Forest vegetation in the southeastern United States extends westward beyond the Ozark and Ouachita plateaus in Arkansas and Missouri into the Central Plains. Along this transect, luxuriant forests give way to mixed forests and grasslands that include smaller trees and progressively fewer tree species and eventually to grassland-dominated landscapes in central Kansas, Oklahoma, and Texas. This transition is directly related to decreasing precipitation with distance to the west of the Mississippi River valley. Many species, however, have abrupt western range boundaries related to physiography and hydrogeomorphic processes. The western range limits for many trees correspond to Coastal Plain boundaries that at a regional scale impose sharper range boundaries than would be expected on the basis of decreasing precipitation. Also, riparian habitats within stream valleys extending westward from the Coastal Plain provide suitable habitats for trees in the dry regions of the Great Plains. The presence of riparian trees in this region is determined largely by the presence or absence of groundwater conditions necessary for survival. For floodplain trees, then, it is primarily habitat—not climate—that determines the location of range boundaries.