this flora extended from about the latitude of San Francisco weU 

 northward into what are now arctic regions. This forest has been 

 termed the Arcto-Tertiary geoflora. "Arcto" refers to its north- 

 ern distribution; "Tertiary" refers to the time period during 

 which it flourished; and a "geoflora" is a major vegetation unit 

 that has continuity in space and time. In recent years, the concept 

 of the Arcto-Tertiary geoflora has been a subject of some dis- 

 pute; nevertheless, it does help to bring home the idea that the 

 contemporary flora of California has had a long history in time 

 and space. 



The Arcto-Tertiary geoflora contained a number of tree genera 

 that have persisted on the Pacific coast until the modern day. 

 These include such genera as Tsuga (hemlock), Thuja (Canoe or 

 Western Red Cedar), Picea (spruce), and deciduous trees such as 

 maples (Acer) and dogwoods (Comus). However, in response to 

 climatic changes that occurred over a long period of time, other 

 genera in this forest disappeared from the western part of the 

 continent, although they still persist in the eastern part of the 

 continent. These include such genera as beech (Fagus), chestnut 

 (Castanea), elm (Ulmus), sweet gum (Liquidambar), and some 

 other hardwood genera that make up the modern eastern decidu- 

 ous forest. It is interesting that the remnants of the Arcto-Ter- 

 tiary geoflora that persisted in the west are largely coniferous 

 genera - the deciduous genera mostly disappeared from this re- 

 gion — while in the eastern portion of the continent the reverse 

 was true - the conifers largely disappeared and the hardwoods 

 remained. Some genera that still occur in either the eastern or the 

 western forests of North America also survived in the Old World, 

 although there they are now represented by different species from 

 those present in North America. Other genera, such as Ginkgo 

 (the maidenhair tree) 2ind Ailanthus (the tree of heaven) disap- 

 peared completely from North America, whereas others, such as 

 Sequoia, disappeared completely from the Old World. 



Since the middle of the Miocene there have been three major 

 climatic and geological changes in the western United States that 

 have been responsible for the elimination of the old Arcto-Terti- 

 ary forest over much of the area and its replacement by other 

 plant communities: 



115 



