282 PLANT PHYSIOLOGY AND ECOLOGY 



ceeding stages contain an increasing number of species found 

 in the original forest, until, in the final formation, the original 

 facies have returned. The reconstruction of a mesoph3'tic forest 

 regularly takes place by means of mesophytes, owing to the fact 

 that the change in soil is slight. 



303. Succession in lumbered areas. Lumbering often re- 

 sults in complete or nearly complete destruction of the vegeta- 

 tion through removal, or through the action of erosion upon the 

 exposed surface. In the first case a short mesophytic succession 

 results. In the second the succession is long and complex, pass- 

 ing through decreasingly xerophytic conditions to a stable meso- 

 phytic forest. Where a forest is cut over for certain species 

 alone, the undisturbed trees usually take full possession. In some 

 instances a newcomer usurps the first place, while in others the 

 original species ultimately return. 



304. Succession by cultivation. The clearing of forests and 

 the "breaking" of grassland for cultivation destroy the original 

 vegetation. The temporary or permanent abandonment of cul- 

 tivated fields then permits the entrance of weeds, which are the 

 pioneers of new successions. This occurs annually in fields after 

 harvest, the same species reappearing year after year. In fields 

 that lie fallow for several years or are entirely abandoned, the 

 first ruderal plants are displaced by newcomers, or certain ones 

 become dominant at the expense of the others. In a few years 

 these are replaced by invaders from adjacent formations, and 

 the field is ultimately reclaimed by the original vegetation, unless 

 this has entirely disappeared from the neighborhood. 



Other activities of man, such as the construction of buildings, 

 roads, railways, canals, etc., remove the native vegetation and 

 make room for the rapid development of weed formations 

 In and about cities, where the original formation has entirely 

 disappeared, such initial stages persist as permanent formations. 

 Elsewhere the usual succession takes place, and the ruderal vegeta- 

 tion is finally replaced by the original one. In mountain and 

 desert regions, where ruderal plants are rare or lacking, their place 

 is taken by native species of large seed production and much 

 mobility. These are gradually replaced by other native species 

 of less mobility but greater persistence. 



305. Reactions of plants upon the habitat. _A succession 

 starts wherever an area is unoccupied, denuded, or otherwise 



