Vegetation of Northern Mexico

YEARBOOK OF THE ASSOCIATION OF PACIFIC COAST GEOGRAPHERS Volume 8 1942 VEGETATION OF NORTHERN MEXICO* Forrest Shreve Desert Laboratory, Carnegie Institution, Tucson, Arizona The topography of Mexico north of the Tropic of Cancer is simple in its general pattern but complicated in many of its details. The Eastern and Western Sierra Madre Mountains, which lie parallel to the coasts, rise to maximum elevations of about 3000 m. and dominate the topography, determine the drainage, influence the climate and fundamentally affect the distribution of the vegetation. Narrow piedmont and coastal plain areas skirt the coasts of the Gulf of California and the Gulf of Mexico. The broad central plateau rises irregularly from the Rio Grande to elevations of 1700 to 2300 m., its drainage being either into the Rio Grande or into enclosed basins. The peninsula of Baja California has an irregular mountain axis and only small areas of coastal plain. The climate of northern Mexico is mild in its temperatures and arid or semi-arid. The lightly forested areas are visited only by seasonal rains and are semi-arid in many of their biological features. Only in the highest mountains and in favored localities on the coasts are mesic conditions found. The prevailing vegetation is desert, with fringes of grassland or chaparral. The most arid parts of northern Mexico are central Baja California, the plains on the northwestern mainland, the floor of the central plateau, and the extreme northern part of the eastern coastal plain. In the first three of these areas there is a broad similarity in the vegetation, dominated by a few species of shrubs and numerous cacti, yuccas, agaves and other specialized plants. In spite of similarity in the physiognomy of the vegetation there are strong differences in the composition of the flora which forms it. In the extreme northeast the conditions are somewhat less arid and both vegetation and flora differ widely from those of the other areas. The principal grassland areas are on the benches which lie immediately east of the Western Sierra Madre and those which surround the northernmost part of the Eastern Sierra Madre. In their composition and structure the grasslands are closely related to those of the southwestern United States. On the edges of the plateau, and in or above the grassland, are ex- *Abstract of Presidential address before the Association of Pacific Coast Geographers , at Salt Lake City, Utah, June 18, 1942. (Note: The author will be glad to send reprints of several short papers on Mexican vegetation to anyone who wishes them. Address: Box 471, Tucson, Arizona). 3 4 Yeartoofc o/ the AssociationVol. 8 tensive areas of evergreen oak woodland, with local admixture of juniper and pinyon. At the same elevation, between 1500 and 2200 m., grassland occurs on deep soil and woodland on hills and rocky soil, the climatic requirements of the two types of vegetation being very similar. Pine forest clothes the western Sierra Madre above 2200 m. almost continuously from the International Boundary to the Tropic. In addition to western yellow pine there are several other abundant pines, as well as a few evergreen oaks and deciduous trees. There are small areas of fir forest on the highest peaks. On the Eastern Sierra Madre the area of pine forest is smaller and more interrupted in distribution than it is in the west. On the west coast there is a rapid transition in southern Sonora from desert to thorn forest, which is characterized by a thin stand of small leguminous and other trees, with many columnar cacti of species not found in the desert. The well watered canyons on the western slopes of the Western Sierra Madre and on the eastern slopes of the Eastern Sierra Madre are heavily wooded with deciduous trees together with some evergreen oaks and a few pines. The affinities of the floras of the canyons are distinctly sub-tropical, especially in the southernmost canyons of the east coast. In its larger features the distribution of the several types of vegetation in northern Mexico appears to be coirtroHed by climatic conditions in i. manner similar to that which has been demonstrated for the western United States. Detailed study of smaller areas reveals, however, many...