Axelrod: The Pliocene Verdi Flora of Western Nevada 113 



Populus tremuloides (P. pliotremuloides) is the only Sierran forest species hav- 

 ing an equivalent in the Verdi flora that does not range down into oak woodland 

 country today. In recent years, however, it has been found at no great distance 

 from oak woodland. The occurrence of aspen on the north rim of Hetch Hetchy 

 valley where it is within half a mile of digger pine, live oak, toyon, buckbrush, and 

 their regular associates has been noted (Axelrod, 1944&, p. 153). More recently, 

 it has been observed in the middle part of the Sierran forest mixed in with black 

 oak, incense cedar, Douglas fir, Pacific dogwood, big-leaf maple, yellow pine, sugar 

 pine, and other common Sierran species.' A mile southward, across a low ridge, live 

 oaks and chaparral occur in abundance. During the Pliocene, aspen has been, re- 

 corded with redwood forest, oak woodland, chaparral, and desert-border vegeta- 

 tion. On the basis of such present and past occurrences, it has been suggested that 

 P. pliotremuloides was represented by one or more extinct ecotypes which extended 

 its range well into mild-winter regions where it lived with communities with which 

 its modern counterpart is no longer found (Axelrod, 1941). 



The climatic contrast between the Verdi area today, and that on the west slope 

 of the Sierra Nevada where 14 of the 16 (87 per cent) woody Verdi species find 

 their nearest relatives living in close association, is suggested by the data in table 5. 

 The precipitation figure is somewhat misleading, however, because rainfall at the 

 lower margin of the Sierran forest varies greatly. In areas north of Placerville it 

 exceeds 50 inches in the zone of overlap between forest and woodland. To south- 

 ward, the ecotone occurs in areas where rainfall is 25 inches, and in favorable 

 topographic situations tongues of forest often extend down the valleys into wood- 

 land country where rainfall is only 20 inches. In evaluating modern environments 

 in terms of Verdi precipitation, the small-leafed ecotype of Populus trichocarpa, 

 which is the nearest analogue of the dominant Verdi P. alexanderi, probably pro- 

 vides the best indication of rainfall. At one extreme, as in the south Coast Ranges 

 and in southern California, it occurs in mild-winter, lowland areas far removed 

 from forest where rainfall varies from 15 to 18 inches yearly. At the other, it 

 ranges up to near the lower margin of yellow pine forest where precipitation is 

 approximately 25 inches. In areas of higher rainfall, which is correlated with in- 

 creased altitude and lower temperature, the small-leafed ecotype is replaced by 

 the larger-leafed form of P. trichocarpa which is not as closely related to the fossil 

 P. alexanderi. It would appear, therefore, that at a maximum, rainfall at the lower 

 edge of forest in the Verdi area probably was not in excess of 25 inches. Since the 

 Verdi forest appears to have lived chiefly in valleys flanking the floodplain where 

 the flora accumulated, it is concluded that precipitation over the Verdi basin 

 averaged 18 to 20 inches. This estimate for the Verdi site of deposition is approxi- 

 mately 5 inches more than that at the Verdi locality today. The decrease can be 

 ascribed chiefly to the rain-shadow cast by the mountainous region to the west 

 which was elevated after the Coal Valley formation was deposited. When the topo- 

 graphic factor is taken into account, there appears to have been no significant 

 difference between total rainfall in the Verdi area and that which might otherwise 

 now be expected in the area. In fact, if the regional topography during Verdi 

 deposition could be restored, it seems probable that precipitation there would now 



s These aspen groves are on State Highway 20 just south of Lake Spaulding at an altitude of 

 5,200 feet. 



