The highest elevation forests contain whitebark pine, Engelmann spruce, subalpine fir, and limber pine, including the largest individual whitebark pine in North America. Based on tree ring chronologies, some of the whitebark pines are believed to be 700 to 1000 or more years old. The highest elevation forests typically have understories of grasses and forbs that are resistant to freezing at any point of the growing season. Willows, alders, cottonwoods, and sedges are found in riparian areas. Ponderosa pine occupy the dry, lower elevations near the western edge of the forest and historically persisted due to the occurrence of frequent non-lethal fires. Ponderosa pine forest understories typically consist of perennial grasses such as Idaho fescue and bluebunch wheatgrass. In the slightly moister ponderosa pine forests grasses such as pinegrass are found with a cover of shrubs including white spirea, common snowberry, and mallow ninebark.