Boreal kingdom

Good noted that many plant species of temperate North America and Eurasia were very closely related, despite their separation by the Atlantic Ocean and the Bering Strait.

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.

Others, based on the distribution of related plant and animal families, include the Palearctic and Nearctic in a single Holarctic 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.