populations of Achillea has the potential for responding to grad- 

 ual changes to its environment, that is, these populations have 

 not become so closely and invariably adapted to their immediate 

 habitat that they would become extinct as a result of slight 

 environmental change.) 



3. A species is widespread only if diversified into local eco- 

 logical races or ecotypes. (This generalization may not be true of 

 weedy plant species or some forest trees.) 



One practical consequence of an understanding of the genetic 

 complexity of wide-ranging plant species is that now, when 

 foresters choose seeds of forest trees to provide seedlings for re- 

 forestation of logged or burned areas, an attempt is made to 

 select seeds originating from populations growing near the area 

 to be re-seeded, or at least from a population occurring under 

 similar ecological circumstances. 



Life Zones of California 



Much of the preceding discussion has been concerned with 

 factors that affect the distribution of plant species in California. 

 Any factor that influences the distribution of a plant species will 

 also have an influence on the distribution of plant communities, 

 since a plant community is an aggregation of several plant species. 



An early attempt to describe the bio tic composition of North 

 America was made by the American biologist C. Hart Merriam in 

 the 1890's. Merriam recognized that as one ascends a mountain 

 the vegetation is stratified into horizontal zones or bands that 

 are characteristic of certain elevations. In a very general way, 

 these changes in the vegetation that are observed as one ascends 

 a mountain are similar to the latitudinal vegetational changes that 

 occur from sea level in the southern parts of North America to 

 sea level in the northern part of the continent. For example, the 

 tundra vegetation of mountain tops in the central Rockies bears 

 a general similarity to the vegetation of the arctic coast of 

 Alaska — or at least this similarity seemed to be a real one to 

 Merriam. As a consequence of these general observations, Mer- 

 riam formulated the Life Zone concept. The essence of this idea 



54 



