THE PHYSICAL CONTROL OF VEGETATION IN 

 RAIN-FOREST AND DESERT MOUNTAINS 



FORREST SHREVE 



The Desert Laboratory, Tucson, Arizona 



It is possible to compare the physical conditions of two widely 

 separated localities with respect to the influence which these 

 conditions exert upon their respective vegetations without at 

 the same time entailing any comparison of the vegetations them- 

 selves. It is the aim of this paper to bring out some of the 

 contrasts between the manner in which vegetation is controlled 

 by the conditions in the humid mountains of a tropical island 

 and in the arid mountains of a temperate continental region. 

 The details upon which these generalizations are based are to 

 be found in the author's publications on the montane rain- 

 forests of Jamaica' and on the Santa Catalina Mountains of 

 Arizona. 2 



The point of view of this paper may be indicated by calling 

 attention to the fact that two mountain ranges may differ ut- 

 terly in flora, and may differ very greatly in vegetation at the 

 same time that the controlling environmental factors are iden- 

 tical in the two. For example, there is a sharp dissimilarity 

 between the San Bernardino Mountains of southern California, 

 and the Santa Catalina Mountains. The latter have only 2% of 

 the species found in the former. The latter have desert and 

 evergreen oak scrub (encinal) where the former have chaparral 

 and nut pine scrub. In spite of these floristic and vegetational 

 differences, however, there is a very close agreement between 

 the relative importance of the various physical factors in these 



^ Shreve, Forrest, A Montane Rain-Forest: A Contribution to the Physiologi- 

 cal Plant Geography of Jamaica. Carnegie Inst. Wash. Publ. 199, 1914. 



2 Shreve, Forrest, The Vegetation of a Desert Mountain Range as Conditioned 

 by Climatic Factors. Carnegie Inst. Wash. Publ. 217, 1915. 



135 



THE PLANT WORLD, VOL. 20, NO. 5 

 MAT, 1917 



