also in relatively recent times that the zonation of forest com- 

 munities took place on the western slopes of these mountains. It 

 has been suggested that the major reorganization of the Arcto- 

 Tertiary forest in California occurred on the western slopes of the 

 Sierra Nevada and that the forest communities that developed in 

 this area subsequently invaded the Coast Ranges, the mountains 

 of southern California, and the mountains of Baja California. It 

 has even been suggested that some species of conifers now con- 

 sidered typical of the North Coast Ranges (such as Coast Red- 

 wood, Port Orford Cedar, and Canoe Cedar) are probably fairly 

 recent immigrants into this area from the western slopes of the 

 Sierra Nevada, where they have subsequently become extinct. 



Many of the modern plant communities in California contain 

 vestiges of the Arcto-Tertiary geoflora. It seems probable that 

 some of these communities represent local aggregations of species 

 whose immediate ancestors occurred with the ancestors of 

 species that are present in other remnant plant communities. In 

 recent times there has been a segregation of the descendants of 

 Arcto-Tertiary tree species into smaller, more homogeneous plant 

 communities than were occupied by their forebears. Some of 

 these derivative plant communities (such as the Closed-Cone 

 Pine Forest) are not very successful in coping with contemporary 

 biotic and climatic conditions of the Pacific coast and seemingly 

 are on their way to complete disappearance as communities. 

 Other of these communities (such as most phases of the North 

 Coastal Forest) seem to be well adapted to current ecological 

 conditions and are therefore successful. 



The Neotropical Tertiary Geoflora 



At the southern edge of its range, the Arcto-Tertiary geoflora 

 merged into the Neotropical Tertiary geoflora. This second geo- 

 flora covered the southern portion of North America, although 

 its boundaries with the Arcto-Tertiary geoflora oscillated in re- 

 sponse to long-term climatic changes. In the Eocene, for example, 

 segments of the Neotropical Tertiary geoflora extended as far 

 north as Alaska, at which time the Arcto-Tertiary geoflora must 

 have been squeezed into a rather small area along the northern 



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