Three major forest types converge in the Niobrara River valley: ponderosa pine, eastern deciduous, and northern boreal. Ponderosa pine forest is at its eastern limit in the river valley. The nearest designated forest area containing ponderosa pine is located in the Black Hills in South Dakota. Eastern deciduous forest has extended up the valley and includes bur oak, American elm, black walnut, green ash, basswood, and hackberry. Broadleaf shrubs and vines include sumac, western snowberry, gooseberry, wild plum, and wild grape. Northern boreal forest is found on cool, moist, north facing slopes and includes paper birch, a hybrid species of quaking and bigtooth aspen (the Niobrara Valley is the only known area where this species occurs in Nebraska), ferns, and several species of club mosses. These plants apparently have survived as relics of the Pleistocene ice age, when they were more widely distributed on the Great Plains.
For more information regarding Nebraska forests and species distributions you can click on the links below.
Nebraska Forest Service
Native vegetation map - Univ of Nebraska State Museum
Index of native plant species – University of Nebraska State Museum