The Edge of the Desert

Abstract

THE EDGE OF THE DESERT Forrest Shreve Desert Laboratory, Carnegie Institution of Washington, Tucson, Arizona SOME GEOGRAPHERS and plant ecologists apply the term desert only to the most barren and extreme types of arid country. From the I iiological standpoint it seems desirable to adopt a broader conception of desert, as is done in the use of the word forest. The botanical earmarks of desert are the low stature and open spacing of the plants, and the presence of a few or many type > of highly specialized plants. It is the many features of specialization in form, structure, physiological, performance, and ecological behavior which identify the plants and animals of the desert in the broader sense of the word. The name "bush steppe" has been used for certain shrubby communities in the North American Desert, but is an unfortunate term which should be discarded. Such communities are in no sense steppe, and the presence of bushes as the matrix of the vegetation is characteristic of perhaps 90% of the North American Desert. Both the physical and the biological features of the Narth American Desert suggest its subdivision into four areas: the Great Basin Desert, the Mojave Desert, the Sonoran Desert, and the Chihuahuan Desert. South of the last lies a region in which there are discontinuous areas of desert of a distinct type, for which the designation Hidalgan Desert is proposed. Parts of this area have been studied by Ochoterena(7), Bravo (2, 3), and Batalla and Cantu(l), but its exact limits have not yet been drawn. The Great Basin, Mojave and Chihuahuan areas are entirely continental, while the Sonoran area is partly continental and partly coastal The periphery of the North American Desert, exclusive of the Hidalgan area, is approximately 9,500 miles. Of this distance about 7,700 miles are on land and constitute the boundary along which desert comes in contact with other types of vegetation. Certain parts of the edge of the desert are sharply drawn, especially where high mountains cause a marked change of environment within a short distance. Other parts of the edge are broad; and are occupied by transitional belts in which slope exposure, character of soil, and other local conditions may favor one or the other of the two adjacent vegetations. If the edge of the desert is followed for a long distanc; it becomes obvious that the particular physical conditions which limit the desert in one place are diffère it from those that limit it in another. The character of the vegetation bordering the desert changes radically and the species of plants involved in the transitions are not the same. Throughout the entire periphery of the North American Desert it is in contact with seven of the major types of vegetation of North America, as follows: chaparral, coniferous woodland, evergreen oak woodland, grassland, cactus savanna, semi-arid bushland, and thorn forest. The writer has published descriptions of the transition from desert to chaparral in Baja California(lO), from desert to grassland in Chihuahua (9), and from desert to thorn forest in southern Sonora . (8) The transition to chaparral is found in Baja California and southern California, as well as along the western base of the Sierra Madre Oriental in Nuevo Leon, as described by Müller. (6) The transition to coniferous woodland (juniper and pinyon ) is found on all sides of the Great Basin and locally in Arizona. The meeting of desert and evergreen oak woodland occurs in southern Arizona, northern Sonora and Chihuahua, but is usually local, since these vegetation ; are commonly separated by grassland. The transition to grassland is found in Arizona, New Mexico, 6 1940of Pacific Coast Geographers7 Texas, and Chihuahua, and is commonly the most gradual of all of them. The transition from desert to cactus savanna may be seen in Durango and Zacatecas, where cactus savanna lies between desert and grassland. The transition to semiarid bushland is found in Texas, Coahuila, and San Luis Potosí along most of the eastern edge of the desert. The transition to thorn forest is found only in southern Baja California and on the coastal plain of southern Sonora. It is not possible here to go into the details...