The region's long ridges and valleys serve both as barrier and corridors, and their general north-south alignment allowed habitats to shift southward during ice ages. The geography also contains a large number of diverse microclimates and microhabitats. These factors resulted in many refugia areas and relict species surviving and thriving in the Appalachians. The long-term geologic stability of the region, in addition to its long ridges and valleys, has resulted in one of the world's richest temperate deciduous forests in terms of biodiversity. There are an unusually high number of species of both flora and fauna, as well as a high number of endemic species.
In terms of biodiversity, the only comparable temperate deciduous forest region in the world is that of central China. Both the Appalachians and central China contain relict habitats of an ancient forest that was once widespread over the Northern Hemisphere. There are species, genera, and families of plants that occur only in central China and the Appalachians.