32 University of California Publications in Botany [Vol.9 



The local climates so far considered have but one major variable, 

 temperature. At all four stations, Tahoe, Fordyce, Summit, and 

 Tamarack, the total precipitation is abundant though varying within 

 rather wide limits. This large annual rainfall (or snowfall) with 

 accompanying cloudiness reacts upon temperature, reducing 'the 

 extremes. Wlien, however, we study the local climate of Bridgeport, 

 the always deficient and irregular rainfall is attended by the greatest 

 temperature ranges, both daily and monthly, found within the series. 

 When the rainfall (snowfall) at Bridgeport sharply declines by the 

 end of February, the thin snow-cover is entirely melted in March and 

 relatively high temperature immediately follows in this mountain 

 valley of the eastern slope (to a less degree, the same thing occurs in 

 the valley of the Truckee at Tahoe, also on the east slope). Bridge- 

 port during the next four months (April- July) is the warmest station 

 but it will be observed by study of the temperature data throughout 

 this period when its mean is the highest, low temperatures are con- 

 stantly recorded (6 to 20 degrees of frost). The vegetation at Bridge- 

 port and on the east slope generally must adjust itself to two sets of 

 extremes: temperature and precipitation. It is this necessity for a 

 twofold accommodation which explains the poverty of the east slope 

 flora at an elevation which, on the more favored western side, supports 

 a plant population both floristically and ecologically^ more highly 

 diversified. 



We have so far mainly considered the mean temperatures of this 

 series of stations, but localized plant populations are believed to be 

 more directly infiuenced by temperature extremes ; at least it has been 

 known for a long time that the several functions of the single plant 

 have different temperature ranges and that a station may be subject 

 to such a temperature range that a given species may be excluded, 

 or, if admitted, one or more of its functions impeded or prevented. 

 Unfortunately, within our region little has yet been done to exactly 

 determine the effects upon the plants of the temperature extremes 

 known to occur but a study of the possible temperatures at a given 

 station with a definite plant population may be suggestive. In this 

 connection it should be remembered that temperature ranges have 

 very unequal effects upon plant life accordingly as they include or 

 exclude the freezing point. Wliile many boreal plants at the height 

 of the growing season can withstand freezing and thawing, other 

 kinds are not so tolerant and may not survive such change. 



