Axelrod: The Pliocene Verdi Flora of Western Nevada 1 15 



rarely reach 10°, as compared with the Sierran slope where extremes in the eco- 

 tone commonly fall below 10° F. in winter. It is pertinent that black cottonwood 

 is rarely found on the lower Sierran slopes in woodland country, but occurs usu- 

 ally in the pine-fir forest above woodland, where its leaves are much larger than 

 those of the mild-winter ecotype which so closely resembles the fossil P. alexanderi. 

 It is in the mountains of southern California that this small-leafed ecotype 

 ranges up the canyons to within a few hundred feet (altitudinally) of the lower 

 edge of pine-fir forest. Although the forest generally occurs at levels above 4,500 

 feet, stringers regularly extend down the deeper canyons to 4,000 feet where they 

 interfinger with woodland and chaparral. Many species that have close equiva- 

 lents in the Verdi flora occur in such areas, as shown on plates 15 and 16, which 

 are views near the lower margins of forest in the valleys of Mill Creek and the 

 Santa Ana River in the San Bernardino Mountains east of Redlands. Here the 

 forest dominants are Abies concolor (A. concoloroides) ,"' Pinus ponderosa (P. flo- 

 rissanti), and P. lambertiana (P. prelambertiana) , associated with Prunus emar- 

 ginata (P. moragensis) , Bibes roezlii (P. galeana), and Salix scouleriana (8. 

 boisiensis), all of which also live in the Verdi area. Arctostaphylos nevadensis 

 (A. verdiana) and Populus tremuloides (P. pliotremidoides) are in the nearby 

 area at higher levels. Populus trichocarpa (P. alexanderi) occurs on the stream 

 margin in the forest zone, but there its leaves are larger than those of the domi- 

 nant Verdi species, and are more lanceolate. It is only at lower levels in the oak 

 woodland and chaparral country that black cottonwood produces leaves closely 

 similar to the Verdi dominant. Quercus wislizenii (Q. wislizenoides) is abundant 

 in this region, and extends upward to mingle with forest species on exposed, south- 

 facing slopes. Salix gooddingii (S. truckeana) is also more common at lower levels, 

 but is found locally near the pine-fir forest. Three other plants having close rela- 

 tives in the Verdi flora are in the nearby region. A large stand of Pinus attenuata 

 (P. pretuberculata) is a few miles west of Mill Creek on the City Creek road in the 

 transition from chaparral to yellow pine forest, as illustrated in figure 1, plate 16. 

 Quercus engelmannii (Q. renoana) is in the foothills of the eastern San Gabriel 

 Mountains near Claremont, 25 miles from the Mill Creek area ; in the central part 

 of its range, as on the slopes of Mount Palomar, it extends up to 4,000 feet, where 

 it is at no great distance from yellow pine forest. Quercus lobata (Q. prelobata) 

 has its southern outpost 50 miles northward in the foothills of the San Gabriel 

 Mountains where most of the plants mentioned above also occur. 



Present climatic conditions near the lower margin of the yellow pine forest in 

 southern California are suggested by the meteorological data for Mount Wilson 

 and Idyllwild, as listed in table 5. It is apparent that winter temperatures are gen- 

 erally higher than those in the central Sierran area at the lower margins of yellow 

 pine forest. If we recall that forest was confined chiefly to hills surrounding the 

 Verdi basin, and that woodland was in the lowlands on the Verdi floodplain, then 

 temperature at the Verdi site of deposition must have been more mild than that 

 at Mount Wilson and Idyllwild for they are well up in the lower part of the forest 

 zone. Furthermore, at these levels Populus trichocarpa produces larger leaves than 

 the fossil P. alexanderi; the small-leafed ecotype of P. trichocarpa which most 



4 Species in parentheses are fossil species most nearly related to the living plants. 



