12 MISC. PUBLICATION 303, U. S. DEPT. OF AGRICULTURE 



relation between soil, vegetation, and climate has long been known. 

 Similarly, the accumulation of debris causing submerged water plants 

 eventually to give way before the advancing shore plants, and these 

 in turn before the successful competition of swamp forests and later 

 dry land forests, has been studied in considerable detail. 



SECONDARY SUCCESSION AND PIONEER SPECIES 



In primary successions, as delineated above, the orderly and slow 

 process of replacement of one association by another proceeds under 

 natural conditions. As a consequence of the activities of man, how- 

 ever, these successions are often violently interrupted for shorter or 

 longer periods, and new plant associations very different in composi- 

 tion from the original ones may initiate secondary successions. These 

 may be started as a result of fire, overgrazing, logging, cultivation, 

 road building, coal stripping or other digging operations, blowing of 

 sand, and even the activities of mo'und-building animals. The climax 

 toward which they develop, however, is the same as would have been 

 developed by the original vegetation. It is with these secondary 

 successions that technicians have most to deal. 



It is no longer new to point out that we are now dealing not with 

 biologic areas as they were, but as man's activities have changed 

 them. Students of plant ecology and geography have long recognized 

 that existing vegetation and fauna do not represent original condi- 

 tions. As time goes on, studies of original vegetation must deal more 

 and more with old records, historical documents, travel sketches_ of 

 earlier periods, early surveys, and consideration of the few remaining 

 virgin areas. Original f aunal and floral areas can be determined only 

 in this manner. Man has been relocating species of animals and 

 plants to suit himself from the time the first settlers arrived in 

 the country. Brief consideration of the changing face of the land 

 surface of the country shows this very clearly. Furthermore, it is 

 obvious that because most vegetation has been disturbed by man, 

 associations comprising secondary successions are the rule rather than 

 the exception. 



It is regrettable but true that it is not known with certainty just 

 why one association gives way to another. It seems probable that 

 changes in the substratum play a considerable part, but those changes 

 cannot be measured accurately enough to say that at one time certain 

 plants will succeed; at another, different plants will thrive. So far 

 it has simply been noted that in any type of succession a series of 

 what appears to be preparatory stages must precede the climax. It 

 may also be noted that as a consequence, or perhaps as a parallel 

 phenomenon, the food of wildlife and the fauna itself changes. Cer- 

 tain pioneer plants lead the way and very rarely persist through all 

 stages to the climax. Conversely, climax species almost never act as 

 pioneers. Edaphic factors appear to be very important in the pre- 

 liminary stages of secondary successions. 



In applying these facts to planting, where no further care is to be 

 given to the plants, one may with reason select for use pioneer species 

 that have demonstrated their ability to succeed on areas variously dis- 

 turbed by man. This is of great importance. We see that under 

 natural conditions the preparation of an area for occupation by climax 

 species is undertaken by pioneer species. They do not last long, con- 



