336 BRAUN 



are subject to (see maps by Thornthwaite, 1941). It became apparent that 

 here was the center of best development (not center of dispersal) of mixed 

 mesophytic forest. Extensive travel disclosed the prevalence of mixed meso- 

 phytic forest communities over a considerable geographic area, which I later 

 designated as the Mixed Mesophytic Forest region. Beyond this region — in 

 any direction — such communities are more limited in extent. Other climax 

 types prevail. On the basis of prevalence of a particular climax type (and 

 prominence of other community types) other forest regions were designated 

 (Braun, 1950). 



The prevalence of any particular climax type may be due to the control 

 of climate ; it may be related to the history of erosion cycles and past climates ; 

 it may be due to state of development of climax communities ; or, more likely, 

 to a combination of all. 



Prevalence of a climax type should not be construed as uniformity of 

 vegetation. Great diversity of vegetation is a characteristic of all areas of 

 diverse topography. And each area may have its own peculiar vegetation 

 types as well as some common to other areas of the region. In the intensive 

 study of any local area, all community types should be distinguished, and the 

 status (climax, developmental, primary, secondary) be determined. In an 

 extensive approach, the need for classification is as evident as is the need 

 for classifying species into genera and families — the number of community 

 types would doubtless be in the thousands. 



The scheme of classification most used by American ecologists in con- 

 nection with the Eastern Deciduous Forest is division into what have been 

 called climax associations. For many years a threefold concept prevailed 

 which recognized three climax associations: beech-maple, oak-chestnut, and 

 oak-hickory (Clements, 1916, 1928). In 1916, a fourth climax, mixed meso- 

 phytic, was proposed for a mesic climax type with several dominants, among 

 which beech was most important (Braun, 1916). Expansion of the concept 

 became necessary, for beech is not always an important species, in fact is 

 not always present. Two species, Tilia hcterophylla and Aesculus octandra, 

 are the most characteristic species of the mixed mesophytic association 

 (Braun, 1947, 1950). This does not mean that other species cannot be 

 thought of as characteristically present in mixed mesophytic communities, but 

 that these two are essentially confined to such communities and do not occur 

 in any other climax association than mixed mesophytic. 



If the four major climax associations mentioned above are recognized, 

 then a geographic region in which the mixed mesophytic association prevails 

 can be distinguished as the Mixed Mesophytic Forest region; this occupies 

 a central position. A region in which an oak-chestnut association prevails 

 occupies a large area bordering it to the east and southeast and is called 

 the Oak-Chestnut Forest region. Farther west, centering in the Ozarks, is the 

 Oak-Hickory Forest region. The Beech-Maple Forest region lies to the north; 



