The Niobrara ecotone, along the Niobrara River in north-central Nebraska, is an area of transition between habitats, plants, and animals with more eastern, western, southern, and northern distributions. The ecotone is most dramatic from the mouth of Minnechaduza Creek on the Fort Niobrara National Wildlife Refuge to the mouth of Plum Creek north of Ainsworth. In this fifty-mile stretch, the Niobrara flows through a gently rolling grassland in a wooded canyon nearly 400 feet deep. The canyon has developed during the last 20,000 years as local uplifting of the earth's surface has been countered by the actively downcutting Niobrara River. Several geological formations are exposed, leading to an array of soils, slopes, and sun angles. The ecotone is a mosaic of habitats on wet-to-dry and warm-to-cool sites, with shallow-to-deep and fertile-to-sterile soils.
South of the river, paper birch (Betula papyrifera) and aspen (Populus tremuloidesgrandidentata) stands, with northern mannagrass (Glyceria borealis) and green orchids (Habenaria hyperborea) in the understory, grade into eastern deciduous woodland featuring basswood (Tilia americana), hop hornbeam (Ostrya virginiana), and hazelnut (Corylus americana). These woodlands are replaced by mesic tallgrass prairie stands on subirrigated low terraces, and bur oak (Quercus macrocarpa)– ponderosa pine (Pinus ponderosa) savanna and then Sandhills prairie on the dry upper slopes. North of the river the ecotone is a transition from western coniferous woodland dominated by ponderosa pine to northern mixed-grass prairie. Riparian woodlands, shrub lands, wetlands, and riverine habitats occur on the valley floor.
The Niobrara ecotone is particularly rich in species because of the many transitions between the distinct woodland, grassland, and wetland habitats in a relatively small area. The ecotone supports nearly two-thirds of the vascular plant species, more than half of the mammal and bird species, more than a third of the butterfly species, and more than a quarter of the fish species found in Nebraska. Fifty-five moss and eighty-six lichen species have been collected within the ecotone.
Whether on the canyon slopes and terraces or in the valley bottom, the forces of moving water, fire, and grazing continue to maintain this dynamic mosaic. Interestingly a northwest to southeast flowing river of the Miocene epoch (5 to 24 million years ago) also supported an impressive ecotone in what is now the middle Niobrara Valley. The fossil remains of a diverse group of browsers, grazers, predators, and aquatic species are represented in numerous quarries excavated in the contemporary Niobrara ecotone.
Allen A. Steuter The Nature Conservancy
Kaul, Robert B., Gail E. Kantak, and Steven P. Churchill. "The Niobrara River Valley, a Postglacial Migration Corridor and Refugium of Forest Plants and Animals in the Grasslands of Central North America." The Botanical Review 54 (1988): 44–81.
Steuter, Allen A., Bruce Jasch, Joel Ihnen, and Larry L. Tieszen. "Woodland/Grassland Boundary Changes in the Middle Niobrara Valley of Nebraska Identified by Delta Carbon 13 Values of Soil Organic Matter." American Midland Naturalist 124 (1990): 301–8.