Phytogeography of higher units of forests and krummholz in North Asia and formation of vegetation complex in the Holocene
Krestov, Pavel V.; Omelko, Alexander M.; Nakamura, Yukito
The modern vegetation pattern of Northern Asia has an uninterrupted history of development from Tertiary boreo-nemoral vegetation complex. The most dramatic transformation of this complex took place in the Pleistocene Maximum, when the low temperatures along with severe climatic aridisation caused a qualitative change of the vegetation in the broad areas of mainland Asia and provided an opportunity for migrations of drought- and cold-tolerant species. This paper focuses on the problem of the contributions of major Asian floristic centres to the formation of major vegetation types of Northeast Asia in the changing climatic conditions of the Pleistocene and Holocene. The increasing temperature and humidity in the beginning of Holocene caused the formation and expansion of nemoral forests belonging to the order Tilio-Pinetalia koraiensis composed mainly of humidity-dependent Sino-Japanese species, and nemoral Quercus mongolica forests that included drought-tolerant species of Central Asian origin. In the boreal zone, the larch forests reached the coast of the Arctic Ocean in a relatively short period between 10000 and 6000 years BP. Perhaps this period is the time of formation of the order Ledo-Laricetalia cajanderi composed of species adapted to extremely cold winters, hot summers and cold soils lying on top of the layer of permafrost. The ultra-continental climate in conditions of relatively high temperatures caused deepening of the active soil horizons and differentiation of the order Lathyro-Laricetalia, characterised by the presence of drought-tolerant species of Central Asian and Sino-Japanese origin. These climatic and edaphic conditions appeared to be unfavorable for mesic species. Therefore, the areas with an ultra-continental climate represent an important biogeographical limit for mesic Beringian and Sino-Japanese species. Increasing oceanicity in the period of 5000–3500 years BP caused forest retreat that led to the formation of the youngest zonal order of Northern Asia, the Larici-Betuletalia divaricatae characterised by domination by cold tolerant shrubs from the genera Betula, Alnus and Salix.