140 FORREST SHREVE 



by lower temperatures, and by a higher soil moisture at the out- 

 set of the dry seasons. The low humidities of the desert are to 

 be found with only slight ameUoration at the highest mountain 

 altitudes, and the differences of total wind movements and of 

 the character and intensity of insolation are nearly identical. 

 There are many respects, therefore, in which the controlling 

 physical conditions of the desert are carried with little change 

 to the mountain tops. The forest of the desert mountain does 

 extremely little to make its own climate except in so far as it 

 affords a shade for the herbaceous plants of the closed stands of 

 fir and spruce. In the pine forest of the Santa Catalinas, and 

 to some extent in the fir forest, the herbaceous plants are of a 

 notably xerophilous stamp and with the exception of a very few 

 annuals they are characterized by deep-seated root systems. 

 The lowest stratum of the pine forest is subjected to all of the 

 adverse conditions of water supply and water loss that from 

 time to time affect the trees themselves. The stabilization of 

 climate that is effected by a heavy rain-forest in a moist climate 

 is totally lacking in a desert mountain forest. 



An attempt to segregate the various plant communities that 

 make up the collective vegetation of the two mountains under 

 consideration will reveal the fact that in each of them the topo- 

 graphic relief is the first-hand basis upon which such segregation 

 can be most naturally made. In the Santa Catalinas and in 

 the Blue Mountains it is the valley-bottoms, the slopes and the 

 ridges which present the most striking vegetational differences. 

 The vegetational differences correlated with topography are 

 much more striking in the Santa Catalinas than in the Blue 

 Mountains, but this is largely due to the altitudinal interdigita- 

 tion of the desert, encinal and forest on the Santa Catalinas, 

 as contrasted with a mere difference of composition and stature 

 in a forest of uniform general type in the Blue Mountains. 



The great irregularity of the precipitation in Arizona registers 

 its most important effect upon the vegetation in causing great 

 annual fluctuations in the evaporative power of the air and in 

 the moisture of the soil. In Jamaica the soil moisture is uni- 

 formly and almost constantly high, so that it fails to operate 



