1921] Smdley: Flora of the Sierra Nevada of California 49 



a distinctly boreal flora, the genera and species identical with or allied 

 to high mountain or northern types; Bridgeport's flora, on the other 

 hand, is allied to the flora of the Great Basin and to the desert flora 

 of southern California. Again, the data show that two stations (For- 

 dyce and Summit) may have nearly similar floras and so be assigned 

 to the same life-zone, yet one (Summit) be referred to the warmer 

 division of that zone (Jeffrey Pine division), while the vegetation of 

 the other (Fordyce) be dominated by plants able to thrive with 

 slightly lower temperature ; and this in spite of the fact that the 

 station with the microthermic plants (Ahies magnifioa, Pimis Murray- 

 ana and associated species) is actually some 500 feet nearer sea level 

 than the place having less tolerant vegetation. These facts of distri- 

 bution indicate that life-zones are not to be thought of as continuous 

 belts of vegetation reached everywhere at successive levels within the 

 mountains. Life-zones are rather to be considered as areas of greater 

 or less extent with definite plant populations, which are able to grow 

 where we find them because the local climate admits the species present 

 and may exclude others. We are here considering only climate and 

 therefore, for the present, no mention is made of other factors which 

 also play a part in the distribution of plants. In mountainous regions, 

 like the northern Sierra Nevada, where the general elevation is mod- 

 erate, not all the boreal life-zones will be represented, or, if present, 

 they will be distributed irregularly and in conformity with topo- 

 graphy. The factors controlling the limits of life-zones have recently 

 been listed by Hall and Grinnell.''^ A consideration of the effects of 

 these factors will explain some of the peculiarities of the local climates 

 of the five stations studied in the preceding section. 



The effect of exposure, i.e., the location of a place with reference 

 to that sector of the environment from which the major effect of the 

 climatic factor under consideration (wind, storm track, sunlight) 

 bears upon the place, has long been known. On open plains, a place 

 can scarcely be said to have exposure since the climatic factors exert 

 their effect from all quarters with nearly equal intensity. In moun- 

 tainous regions, exposure becomes of fundamental importance. Since 

 exposure is conditioned by topography, the bolder the relief, the 

 greater will be the effect of exposure (or its reciprocal "protection"). 

 A given station may be positively exposed to one factor and negatively 

 related to another: a southwest slope in the northern hemisphere 

 receives a maximum insolation (in mountains this may not be quite 

 true because of afternoon cloudiness) and is the warmer and drier 



