I. CONCEPT AND CAUSES OF SUCCESSION. 



The formation an organism. — The developmental study of vegetation neces- 

 sarily rests upon the assumption that the unit or climax formation is an 

 organic entity (Research Methods, 199). As an organism the formation 

 arises, grows, matures, and dies. Its response to the habitat is shown in 

 processes or functions and in structures which are the record as well as the 

 result of these functions. Furthermore, each cUmax formation is able to 

 reproduce itself, repeating with essential fidelity the stages of its development. 

 The life-history of a formation is a complex but definite process, comparable 

 in its chief features with the life-history of an individual plant. 



Universal occurrence of succession. — Succession is the universal process of 

 formation development. It has occurred again and again in the history of 

 every climax formation, and must recur whenever proper conditions arise. 

 No climax area lacks frequent evidence of succession, and the greater number 

 present it in bewildering abundance. The evidence is most obvious in active 

 physiographic areas, dunes, strands, lakes, flood-plains, bad lands, etc., and 

 in areas disturbed by man. But the most stable association is never in com- 

 plete equilibrium, nor is it free from disturbed areas in which secondary succes- 

 sion is evident. An outcrop of rock, a projecting boulder, a change in soil or 

 in exposure, an increase or decrease in the water-content or the light intensity, 

 a rabbit-burrow, an ant-heap, the furrow of a plow, or the tracks worn by 

 wheels, all these and many others initiate successions, often short and minute, 

 but always significant. Even where the final community seems most homo- 

 geneous and its factors uniform, quantitative study by quadrat and instru- 

 ment reveals a swing of population and a variation in the controlling factors. 

 Invisible as these are to the ordinary observer, they are often very consider- 

 able, and in all cases are essentially materials for the study of succession. In 

 consequence, a floristic or physiognomic study of an association, especially in 

 a restricted area, can furnish no trustworthy conclusions as to the prevalence 

 of succession. The latter can be determined only by investigation which is 

 intensive in method and extensive in scope. 



Viewpoints of succession. — ^A complete understanding of succession is pos- 

 sible only from the consideration of various viewpoints. Its most striking 

 feature lies in the movement of populations, the waves of invasion, which rise 

 and fall through the habitat from initiation to climax. These are marked by 

 a corresponding progression of vegetation forms or phyads, from lichens and 

 mosses to the final trees. On the physical side, the fundamental view is that 

 which deals with the forces which initiate succession and the reactions which 

 maintain it. This leads to the consideration of the responsive processes or 

 functions which characterize the development, and the resulting structures, 

 communities, zones, alternes, and layers. Finally, all of these viewpoints are 

 summed up in that which regards succession as the growth or development 



