International audienceFossil pollen data supplemented by tree macrofossil records were used to reconstruct the vegetation of the Former Soviet Union and Mongolia at 6000 years. Pollen spectra were assigned to biomes using the plant-functional-type method developed by Prentice ct al. (1996). Surface pollen data and a modern vegetation map provided a test of the method. This is the first time such a broad-scale vegetation reconstruction for the greater part of northern Eurasia has been attempted with objective techniques. The new results confirm previous regional palaeoenvironmental studies of the mid-Holocene while providing a comprehensive synopsis and firmer conclusions. West of the Ural Mountains temperate deciduous forest extended both northward and southward from its modern range. The northern limits of cool mixed and cool conifer forests were also further north than present. Taiga was reduced in European Russia, but was extended into Yakutia where now there is cold deciduous forest. The northern limit of taiga was extended (as shown by increased Picea pollen percentages, and by tree macrofossil records north of the present-day forest limit) but tundra was still present in north-eastern Siberia. The boundary between forest and steppe in the continental interior did not shift substantially, and dry conditions similar to present existed in western Mongolia and north of the Aral Sea

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Pollen stratigraphy of an extrazonal palsa bog in the middle taiga of the West Siberian Plain is dated by radiocarbon at five levels. Local pollen assemblage zones (LPAZ) are the basis for palaeogeographical reconstructions. Tundra-steppe plant communities with shrub birch (Betula nana) dominated in the latest Pleistocene. Warming after 10 000 14 C yr BP caused the local thawing of permafrost, forming shallow lakes. Larix and then Picea spread along river valleys and depressions. Steppe plant communities existed on the dry interfluves. Further climatic warming and drying caused Picea to retreat, and Betula forest-steppe dominated 9500-8900 yr BP. Dense Pinus sylvestris and Pinus sibirica forests then spread over the whole area, and steppe communities decreased about 8300 yr BP. The Holocene climatic optimum (6000-5000 yr BP) was characterized by warm and wet conditions and Abies was widespread. Cooling then caused retreat of Abies forests to the south and the expansion of Pinus sibirica forests on clay soils and Pinus sylvestris forests on sandy soils. Cooling about 4300 yr BP caused the peat to freeze and the palsa to form by bulging. Peat accumulation on the Bugristoye bog stopped at this time.

In the southeast of Western Siberia within the zone of birch and pine forests in the Ob'-Tom' interfluve area, four new pollen diagrams from sites in different landforms (interfluve, terraces, floodplain, and ancient drainageways) with sandy and clay soils were correlated with two published pollen diagrams. Two main types of pollen stratigraphy are characteristic for areas with clay and sandy soils, reflecting different vegetational developments during the Holocene that were controlled by changes in climate and soil properties. Open birch forest-steppe with Artemisia and Chenopodiaceae was widespread in the Ob'-Tom' interfluve during the early Holocene, whereas the young floodplains had flooded meadows and willow scrub. The principal vegetation change from open steppe and forest-steppe to dense forest took place about 7000 yr BP, after which forest vegetation has prevailed. On sandy soils the dominant tree species was Pinus sylvestris throughout, but on clay Pinus sibirica and Abies sibirica together with Betula pendula had more important roles, alternating periodically with Pinus sylvestris. Expansion of Pinus sibirica and Abies sibirica in this area started about 4500-4000 yr BP, but in some places they declined markedly during recent centuries due to human influence. Based on the vegetational development, the climate in the southeast of Western Siberia changed from colddry to warm-dry (9500 yr BP) then to warm-wet (7000 yr BP), and finally to cool-wet (4000 yr BP).

Westover, Karlyn S.; Fritz, Sherilyn C.; Blyakharchuk, Tatyana A.; and Wright, Herbert E., "Diatom paleolimnological record of Holocene climatic and environmental change in the Altai Mountains, Siberia" (2006).