1921] Smiley: Flora of the Sierra Nevada of California 33 



Considering that part of the year within which the mean monthly 

 temperature is above freezing, we see that it is of unequal length : 



Tahoe, March to November inclusive. 

 Fordyce, April to November inclusive. 

 Bridgeport, April to November inclusive. 

 Summit, April to November inclusive. 

 Tamarack, April to October inclusive. 



But the vegetative season is more limited, especially for low shrubs 

 and herbs ; in spring, limited by the disappearance of the snow-cover ; 

 in fall, by the general fall in temperature combined with the scant 

 water supply that, as a rule, then exists in the Sierra. "With regard to 

 the disappearance of the snow-cover, the data show that at Tahoe, in 

 two years of the quadrennium, the ground was already bare by the 

 end of May and in the other two years (1915, 1917) seven inches and 

 one inch lay on the ground; by the first week in June "spring" is 

 well advanced at Tahoe and vegetation has resumed active growth. 

 In this same period (1914—17), the end of May found on the average 

 12.75 and 10.25 inches of snow on the ground at Fordyce and Svimmit, 

 respectively; a month later the ground is practically bare.*^ At 

 Tamarack also the end of June finds the winter's snow about to dis- 

 appear and active growth initiated. It appears that the station at 

 Tahoe, near the upper edge of the Transition life-zone, enjoys a 

 vegetative period approximately a month longer and that this exten- 

 sion comes when the conditions for plant growth are best : abundant 

 moisture and most daj'light. In the higher mountains, the brevity of 

 the period of growth is, in part, made up by the higher temperature 

 which prevails when growth is resumed — in June, the mean tempera- 

 ture of Tahoe is 50.6° ; in July, at Tamarack the mean temperature 

 is 55.5° — resulting in an acceleration of the life processes in the higher 

 mountains. It is this acceleration which causes the boreal vegetation 

 to pass from a dormant condition to the state of active growth so 

 rapidl}", changing the aspect of the high mountain region with abrupt- 

 ness often astonishing to the visitor. Just as it is necessary to bear in 

 mind that the climate of the Sierra is a composite, made up of many 

 local varieties of the general climate of the range, so upon smaller 

 areas defined by topographic details, the resident plant populations 

 are subject to more or less peculiar very local climates determined by 

 inequalities of slope and exposure, the distribution of the plant com- 

 munities being correspondingly diversified. 



