The USGS defines ten forested zones in the Rocky Mountains. The more southern, warmer, drier zones are defined by the presence of pinyon pines/junipers, ponderosa pines, or oaks mixed with pines. The more northern, colder, wetter zones are defined by Douglas-firs, Cascadian species (such as western hemlock), lodgepole pines/quaking aspens, or firs mixed with spruce. Near the treeline, zones can consist of white pines (such as whitebark pine or bristlecone pine); or a mixture of white pine, fir, and spruce that appear as shrub-like krummholz. Finally, rivers and canyons are home to unique forest habitats even in the more arid parts of the mountain range.