North American Atlantic Region is a
floristic region within the
Holarctic Kingdom identified by
Armen Takhtajan and
Robert F. Thorne, spanning from the
Atlantic and
Gulf coasts to the
Great Plains and comprising a major part of the
United States and southeastern portions of
Canada. It is bordered by the Circumboreal floristic region in the north, by the Rocky Mountain and Madrean floristic regions in the west and by the Caribbean floristic region of the
Neotropical Kingdom in the south of
Florida. The
flora of the region comprises two
endemic monotypic families, Hydrastidaceae and
Leitneriaceae, and is characterized by about a hundred of endemic
genera (such as
Sanguinaria,
Leavenworthia,
Gillenia,
Neviusia,
Dionaea,
Yeatesia,
Pleea). The degree of
species endemism is very high, many species are
Tertiary relicts, which survived the
Wisconsin glaciation and are now concentrated in the
Appalachians (esp.
Blue Ridge Mountains) and the
Ozarks. A number of genera (
Sarracenia,
Uvularia etc.) are shared only with the Canadian floristic province of the Circumboreal region. Moreover, as has long been noted (e.g. by
Joseph Gerhard Zuccarini and especially by
Asa Gray), a large number of relict genera (
Liriodendron,
Hamamelis,
Stewartia etc.) are shared with the relatively distant
Eastern Asiatic Region (comprising
Japan and the east of
China) and sometimes
Southeast Asia. R. F. Thorne counted at least 74 genera restricted to eastern
North America and
Asia (mostly eastern and southeastern Asia). The fossil record indicates that during the
Tertiary period a warm temperate zone extended across much of the
Northern Hemisphere, linking America to Asia.