 10 cm. The results of the study of the botanical composition of peat are described in an earlier publication [Author, 2023].

The humified part of the samples was washed under running water through a sieve with a mesh diameter of 0.25 mm. The prepared sample was then examined under a microscope. Macroscopic remains were identified using specialized atlases [Dombrovskaya et al., 1959; Katz et al., 1977]. The degree of peat decomposition was assessed using macroscopic and microscopic methods. The classification proposed in 1976 by S. N. Tyuremnov was used as a methodological basis for peat classification [Tyuremnov, 1976].

Core sampling «И1». It is located in the near-terrace part of the massif. The depth of peat here is 4.5 m. Mire formation in this area differs from others. It began with Carex-Menyanthes communities. Tree species did not play a primary role in the formation of phytocenoses throughout the development of the mire (Fig. 2). For this site, the most frequent changes in the directions of transitions of peat types were noted (31 out of 61). The change of plant communities in response to changing environmental conditions occurred here more intensively.

Core sampling «И2». It is located in the central part of the mire massif, occupied by a community dominated by Betula fruticosa and Carex lasiocarpa. The beginning of mire formation in this area is associated with tree (coniferous) communities (Fig. 3). It is characterized by high stability and long-term dominance of trees. With a change in conditions (primarily moisture), sedges and pines settled on the site. As a result of further depletion of conditions, a period begins when the role of sphagnum mosses increases (up to 15% of the cover). Subsequently, the development of a grass community dominated by Carex lasiocarpa was discovered on the site. However, a gradual change in the structure of phytocenoses ultimately led to the formation of a complex multi-tiered community.

Core sampling «И3». It is located in the center of the forested zone of the mire, in an area where the highest plant species diversity was noted during geobotanical studies. The depth of peat here is 3.5 m. Communities with a developed tree layer occupy more than half of the mire area. Here, the greatest role of woody plants in the formation of phytocenoses is indicated throughout the history of the mire development (Fig. 4). Stable communities of woody (mainly coniferous) plants disappeared only once. In that case, they were replaced by a monodominant community of Menyanthes trifoliata. For this type of mire, Menyanthes trifoliata is an important peat-forming plant. According to our results, Menyanthes remains are the most common macrofossils in peat. In these types of mire, they are often found together with woody plants.

It can be concluded that the mire-forming process in most of the territory began with water-logging of coniferous tree communities. With the overall high dynamics of phytocenosis change, this process was most clearly manifested in the near-terrace part. We associate intensive structural transformations with the influence of flood processes, with the active approach/removal of the Ob River bed and flooding of the territory. The active influence of the river is also indicated by the shells of freshwater mollusks, often found in peat at different depths. Based on our research, we come to the conclusion that the vegetation in the mires of river valleys developed both in the direction of increasing the complexity of the phytocenotic structure (for example, increasing species diversity) and in the direction of depletion of the floristic composition (the appearance of monodominant communities with Carex lasiocarpa or Menyanthes trifoliata).