Comparative Age of Grassland and Steppe East and West of the Northern Rocky Mountain

Abstract

Given the taxonomic and biogeographic differences between dominant species in grasslands of the Great Plains and the palouse grasslands and steppe of the Pacific Northwest, it seems likely that the two biomes have had separate origins. Fossil leaf and seed floras and pollen data from east and west of the Rocky Mountain cordillera and pollen data from the cordillera area suggest that three distinct floristic provinces were in existence by the beginning of mid-Miocene (Barstovian) time. Montane conifer forest poor in genera typified the Rocky Mountain foothills and nearby basins, while mixed conifer-deciduous hardwood forest and Taxodium swamps rich in woody genera occurred in the Columbia Basin. On the Great Plains the Kilgore flora (Barstovian of Nebraska) indicates deciduous open forest and prairie dominated by species of eastern and southern affinities. Younger Neogene floras in the Pacific Northwest suggest that steppe and local grassland were beginning to be important in the Pliocene of the Pacific Northwest, while grasslands and open forest became widespread on the Great Plains somewhat earlier (late Miocene or Clarendonian-Hemphillian time). The persistence of mixed conifer and broad-leaved forest on the Columbia Plateaus suggests that this area was open to the west through about 8 Ma. Due to the increasing height of the Cascade Range and/or regional upwarping, the rain shadow east of the Cascades became increasingly effective after the Clarendonian. Many of the terrestrial herbaceous groups and some shrub taxa of xeric environments reported here from a variety of sites are unknown in the Paleogene and may have evolved during the Neogene. According to the regional pollen record, terrestrial herbs become more diverse in the late Neogene.