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 cafions 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 emoryt, 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 
wrightit, 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 Dasylirion), 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 
ALTITUDINAL 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 
