62 INITIAL CAUSES. 



and dunes, the reaction is slight, but it seems probable, however, that these, 

 too, must follow the general rule, namely, that the removal of the soil built 

 up bjr reaction must necessitate a return to primary conditions. In the vast 

 majority of cases a secondary area is formed whenever removal operates 

 within the root layer of soil. This may be readily tested by instrumental 

 methods or by experiment. In general, the composition of the initial stage 

 of the sere indicates this clearly enough. The removal of this layer to different 

 depths is reflected in the composition and length of the resulting secondary 

 sere or subsere. 



In cases where the destruction of the vegetation is accompanied or followed 

 by the deposition of soU, the nature of the bare area will be decided by the 

 kind of soil deposited as well as by its depth. If sand or gravel are laid down 

 over loam to a sufficient depth, the water relations of the area may be moved 

 to one extreme and a primary habitat result. Here the depth must approxi- 

 mate the length of the root system of the species of the initial stages. Other- 

 wise the roots will reach the original soil and the development will be controlled 

 in some degree by the latter. When the depth of added soil exceeds 1 or 2 

 meters, a secondary succession can result only when the soil is essentially 

 similar in texture and water relations to the original. This is apparently 

 true in the majority of cases (plate 13, a, b). 



The effects of the removal of water by drainage or of the addition of water 

 by flooding may be alike or unlike. Either flooding or drainage may destroy 

 a plant population and yet leave the area Uttle changed, thus initiating a 

 secondary succession. This is the regular effect of drainage when it does not 

 merely modify the existing vegetation. In the case of ponding, however, the 

 water produces a new set of extreme conditions, and this constitutes a primary 

 area. 



Bate and extent of removal. — Destruction of a community with accompany- 

 ing or subsequent removal of the soil is the general process of which topographic 

 erosion is much the most important part. In fact, erosion may well be regarded 

 as the general process, which is produced by topographic, cUmatic, or biotic 

 forces. While depth is the final criterion of the effect of erosion, both its 

 rate and extent have an influence. Erosion to a depth of a foot would produce 

 different conditions when produced by a single torrential rain from those due 

 to gradual erosion spread over several years, though in both cases the resulting 

 area would normally be a secondary one. The differences would consist as 

 much in the stability of the surface for migration and ecesis as in the water 

 relation. The extent of the denuded area is closely related to depth of erosion. 

 When the latter is local, it is less apt to depart widely from the normal condi- 

 dition, and its invasion is controlled almost completely by the parent area. 

 This matter is discussed in detaU in the section on cycles of erosion. 



