CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 247 



and in the mountains and becomes decreasingly favorable ap- 

 proaching the transition to prairie. 



The southern Appalachians represent the oldest exposed land 

 surface in the region. Here the deciduous forest is more complex 

 than in any other part. Practically all of the species found else- 

 where in the deciduous forest are represented, as well as several 



FlG. 119. Sugar maple-basswood forest, illustrating the climax for much 

 of southern Wisconsin and Minnesota— U. S. Forest Service. 



others. Numerous endemics occur as associates. Most of the trees 

 also attain their greatest size here. Away from the mountains, the 

 number of species declines, and habitat requirements become of in- 

 creasing importance. It is believed that a forest similar to the pres- 

 ent one has existed here since Tertiary time. Such evidence is 

 taken to mean that the southern Appalachians are a center of origin 

 for much of the widespread deciduous forest. The distribution and 

 nature of the several associations of the formation give additional 

 supporting evidence. In general, with increasing distance from 

 the center, the associations are made up of fewer species and yet 

 all are bound together or interrelated by several species that range 

 throughout. 



Mixed Mesophytic Forest Association.-Thtonghoxit the Ap- 



