44 University of California Publications in Botany [Vol. 9 



LIFE-ZONES OF THE SIERRA NEVADA 



Though it is not proposed in this paper to consider the different 

 aspects or types of vegetation present in the high mountain region, 

 it seems desirable to define the several life-zones to which plants are 

 assigned in the following annotated list.* 



LIFE-ZONES PEESENT IN THE SIEEEA NEVADA 



Perhaps no single phase of his problem is more perplexing to the 

 student of plant distribution than that of attempting to delimit accu- 

 rately the life-zones of the region the flora of which he would describe 

 and compare with the floras of other regions that have contributed 

 elements to and received immigrants from his own. No single student 

 possesses the minute knowledge of the whole field roughly designated 

 the Cordilleran section of North America, which would make him com- 

 petent to survey this immense extent of territory. Even within much 

 narrower limits, when attempt is made to particularize and precisely 

 define zonal boundaries, the investigator is compelled to rely in no 

 small part upon the results achieved by workers outside his own field 

 of observation. By very general agreement among western students of 

 both animal and plant distribution, the basis for current work starts 

 with the system of life-zones formulated some twenty-five years ago 

 by Merriam*^ and corrected in the subsequent period by its author 

 and his co-workers on the Biological Survey. 



The fundamental postulate of Merriam's system of life-zones is 

 the assumption that for each species there exists certain fixed tem- 

 perature limits, which inhibit the spread of the species into other 

 regions where those temperature summations do not obtain. In the 

 Cordilleran section, with its diversified topography and profound 

 differences of altitude and therefore of climate, the limits of the life- 

 zones are subject to many controlling factoi*s whose importance and 

 significance must be understood before assignment of particular plants 

 to definite life-zones may be undertaken. The data underlying the 



* The ecologic types present in the higher Sierra were investigated and the 

 results in part published in a preliminary paper on the Tahoe region, which may 

 be regarded as a typical cross-section of the range, and to that paper reference 

 is here made concerning the general features of the high mountain vegetation.39 



