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University of California Publications in Geological Sciences 



be several inches more than that under which the Verdi flora lived. Similar cli- 

 matic relations have previously been reported for several Middle Pliocene floras 

 in the western United States (Axelrod, 1948). 



The temperature contrasts between the Verdi area and those which commonly 

 prevail in central California in areas where the flora finds many of its nearest 

 related species are shown in table 5. Summer temperatures are broadly similar, 

 though they are higher on the west front of the range. The major difference is in 

 winter temperature. Both the average January minimum (31° F. vs. 20° F.) and 

 the average January temperature (42.5° F. vs. 31° F.) are from 10° to 12° higher 



TABLE 5 



Comparative Climatic Data for Verdi Area and for Regions Where the 



Verdi Flora Has Most of Its Modern Relatives 



Central California 



Verdi 



Grass 

 Valley 



Placerville 



Southern California 



Mt. Wilson 



Idyll wild 



Mill Creek 



Yearly rainfall (inches) 



Average annual temperature (°F.) . 

 Summer temperature (°F.) 



Average July 



Average July maximum 



Extreme high 



Winter temperature (°F.) 



Average January 



Average January minimum 



Extreme low 



Growing season (days between 



last killing frost in spring and 



first in the fall) 



12 



48 



70 



86 



102 



31 



19 



-19 



145 



51 

 57 



75 



89 



104 



43 



34 



-1 



211 



35 

 55 



72 



91 



114 



41 



31 



-9 



182 



32 



55 



73 



83 



101 



43 

 34 



7 



176 



26 

 51 



67 

 86 

 99 



40 



27 s 



6 



151 a 



23 

 62 



77 



90 



108 



49 

 41 

 16 



247 



» These low figures are due to the station's site in a valley with cold air drainage. 



in areas where the Sierran forest mingles with oak woodland and chaparral. The 

 milder winters on the west slope are reflected in a six-week longer growing season 

 as compared with the Verdi area today. 



That winter temperature at Verdi may have been even more mild than that now 

 found on the lower west Sierran slope is suggested by the temperature require- 

 ments of the modern ecotype of Populus trichocarpa which most closely resembles 

 the dominant Verdi Cottonwood, P. alexanderi. The small-leafed ecotype of black 

 cottonwood is found chiefly in areas below the Sierran forest. As judged from 

 field and herbarium studies, it ranges through the south Coast Kanges (lower 

 Carmel River valley; Salinas Valley; Santa Ynez River; Sisquoc River) and into 

 southern California (Santa Clara Valley; Ventura River; lower Sespe Creek; 

 lower Piru Creek; canyons of Santa Monica and Santa Ana mountains; lower 

 canyons of San Gabriel and San Bernardino mountains). In these areas the mean 

 January temperature is from 45° to 52°, or about 5° to 10° higher than on the 

 west Sierran slope in the forest-woodland ecotone. Furthermore, extreme low 

 winter temperatures in its area fall in the range of from 15° to 30°, and only 



