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. Recent fossil evidence has helped substantiate that the grasslands and steppe west of the Continental Divide in the Columbia Basin area are about three million years (Ma) old, about ten million years younger than similar vegetation types east of the Divide in the Rocky Mountain foothills and in the Great Plains. Biogeographic differences between the two areas are reflected in species composition and in the dominant habits of grasses. Some climatic differences between 'east' and 'west' exist, but they do not seem sufficient to account for the vegetation contrasts. We propose that an examination of the Late Cenozoic history (Miocene to present) of the northern Rocky Mountains may help illuminate the nature of origins of the grassland and steppe east and west of the Divide. The region to the west of the Rocky Mountains, the Columbia Plateaus3, is endowed with some of the finest and best-documented Miocene leaf floras in North America, while the region to the east, the Rocky Mountain foothills and the Great Plains, have Mio-Pliocene deposits for which pollen, leaf, and seed data are now available. To understand the history of grassland and steppe development, be