Flora of the Sonoran Desert includes six subdivisions based on vegetation types. Two are north of the boundary between the United States and Mexico, and four are south of the boundary. The flora of the Colorado Desert are influenced by the environment of the very dry and hot lower areas of the Colorado River valley, which may be barren, treeless, and generally have no large cacti. Flora of the Arizona Upland are comparatively lush, with trees and large columnar cacti that can withstand winter frosts. South of the border subdivisions are characterized by plants that cannot withstand frost.