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TheBoreal kingdom orHolarctic kingdom (Holarctis) is afloristic kingdom identified (1947) by botanistRonald Good[1] (and later byArmen Takhtajan), which includes thetemperate toArctic portions ofNorth America andEurasia. Itsflora is inherited from the ancientsupercontinent ofLaurasia. However, parts of the floristic kingdom (and most of itsCircumboreal Region) were glaciated during thePleistocene and as a consequence have a very young flora.Cenozoicrelicts found refuge in the southern and mountainous parts of the kingdom, especially in theEastern Asiatic Region and southernNorth American Atlantic Region.
Good noted that many plant species of temperate North America and Eurasia were very closely related, despite their separation by theAtlantic Ocean and theBering Strait.
Millions of years ago, before the opening of the Atlantic Ocean, North America and Eurasia were joined as a single continent, Laurasia. After the opening of the Atlantic, the continents were connected to one another periodically via land bridges linkingAlaska andSiberia. Until a few million years ago, the globalclimate was warmer than at present, especially at higher latitudes, and many temperate climate species were distributed across North America and Eurasia via Alaska and Siberia. The sharply cooler climate of the past few million years eliminated a temperate-zone connection between North America and Eurasia, but common Laurasian origins and a long history of temperate-climate land bridges account for the botanical similarities between the temperate floras on the two continents.
A floristic kingdom is the botanical analogue to anbiogeographic realm, which takes into account the distribution of animal as well as plant species. Manybiogeographers distinguish the Boreal kingdom as comprising two realms, theNearctic (North America) andPalearctic (North Africa and Eurasia). Others, based on the distribution of related plant and animal families, include the Palearctic and Nearctic in a singleHolarctic realm, which corresponds to Good's Boreal kingdom.
According to Takhtajan (1978), the following families are endemic to this kingdom:Ginkgoaceae,Cephalotaxaceae,Glaucidiaceae, Hydrastidaceae (Hydrastis),Trochodendraceae,Tetracentraceae,Cercidiphyllaceae, Euptelaceae (Euptelea),Platanaceae,Eucommiaceae,Rhoipteleaceae,Leitneriaceae,Paeoniaceae,Crossosomataceae,Stachyuraceae,Fouquieriaceae,Diapensiaceae,Simmondsiaceae,Pterostemonaceae,Penthoraceae,Bretschneideraceae,Limnanthaceae, Davidiaceae (Davidia),Toricelliaceae,Helwingiaceae,Theligonaceae,Dipentodontaceae,Cynomoriaceae,Adoxaceae, Trapellaceae (Trapella),Butomaceae,Scheuchzeriaceae,Aphyllanthaceae.[2][3]
According to the same author, the following families are well represented:Magnoliaceae,Lauraceae,Ranunculaceae,Berberidaceae,Hamamelidaceae,Fagaceae,Betulaceae,Juglandaceae,Caryophyllaceae,Chenopodiaceae,Polygonaceae,Plumbaginaceae,Theaceae,Salicaceae,Brassicaceae,Ericaceae,Primulaceae,Malvaceae,Euphorbiaceae,Thymelaeaceae,Rosaceae,Fabaceae,Cornaceae,Araliaceae,Apiaceae,Rhamnaceae,Gentianaceae,Boraginaceae,Scrophulariaceae,Lamiaceae,Campanulaceae,Asteraceae,Liliaceae,Iridaceae,Orchidaceae,Juncaceae,Cyperaceae,Poaceae,Pinaceae,Cupressaceae,Aspleniaceae,Polypodiaceae.
The kingdom is subdivided into three floristic subkingdoms and ninefloristic regions.