FOREST RESPONSES TO CLIMATIC COOLING ACROSS THE EOCENE-OLIGOCENE BOUNDARY IN WESTERN NORTH AMERICA

Paleofloras of western North America provide evidence for forest responses to the significant cooling during the Eocene-Oligocene transition. Floras of late Eocene age range from broadleaved evergreen and paratropical rain forests of the Pacific coast lowlands to warm temperate upland forests in the continental interior. Late Eocene upland floras such as Florissant provided sources of temperate taxa that dispersed or evolved to form new forest types in cooler climates of the Oligocene lowlands. Oligocene floras of the Pacific coast (e.g., Bridge Creek flora of Oregon) show strong similarity to the deciduous hardwood forests of eastern Asia and eastern North America but less resemblance to modern west coast forests, whereas interior floras of the southern Rocky Mountains and Rio Grande Rift (e.g., Creede flora of Colorado, and Hermosa and Hillsboro floras of New Mexico) “modernized” within that region during the Oligocene and developed strong affinities to the region’s modern flora. The early Oligocene Bridge Creek flora (31.8-33.6 Ma) is a classic example of the type of vegetation that became widespread at low elevations following the Eocene-Oligocene cooling. The flora indicates a temperate broad-leaved deciduous forest similar to the modern Mixed Mesophytic forest. By contrast, Oligocene floras of the southern Rocky Mountains have a strong coniferous component and are cool temperate, indicating moderately high elevations. The transitional Eocene-Oligocene Antero flora (33.76 Ma) contrasts with the nearby latest Eocene Florissant flora (34.07 Ma) by having a significant reduction in warm temperate angiosperm hardwoods and an increasing abundance of Pinaceae, implying response to Eocene-Oligocene cooling. The late Oligocene Creede flora (26.8 Ma) represents a cool temperate montane Mixed Coniferous Forest dominated by Pinaceae mixed with angiosperm shrubs and having strong similarities to the modern Rocky Mountain flora. The early Oligocene Hermosa and Hillsboro floras (28.1-33.6 Ma) of the Rio Grande Rift indicate a cold temperate subalpine forest at very high elevation and are dominated almost totally by bristlecone pine. Collectively, these floras provide evidence for dynamic climate and elevation history of western North America during the late Eocene through the Oligocene.