ИСТИНА

The problem of reconstructions of the Late Pleistocene landscapes has long attracted attention of paleogeographers. However, the loess sequence corresponding to the last glaciation has not been adequately studied paleobotanically until now. From this viewpoint, our paleobotanical data recovered from alluvial and loess sediments in the middle reaches of the Desna River may be of special interest. There are several stratigraphic units identified in the Desna drainage basin and studied in a number of key sections (fig.1a); those include Mezin polygenetic soil complex (Eemian, Mikulino Interglacial), three loess horizons (Weichselian, Valdai glacial epoch) separated by Bryansk paleosoil (fig.1b). The reconstructions of early Valdai vegetation are based on results of pollen and plant macrofossil studies of alluvial deposits in the Desna river valley on the Middle Paleolithic site Khotylevo-1. The layers of silt and clay with relatively high organic content are, probably, correlated with early Valdai loess horizon. The pollen diagram (fig.1c) show, that the share of arboreal pollen in these sediments does not exceed 35-40 % from AP+NAP. This pollen group is represented mainly by birch (Betula sect. Alba). Pinus, Salix, Alnus and Betula nana occur in small quantities. Pollen assemblages are marked by high amount and variety of herbaceous pollen (Rosaceae, Ranunculaceae, Asteraceae, Apiaceae, Caryophyllaceae, Fabaceae, etc.), although pollen of Artemisia (up to 20%) and Poaceae (15-20 %) are predominant. The presence of plants typical for birch forest is recorded (Polygonum bistorta, Thalictrum, Valeriana). Pollen of Chenopodiaceae, Ephedra, Echinops represents steppe vegetation. Cichoreaceae pollen became abundant in the upper part of the profile. Spores of clubmosses characteristic for boreal forests (Lycopodium annotinum, L. clavatum, L. selago) were found. Among plant macrofossils of trees few nuts of Betula sect. Alba were identified. The herbaceous plants are represented by remains of Carex cf. pseudocyperus, Ranunculus cf. repens, Potentilla alba, Rorippa palustris, Cicuta virosa, Myriophyllum sp., Rumex sp. According to obtained results complex vegetation of the early Valdai time included open birch woodlands, step-like communities, shrub thickets, meadows and mires. Late Valdai deposits are represented in the Desna R. drainage basin by the loess series up to 8 m thick overlying the Bryansk paleosoil. Pollen analyses of loess are considerably hindered by the low content of pollen that is often insufficient for calculations of percentage composition. In this case our studies were concentrated on fossil flora inferred from four loess sequences: Khotylevo-2, Yudinovo, Pushkari and Pogon (fig. 1a,b). Though each of the sections has no more than 40 species, all of them considered together and supplemented by published materials on sections Arapovichi (Grichuk, 1972), Timonovka (Velichko, et al., 1977), and Eliseevichi (Velichko, et al., 1997) provided a total list of 82 taxa. The composition of the fossil flora, natural habitats and relationships between the species in plant communities, as well as the present-day ranges of taxa identified in the pollen assemblages have been considered. The late Valdai vegetation cover was characterized by mosaic structure. The steppe communities seem to be dominant on flat interfluves (proportion of herb and grass in the pollen assemblages is 60 to 80%). The fossil flora contains typical species of those communities – xerophytes and xerohalophytes (Eurotia ceratoides, Kochia prostrata, Ephedra distachуa, Artemisia); those plants are indifferent to temperatures and grow at present in southern steppes, deserts and semi-deserts. Besides, the periglacial steppe formations included Сhenopodium viride, Ch. vulvaria, Kochia laniflora, Thalictrum contortum, etc. Judging from ecological requirements of the species and characteristics of modern communities including those species, the periglacial steppes of the past suggest a rather simple structure, sparse ground cover, and reduced amount of grass as compared with dwarf shrubs. In typical assemblages with Eurotia, Kochia or Ephedra as edificators the degree of coverage is locally as low as 15–20%. Steppe meadow occurred in shaded and wetter localities on gentle slopes along with periglacial steppe communities. An important component of periglacial environments both in river valleys and flat interfluves were areas with disturbed soils or barren ground. Pioneer species, typical for such habitats, are abundant in fossil flora. The broad bottom of the Desna valley was occupied by meadows, locally wet, with shrubs and isolated tree stands. The valley communities included microtherms which occur at present in tundra and forest-tundra (Betula nana, Botrychium boreale, Selaginella selaginoides). The better protected habitats featured plant assemblages including patches of forest, bush, and mesophilic meadow communities. In the severe environments of the ice age, the tree layer was formed by birch, Scotch pine, Siberian pine, spruce, less common – larch.