The Niobrara River Valley,a postglacial migration corridor and refugium of forest plants and animals in the grasslands of central North America

The Niobrara River Valley of northern Nebraska contains numerous bryophyte, vascular plant, and animal species more typical of forests far to the east, north, and west than to other forests in the grasslands that surround the Valley. Some species are probably relicts of cooler glacial and early postglacial times, when much of what is now grassland was covered by boreal and cool-temperate forests. Others entered the Valley from the east in postglacial times, and some entered from the west as the climate became semi-arid. There is a steep decline in total number of vascular plant species from the mouth of the Missouri River up through the Niobrara Valley, suggesting an environmental gradient and differential migration and extirpation of species at various times since the Pleistocene.