34 University of California Publications. [botany 



The Factors Affecting Plant Distribution on San Jacinto 



Mountain. 



altitude, temperature, and moisture. 



The most potent factor affecting- the distribution of plants on 

 San Jacinto Mountain is altitude. This may be said of all high 

 mountains, since altitude exerts a direct influence on the tem- 

 perature and moisture conditions of any region. On ascending 

 a mountain which rises from an arid plain, as does the one 

 under consideration, we should therefore expect to find a gradual 

 change from a type of vegetation adapted to the hot, dry condi- 

 tions of the lowlands to one suited to the cold, snow-covered 

 slopes of the higher peaks. Thus we might expect to see the 

 chaparral of the lower slopes giving way, at a uniform altitude 

 on all parts of the mountain, to forests of Black Oak, Yellow 

 Pine, and other species of both plants and animals ordinarily 

 associated with these. At a definite altitude this belt would 

 merge into one in which the White Fir would he the dominant 

 tree, and this again into a belt of Murra}' Piue, and so on until 

 the summit were reached. 



Something like this is what we should expect to find on a 

 mountain existing under ideal conditions. But other factors 

 come in to mar this orderh^ succession of biological zones and, 

 since the seemingly promiscuous distribution of plants on San 

 Jacinto Mountain is due to these factors, they will be more fully 

 treated in the following pages ; while the more important factors 

 of altitude, temperature, and moisture will not be further 

 discussed, since their influence is more generally understood and 

 since they usually act in the same direction. 



SLOPE exposure. 



As is well known the inclination of a slope to the sun's rays 

 during the warmest parts of the day may modify largely the 

 amount of heat received and consequently the nature of the veg- 

 etation. As a result of this the life zones are forced up on south 

 and west exposures while they dip down on north and east slopes. 

 Some idea of the extent to which slope ex]30siire may affect 

 the temperature of a surface may be had by calculating the ratio 

 between the number of heat rays received by surfaces equal in 



