Boreal Kingdom

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Good noted that the plant species of temperate North America and Eurasia were very closely related, despite their separation by the Atlantic Ocean and the Bering 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 linking Alaska and Siberia. Until a few million years ago, the global climate 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 an ecozone, which takes into account the distribution of animal as well as plant species. Many biogeographers distinguish the Boreal Kingdom as comprising two ecozones, the Nearctic (North America) and Palearctic (Eurasia). Others, based on the distribution of related plant and animal families, include the Palearctic and Nearctic in a single Holarctic ecozone, which corresponds to Good's Boreal Kingdom.

The kingdom is subdivided into three floristic subkingdoms and nine floristic regions.

ubdivisions

Boreal Subkingdom

* Circumboreal Region
* Eastern Asiatic Region
* North American Atlantic Region
* Rocky Mountain Region

Tethyan Subkingdom

* Macaronesian Region
* Mediterranean Region
* Saharo-Arabian Region
* Irano-Turanian Region

Madrean Subkingdom

* Madrean Region