330 BRAUN 



stated (1926), "a. periodic inspection of foundations is most desirable." Con- 

 cepts are bound to change with progress from local or intensive study to 

 broad and extensive study. This is the reason for change with each individual 

 worker; it has in part been the reason for change in concepts through the 

 past 50 to 60 years of study of vegetation. Geographic location of studies 

 also is a factor in the development of concepts. 



Emphasis of the concepts of association, succession, and climax dates from 

 the work of Henry C. Cowles around the beginning of the century (1899, 

 1901). Communities were recognized and at first called plant societies; later, 

 plant associations. These were seen to be in equilibrium, more or less, with 

 habitat ; however, habitats change through the years, forcing change in vegeta- 

 tion. Changes were recognized as biotic, topographic, and climatic — the biotic 

 due to reactions of the vegetation (and other accompanying life), the topo- 

 graphic to erosional and depositional forces, the climatic to the long-range 

 variation in climate (Cowles, 1911; Clements, 1916, 1928). All groups of 

 forces are at work everywhere, but locally one or another group may seem 

 to determine the nature of vegetational change; all have been operative in 

 determining regional vegetation. 



Awareness of change led to the succession concept, for decades the guiding 

 principle in ecological study and, I believe, the most fundamental of all con- 

 cepts relating to the study of vegetation. 



Succession is vegetational change — the gradual replacement of populations 

 of species by other populations, i.e., of one community by another. The re- 

 sulting plant succession (or sere) is not a series of steps or stages — no serious 

 student of succession (a process) has ever claimed that a succession is made 

 up of "discrete units." However, within any complete or partial sere there 

 may be recognizable communities, communities sufficiently distinct from one 

 another to justify naming them, usually for their dominants. Succession dia- 

 grams naming such "stages" place lines or sometimes arrows leading from 

 one to another, which are intended to indicate gradual change. The whole 

 sere is a continuously but gradually changing complex — the changes forced 

 by biotic, topographic, or climatic factors. It is dynamic. It is a sequence 

 in time, not in space, although spatial relations are often indicative of suc- 

 cession. Given time enough, the process of succession may lead to the estab- 

 lishment of a climax community, a community in which further change awaits 

 major environmental change. It must be recognized that disruptive forces 

 may prevent such progress, that although vegetational change is taking place, 

 successions may be fragmentary and hardly recognizable. 



Let us now apply these ideas to a concrete bit of forest vegetation — as the 

 young student or the local worker must do. I select for this the primary 

 forests of the Illinoian Till Plain of southwestern Ohio (Braun, 1936). Re- 

 connaissance observation reveals a number of communities, among them those 

 dominated by pin oak, by white oak, by beech. Each has certain character- 



