In terms of biodiversity, the only comparable temperate deciduous forest regions in the world are in central China, Japan and in the Caucasus Mountains. Both the Appalachians (along with the neighbouring Appalachian mixed mesophytic forests ecoregion) 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 these two locations. The Great Smoky Mountains are particularly rich in biodiversity.