CORRELATION OF VEGETATION AND CLIMATE. 99 



It is obvious that the importance of slope exposure Hes in the topo- 

 graphic control of the physical factors which form the environment of 

 the plants concerned. It is possible to know, on purely a priori grounds, 

 that two slopes of the same inclination, which lie in opposed positions 

 so that one faces north and the other south, will present to plants two 

 environments differing in almost every essential physical feature. The 

 temperature of the air on two such slopes might be identical as deter- 

 mined by the thermometers of a carefully established meteorological 

 station, but they are distinctly different as they affect the vegetation, 

 for the plants not only receive the direct rays of the sun but receive 

 very different amounts of heat through diurnal terrestrial radiation. 

 This circumstance is of small importance to full-grown trees and large 

 plants, but is of great importance to young plants and seedlings. The 

 soil temperatures of opposed slopes are also widely unlike, even in the 

 presence of the undisturbed cover of natural vegetation. The two 

 opposed slopes would in all likelihood receive the same rainfall, al- 

 though this is not necessarily the case. An equal amount of rain might 

 effect an equal elevation of the soil moisture on the two slopes, and to 

 the same depth, but the soil evaporation of the south slope would 

 greatly exceed that of the north slope, and a lower moisture would soon 

 prevail in the soil of the former. Greater or less differences may thus 

 be shown to obtain between the opposed slopes with respect to the 

 most vital features of plant environment. Any attempt to explain the 

 importance of slope exposure in determining plant distribution is there- 

 fore incomplete unless it takes into account every possible environ- 

 mental difference between the slopes. Some of these dift"erences are 

 undoubtedly of far greater importance than others, but the question 

 of their relative importance is always one that must be asked with 

 respect to a particular species of plant. To make a thoroughgoing 

 answer as to the importance of slope exposure for a single species is in 

 itself a very great undertaking. 



The universal occurrence of a large number of species of plants in 

 the vegetation of the Santa Catalinas, their commonness within their 

 ranges, and the consistency of their distribution with respect to slope 

 exposure, all indicate that there has been ample time in the history of 

 the mountain for all of these species to attain as wide a distribution as 

 it is possible for them to have under existing climatic conditions. It 

 is difficult to conceive of any upward or downward movement being 

 possible for any of the common species of plants, inasmuch as thousands 

 of years have already given an opportunity for such extensions of 

 range. In view of the steep climatic gradient of the mountain it is 

 easy to believe that all of the common species have reached upper and 

 lower limits beyond which their survival is prevented by definite fea- 

 tures of the physical environment. The present vertical hmit of a 

 species, whether upper or lower, must be looked upon as the average 



