DISTRIBUTION OF VEGETATION IN UNITED STATES. 35 



Texas Semidesert {Mesquital-Grassland Complex). — The valley of 

 the Rio Grande below the Balcones Escarpment and the River Neuces 

 presents a region in which the deciduous compound-leaved leguminous 

 shrubs and trees form the dominant vegetation, interspersed with 

 areas of more or less open grassland. The shrubs and trees form a 

 more or less closed scrub from 3 to 6 feet high where the shrubs pre- 

 dominate and a more open one from 15 to 25 feet high where the trees 

 are most abundant. The commonest of the trees is the mesquite 

 {Prosopis glandulosa), which has spread somewhat over adjacent 

 areas since the advent of the white man. With it the evergreen broad- 

 leaved live-oak {Quercus virginiana) is a minor component of the vege- 

 tation. In the scrub the dominant species are huisache {Acacia 

 farnesiana), guajillo {Acacia berlandierei) , and other microphyllous 

 forms {Momesia, Parkinsonia, Condalia, Sesban, etc.), while the succu- 

 lents are confined to the frequent occurrence of a prickly-pear cactus 

 {Opuntia lindheimeri) and a yucca {Yucca treculeana). Ephemeral 

 herbaceous plants are also present here. 



Pacific Semidesert {Chaparral-Encinal-Desert Complex). — The Pacific 

 semidesert region is best designated thus in spite of the fact that it 

 comprises many small areas which are far from being desert. There is 

 no other portion of the United States in which such profound differ- 

 ences of vegetation exist within such small areas, and in which it would 

 be more difficult to map on a small scale the complex allocation of 

 these areas. Over the low hills and around the bases of the Coast 

 Ranges is to be found chaparral, varying from place to place in height 

 and density; in the valleys and on the north faces of some of the hills 

 are to be found groves of evergreen or deciduous oaks; while in other 

 valleys, particularly the broad valleys of the Sacramento and San 

 Joaquin Rivers, and on the interior hills of the Coast Ranges, are to 

 be found some of the most truly desert areas in the United States. 

 The chaparral is sometimes a very low aggregation of shrubs, or some- 

 times reaches a height of 6 or 7 feet. It is also variable in its density, 

 but is commonly so close-set that it can be traversed only very slowly. 

 It is made up predominantly of evergreen shrubs {Ademostoma, 

 Arctostaphylos, Heteromeles), but partly of deciduous shrubs {Ceano- 

 thus spp.). The encinal is made up of evergreen oaks {Quercus agri- 

 folia) and deciduous oaks {Q. lobata, Q. wislizeni), with the digger 

 pine {Pinus sabiniana) an occasional component of it. In the desert 

 of the interior hills and the great valley is to be witnessed a region 

 almost totally devoid of perennial plants, in which the only cover of 

 vegetation is due to the herbaceous annuals that appear in the spring. 



Desert-Grassland Transition. — This transition region comprises the 

 Llano Estacado of Texas and certain plateau lands in Arizona and 

 New Mexico above 5,000 feet in elevation. It is a region that is inter- 

 mediate in all important respects between the Grasslands to the east 



