Origins of the Cordilleran Flora

Axelrod, Daniel; Raven, Peter

The central Cordilleran region has been an ecotone between forest and woodland-scrub vegetation since the middle Eocene. Responding to spreading drought and increasing topographic diversity, many near-modern Cordilleran species were present in both vegetation zones by the late Eocene. Following the elimination of most exotics of eastern affinity during the middle Oligocene, Cordilleran endemic woody species and endemic genera have dominated the forests and woodlands and have changed but little since then. Cordilleran taxa spread into the western Great Basin largely after 14 m.y. ago as climate became drier, summer rains were reduced, and eastern exotic taxa were eliminated. In the late Miocene (5-7 m.y.) some Cordilleran taxa were restricted eastward as summer rains decreased further. Elevation of the southern Rocky Mountain region 1500-3000 m following the later Miocene, together with the global trend to cooler climate, restricted many frost-senstive taxa to the Madrean region farther south. The Great Basin differentiated from the southern Rocky Mountain flora as a double rainshadow developed as the Sierra Nevada-Peninsula Ranges and the Wasatch-Colorado Plateau were elevated in the late Neogene and Quaternary. During this period species in a few herbaceous and perennial groups, such as Astragalus, Eriogonum, and Penstemon, proliferated as new adaptive zones became available at both low and high elevations. The surviving Cordilleran flora in both its Great Basin and southern Rocky Mountain sectors is impoverished, its taxa having adapted to the present more severe continental climate.

Origins of the Cordilleran Flora

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