Arcto-Tertiary Geoflora

Many of the needleleaf (gymnosperm) and broadleaf (angiosperm) genera that today are segregated into the taiga and TBDF biomes, respectively, previously existed together in a near-continuous belt across Eurasia and North America. This distribution pattern occurred some 65 to 15 million years ago, when the northern continents were joined by a Bering land bridge and located at a lower latitude. In geologic time this period is known as the Tertiary, and globally there was a much milder climate than at present. A single flora existed around the earth in the northern latitudes at that time, a flora now designated as the Arcto-Tertiary Geoflora.

About 15 million years ago climatic cooling took place due to the northward drift of both continents, and drying occurred in the center of the North American continent as a consequence of the uplift of the Rocky Mountains. Cooling apparently led to the sorting out of the gymnosperms from the angiosperms. The gymnosperms became concentrated in higher latitudes as a Boreal Forest; angiosperms, preferring a milder climate, became the TBDF. The development of the colder Arctic and Boreal regions and of the drier zones to the lee of the Rockies created disjunctions in the TBDF, leading to the contemporary distribution pattern of the biome and explaining the close relationships (congeners) among the flora of its North American, East Asian, and European expressions.

|TBDF|