1 . Since Tertiary times winter rainfall has become reduced and 

 summer rainfall has essentially disappeared over much of Cali- 

 fornia. 



2. Starting in the Pliocene, the Sierra Nevada and the Cascade 

 Range were uplifted. The result of this major geological event 

 was the formation of a rain shadow to the east of these mountains 

 and the appearance of new upland areas for plant colonization, 

 particularly on the western slopes. Furthermore, the western 

 slopes of these mountains are now comparatively well watered 

 with rain and snow. 



3. During Pleistocene times, much of northern North America 

 was covered with glacial ice sheets. In the west, these barely ex- 

 tended southward across what is now the U.S.-Canadian border, 

 but because of the cooler climatic conditions that prevailed on 

 the continent during this time, many of the upper reaches of the 

 Sierra Nevada in California were covered with glaciers of various 

 sizes. In addition, there was a general cooling trend over much of 

 western North America. 



The biological consequences of these geological and climatic 

 features are complex. The development of a cooler climate and 

 the action of the glaciers in mountainous areas clearly had their 

 effect in eliminating certain species from CaHfornia and in 

 altering the distribution ranges of other species. As an example. 

 Sierra Big Tree (Sequoiadendron giganteum) now occurs only in 

 areas of the western slopes of the Sierra Nevada that were not 

 devastated by the action of glaciers. The reduction in total annual 

 rainfall to the east of the Sierra Nevada eliminated the forest 

 climate and replaced it with a scrubland or grassland climate. The 

 rich Arcto-Tertiary forest was rapidly eliminated from much of 

 the Great Basin because of this climatic change. However, the 

 relatively conservative and moist climate that remained along cer- 

 tain areas of the western slopes of the Sierra Nevada provided 

 pockets in which some of the Arcto-Tertiary forest species took 

 refuge. It was from these pockets that a recolonization of the 

 Sierra Nevada took place following the retreat of the last Pleisto- 

 cene glaciers and the subsequent warming of the cUmate. It was 



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