When Lewis and Clark saw western Iowa in 1804, the land was almost completely covered with prairie grasses and wildflowers, but now very little of this vegetation remains. The largest tracts of undisturbed prairie in Iowa are in the Loess Hills, where these prairie plants have lived for thousands of years. Biologists have identified over 300 species of plants there, many native to Iowa only in the Loess Hills. These prairies are classified as mixed grass prairies, containing tall grass species such as switchgrass, big bluestem, and Indiangrass, mid-grasses such as little bluestem and side oats grama, and short grass plants such as blue grama and buffalograss. Prairie wildflowers, called forbs, also share this habitat, including leadplant, compass plant, coneflower, asters, blue eyed grass, and yucca. Timber is a natural enemy of the sun-loving prairie, as woody plants slowly move up hillsides and grow until their shade eliminates prairie vegetation. Coralberry, sumac, and dogwood begin this process, creating shade and wind protection that allow trees such as red cedar and burr oak to sprout. In Iowa, trees are not rare like the native prairie. The mixture of forbs and grasses in the Loess Hills evolved naturally for thousands of years, and wildfires and herds of buffalo helped control the spread of trees and shrubs in pre-settlement times. Because these natural forces are no longer present, saving the remaining prairie now depends on stopping woody invasion, primarily through periodic burning.