GLOBAL GREENHOUSE TO ICEHOUSE AND BACK AGAIN: THE ORIGIN AND FUTURE OF THE BOREAL FOREST BIOME

The Boreal Forest Biome (Taiga), dominated by evergreen coniferous trees (Pinaceae), is circumpolar in its present distribution, covering a significant part of the total land area of the northern hemisphere and representing perhaps a third of the total forest area of the planet. Nothing comparable to this extant biome could have existed during the global greenhouse interval of the late Mesozoic and Paleogene. Latitudinal temperature gradients would have confined boreal taxa to extremely high latitudes, but evergreen taxa could not survive in the far north due to the extended polar darkness combined with the metabolic costs associated with mild winter temperatures. It is probable that such environmental constraints drove the evolution of the deciduous habit that now characterizes trees and shrubs associated with North American and East Asian deciduous forest biomes.

Probable sources for the pinaceous taxa that now characterize boreal latitudes were the evergreen montane coniferous forests of the North American western Cordillera. Taphonomic factors limit the fossil record for such forests, but assemblages such as the Eocene Thunder Mountain (Idaho) and Bull Run (Nevada) floras are excellent analogs of extant boreal communities. In response to post-Eocene global cooling, such forests would have migrated to lower elevations, eventually spreading across high-latitude North America, subsequently reaching Eurasia via the Beringian corridor. Despite its extensive geographic distribution, the Boreal Forest may be the youngest of the major forest biomes.

If anthropogenic global warming results in a significant redistribution of terrestrial vegetation, the history of the Boreal Forest may well be reversed. Unable to survive at high polar latitudes, evergreen conifers might once again become restricted to New World and Old World montane refugia. Given the biogeographic significance of the Boreal Forest biome, such a consequence would represent a profound ecological transformation.