112 University of California Publications in Geological Sciences 



arid Truckee River valley, where there is a permanent supply of water through 

 the year, and down which cold air drains at night to moderate the high summer 

 temperature of the valley floor. In the bordering hills south of the locality, these 

 hardier Sierran forest species are associated with Abies concolor, Arctostaphylos 

 nevadensis, Pinus lambertiana, and Populus tremuloides, all of which have close 

 relatives in the Verdi flora. 



As shown in table 4, the modern representative species of the Sierra-Cascade 

 Component which live in the Verdi area occur also in the region west of the Sierra 

 Nevada, where they are associated with plants that have fossil relatives in the 

 Verdi flora, but which no longer occur to the east of the range. Clearly, the present 

 environment in the Verdi area differs somewhat from that which prevailed there 

 when the flora was living. Significantly, the dominant species of the flora, Populus 

 alexanderi, has leaves that are much smaller than those produced by its nearest 

 relative (P. trichocarpa) which dominates stream banks in the Verdi area today, 

 as may be seen from a comparison of the figures on plate 19 with those on plate 20. 

 They also differ from them in shape, being typically oval to suborbicular, rather 

 than ovate-lanceolate. It is only in the warmer, subhumid sections of the south 

 Coast Ranges of California and in southern California that P. trichocarpa produces 

 leaves that are closely similar in size and in shape to those of the dominant Verdi 

 cottonwood, as shown on plates 19 and 21. In these warmer areas, the smaller-leafed 

 ecotype ranges from oak woodland and chaparral country of the lowlands up to 

 near the margins of yellow pine forest. Since the small leaves of P. alexanderi 

 dominate the Verdi flora, it is evident that the Sierra Cascade Component was 

 living under conditions more nearly like those now found near the lower margin 

 of yellow pine forest west of the Sierran barrier where black cottonwood produces 

 small, ovate leaves, similar to the fossil P. alexanderi. 



Five of the six modern equivalent Verdi species that are chiefly members of 

 woodland and chaparral vegetation live on the lower west slopes of the Sierra 

 Nevada together with the Sierran forest species that have close Verdi relatives. 

 These woodland plants include Quercus wislizenii (wislizenoidesY and Q. lobata 

 (prelobata) , which form a mixed woodland and savanna that interfingers with the 

 lower margins of the Sierran forest ; by contrast, on the colder, east flank of the 

 range this ecologic position is occupied by the pinon-juniper woodland. Although 

 Quercus engelmannii (renoana) is not represented in the Sierran foothills, it has 

 an ecologic counterpart there in Q. douglasii, which is found commonly with Q. 

 wislizenii and Q. lobata. These oaks regularly mingle with yellow pine, white fir, 

 sugar pine, willow, and gooseberry, which are similar to fossil species comprising 

 the Sierra-Cascade Component of the Verdi flora. Plate 14 shows two areas near 

 Placerville and Jackson where these oak woodland and forest species live in close 

 proximity. Chaparral is common in the region, and regularly interrupts the 

 continuity of woodland and forest. In these areas Ceanothus cuneatus (precunea- 

 tus) is abundant, and black willow, Salix gooddingii (truckeana), is common on 

 stream banks. Pinus attenuata (pretuberculata) is scattered discontinuously in 

 the region, forming relatively pure stands on rocky sites in the forest-woodland 

 ecotone. 



2 Species in parentheses are fossil plants similar to the living species. 



