CORRELATION OF VEGETATION AND CLIMATE. 91 



oblongifolia and Vauquelinia californica. If we except the occurrence 

 of the former in the beds of Sabino and Ventana canons at 3,000 to 

 3,200 feet, its lowest occurrence on slopes is at 3,900 feet and its highest 

 at 5,600, a range of 1,700 feet; while Vauquelinia ranges from 3,900 to 

 5,500 feet, a vertical range of only 1,600 feet. These limits also apply 

 very nearly for Erythrina flabelliformis, Ingenhousia triloba, and several 

 shrubs and shrublets, and are only slightly exceeded by the range of 

 Quercus emoryi, which has already been stated to be 1,900 feet. 



Certain species of plants are confined to arroyos throughout their 

 vertical ranges, as are Vitis arizonica, Amorpha californica, Platanus 

 wrightii, and Juglans rupestris; or are found chiefly in arroyos, as 

 Cupressus arizonica and Acer interior. The great majority of trees, 

 shrubs, and shrublets, as well as the semi-succulents (such as Agave, 

 Yucca, Nolina, and Dasyliriori), are found on slopes and ridges in at 

 least some portions of their ranges, or are chiefly found there. The 

 oaks, the deciduous trees, and most of the shrubs may be found along 

 arroyos, or in flood-plains at elevations from 500 to 1,000 feet below 

 the level at which they become common components of the slope vege- 

 tation. The semi-succulents, like the succulents, are rarely found in 

 arroyos, although they may grow very close to them or may be found 

 in dry flood-plains. Of all species not confined to arroyos, their lowest 

 occurrences are generally to be sought on north slopes or in arroyos 

 at even lower elevations, and their highest occurrences are to be sought 

 on south or southwestern slopes or (particularly in the case of cacti) 

 on rocky ridges. At the vertical center of the distributional range of 

 these species they may be found, as a rule, on slopes of every exposure, 

 and perhaps in flood-plains as well, particularly in the case of the ever- 

 green oaks. The exceptions to the rule are Quercus oblongifolia, which 

 is commoner on south slopes than on north ones at all parts of its 

 vertical range except the very lowest, and Pinus chihuahuana, which 

 is rarely found on north slopes at any part of its range, even its lowest 

 occurrences being on south slopes or on an approximate level. 



PHYSICAL FACTORS INVOLVED IN THE DETERMINATION OF THE NORMAL 

 ALTITUD1NAL GRADIENT OF VEGETATION. 



In order to overlook for the moment all of the subsidiary influences 

 which cause local disturbance of the vegetistic gradient let us consider 

 that the southern slopes at all elevations are representative of the 

 normal altitudinal changes of vegetation, and let us then consider 

 some of the differences of physical conditions that accompany the 

 ascent from 3,000 to 9,000 feet. The differentiations of vegetation 

 which we are accustomed to designate as "due to altitude" are actually 

 due to three groups of physical factors: (a) moisture factors, (6) 

 temperature factors, (c) light factors. It has been customary to regard 

 atmospheric pressure as a negligible agency in relation to plants, but 



