50 University of California Puhlications in Botany [Vol.9 



in so far as maximum insolation can control the local temperature and 

 soil moisture;"'- on the other hand, such a slope might be protected 

 from the effect of the prevailing wind if it blew from the northeast. 

 In the Sierra a further complication follows from the relation of the 

 position of the range across the storm tracks. In the Sierra, the south- 

 west slope is the best insolated but also receives the full effect of the 

 prevailing winds, including storm winds which accompany precipita- 

 tion. Topography reacts upon climate to promote air movements ; an 

 important consequence is the settling of cold air into valleys ; this 

 so-called ' ' air-drainage ' ' operates to cause valleys to show lower tem- 

 peratures than the surrounding ridges. Summit throughout the year 

 is warmer than Fordj^ce, owing, in part, to the fact that the cold air 

 drains down the valley of Fordyce Creek from the flanks of Mt. Lola 

 and Castle Peak, while from the higher station the cold air drains 

 away. This has been recognized as a condition modifying vegetative 

 limits.*"^' ^^ 



Inspection of the temperature table shows that Summit has an 

 average daily range of temperature less than that of any other station ; 

 this small daily range is in accordance with the law of Woeikoff :^* 

 "die Luft iiber einer konvexen Oberflache warmer ist in der Nacht 

 und im Winter und kalter am Tage und im Sommer, dass also die 

 jahrliche wie die tagliche Amplitude kleiner ist, als iiber einer kon- 

 kaven Oberflache. ' ' From this law we see that vegetation growing in 

 montane valleys has an advantage in the matter of receiving new 

 immigrants over a plant population growing on a ridge of equal 

 elevation; the range of temperature is greater in the valleys and the 

 sum total of temperature is larger in valleys than on crest-lines, per- 

 mitting stenothermic plants, with temperature controls near the limits, 

 to gain a foothold in the valleys when they would be excluded from 

 the ridges. Brockmann-Jeroscli^^ has called attention to the impor- 

 tance of possible extremes of temperature in permitting the spread 

 of plants: "Der Temperaturverlauf und nicht eine Durchschnitts- 

 temperatur bedingt, soweit Warme in Betracht kommt, die klimatische 

 Grenze einer Art." 



Attention has been called to the fact that the high mountain region 

 of the Sierra is a lake region with bodies of water of all sizes from a 

 large lake like Tahoe, many square miles in area, to mere pools. The 

 influence of these bodies of water in modifying zonal limits is recog- 

 nized by the plant collector, who commonly finds near their shores 

 species which have their zones of greatest frequency at liiglier levels 



