Given the taxonomic and biogeographic differences between dominant species in grasslands of the Great Plains and the palouse grasslands and steppe of the two biomes have had separate origi l 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 a and nearby pesi 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) iugo 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 Rangea nd/or regional up-According to the regional pollen record, terrestrial herbs become more diverse in the late Neogene.
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Leopold et al. (1987) studied this question.