12 MISC. PUBLICATION 303, U. 8. 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 mound-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and more with old records, historical documents, travel sketches of 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 faunal 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. 
Tt is regrettable but true that it 1s 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 wiil 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- 
Iiminary 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 1s undertaken by pioneer species. They do not last long, con- 
