Abstract: Tundra should be designated as the vegetation in the Arctic treeless area beyond treeline latitudinally, and as the vegetation in alpine treeless areas above treeline which are greatly similar to the Arctic one not only in environment but also in floristic composition and phyto-community. Tundras are limited to the Northern Hemisphere, and is mainly distributed in the circumpolar region (over 95% out of the total). Only a small part of tundra vegetations is scattered in the alpine treeless areas in the middle latitudinal region of the Northern Hemisphere, and is called the alpine tundra and/or mountain tundra. The alpine tundra is greatly similar to the Arctic tundra not only in environment features and vegetation appearance, but in flora as well. Actually, nowaday alpine tundra is directly developed from the remnant segments of the Arctic tundra which migrated southwards to the middle latitudinal region of the Northern Hemisphere in the glacial period of the last Ice-age in the Pleistocene, and later moved up to the alpine areas in the Holocene. The alpine tundras of the Changbai Mountain and Rocky Mountains (middle section)are located on the southern fringes of the tundras in Eastern Asia and Western North America respectively. By means of comparative analyses of the vascular floras of the Chukotka (in NE. Asia) Arctic tundra and Alaska (in NW. America) Arctic tundra (transberingian comparison), the Changbai Mountain (in E. Asia) alpine tundra and Rocky Mountains(in W. North America) alpine tundra( transpacific comparison), and the alpine tundras and the arctic tundras in E. Asia and W. North America, the results show as follows: (1) Because of the fact that Chukotka and Alaska not only share 411 species (making up 83% of the former total species and 72% of the latter total species), but also have many endemic species with a 'Chukotka-Alaska' discontinuous distribution pattern, while there are only 268 species shared by the Arctic tundras of Chukotka and East Siberia and 332 species shared by the Arctic tundras of Alaska and Canada, it seems reasonable to consider the Arctic tundras of Chukotka and Alaska as one floristic province-the Beringian Floristic Province. The existence of the Beringian Floristic Province could at least be traced back to 18,000 B. P. in the Pleistocene. (2) There is a close relationship between the alpine tundras of Eastern Asia and Western North America. This relationship was built up by means of the Bering Land Bridge in Ice-age. (3) In Eastern Asia, 42% species out of the total in the alpine tundra of Changbai Mountains are shared with the Arctic tundra of Chukotka; and in Western North America, 48.9% species out of the total in the alpine tundra of Rocky Mountains (middle section) are shared with the Arctic tundra of Alaska. Therefore, the floristic relationships of the alpine tundras and the Arctic tundras (especially Beringian Arctic tundra) in Eastern Asia and Western North America are very close. (4) The Bering Land Bridge in the Pleistocene became an exchange passageway of the floras between Eastern Asia and Western North America probably only for their Arctic tundra species, but not for the forest tree species nearby the Arctic treeline.Key words Floristics; Arctic tundra; Alpine tundra; Chukotka; Alaska; Changbai Mountains; Rocky Mountains; Eastern Asia; Western North America

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