DEVELOPMENT OF ASSOCIATION AND CLIMAX CONCEPTS 337 



here a beech-maple or maple-beech climax prevails. In addition to these four 

 climaxes, a maple-basswood is now generally recognized (Costing, 1948; 

 Braun, 1950). 



The extent of these regions, and I believe the prevalence of the climaxes 

 which characterize them, is a result of past history as well as present climate. 

 That is why I like to think of these climaxes as regional rather than climatic. 

 Evidence points to mid-Tertiary occupancy of much of the eastern half of 

 the United States by mixed forest. Base leveling and climatic change cur- 

 tailed its extent, leaving remnants, relics, in favorable sites, and giving advan- 

 tage to less mesic types — just as moisture and slope differences in the Cum- 

 berland Mountains cause segregation into types. Glaciation further curtailed 

 the mixed forest. Post-Wisconsin migrations have repopulated the glaciated 

 area, including that part here designated as the Beech-Maple region. Here, 

 more than anywhere else, the importance of succession is evident. Here also 

 the question arises as to the permanence of this classic association. There is 

 evidence that where dissection is beginning, beech-maple dominance is being 

 broken by the entrance of additional mesic species (Braun, 1950, pp. 523, 

 527). Again, question is raised as to the climatic character of this regional 

 climax. 



The associations here distinguished are not concrete pieces of vegetation — 

 they are abstract concepts to which communities and community types can be 

 assigned. The species (or species populations) which make up these com- 

 munities seldom have narrow habitat limits; mostly they are complex species 

 or species made up of a number of ecotypes and hence may range through a 

 number of different community types. A species which in one place may 

 indicate subxeric conditions, in another may grow in hydromesic sites; hence 

 conclusions reached in one intensive study cannot be carried over into an- 

 other. Considering the four major associations, two are mesic, two submesic 

 to subxeric. The area where the Mixed Mesophytic climax prevails is sepa- 

 rated from the area where the Oak-Hickory climax prevails by a broad belt 

 which I call the Western Mesophytic Forest region, characterized by a mosaic 

 of climaxes including rich mixed mesophytic communities, various less mesic 

 types, as beech-chestnut, white oak-black oak-tuliptree, and oak-hickory (of 

 varying composition), pine communities, cedar barrens, and prairies. No 

 existing gradient can account for the distribution of this motley assortment. 

 Late Tertiary, Pleistocene, and post-Pleistocene climates, topography, and 

 migrations must be considered. The distribution of mixed mesophytic com- 

 munities in this region cannot be explained in relation to precipitation alone; 

 rather, they appear to have persisted (with changes of course) from the more 

 widespread mixed Tertiary forest, to have persisted where topography has 

 been continuously favorable, and can and did, in drier times, partly com- 

 pensate for reduced precipitation. 



To summarize — I think of the Eastern Deciduous Forest (ignoring sec- 



