64 ECESIC CAUSES. 



Belation to denuded areas. — ^Aggregation is the normal result of seed-produc- 

 tion in a community. Its importance in secondary areas depends wholly 

 upon whether it occurs before or after the action of the denuding agent. 

 Normally, of course, it occurs before denudation, and the question is wholly 

 one of the kind and number of germules which escape destruction. This is 

 determined by the agent, the position of the germule, and sometimes by its 

 nature. In the case of fire, seeds and fruits on the surface or near it are 

 destroyed, unless they have unusual protection, as in some woody cones. 

 Fruits buried by rodents, or seeds and fruits which become covered with moist 

 duff, often survive. In cultivated areas, seeds often persist for a long time, 

 though they play no part in succession unless they survive until the field is 

 abandoned. On the other hand, intensive cultivation destroys all under- 

 ground parts, while fire has little or no effect upon them. In grassland the 

 effect is merely to modify the population, but in woodland succession results. 



Aggregation occurs after fire only in a few striking instances. It occurs in 

 the case of many conifers with large or hard cones, especially where the fire 

 kills the trees but leaves them standing. This is often true of lodgepole pine 

 (Pinus murrayana) , jack pine (P. divaricata), and all pines in which the cones 

 remain closed and attached to the branches for a long time. 



Interaction of aggregation and migration. — All sterile bare areas owe their 

 pioneers to migration. After the establishment of the first iavaders the 

 development of families and colonies is due primarily to aggregation (plate 

 14 b). The appearance of each successive stage is caused by the interaction 

 of the two processes. Migration brings in the species of the next stage, and 

 aggregation causes them to become characteristic or dominant. Their rela- 

 tion in each stage is shown in the development of the succession as a whole. 

 Migration marks the beginning of the sere, as of each stage. It becomes 

 relatively more marked for a number of stages, and then falls off to a minimum. 

 In dense closed forests it becomes extremely rare, and the ecesis of the migrants 

 impossible. On the other hand, aggregation becomes more marked with suc- 

 cessive stages, and a sere may end in what is essentially a family, e. g., a pure 

 stand of Pseudotsuga or Picea with practically no undergrowth. 



MIGRATION. 



Concept. — The nature of migration as an essential process in succession has 

 been analyzed in detail elsewhere (Clements, 1904:32; 1905:210; 1907:240). 

 It will suffice to summarize the main points in connection with indicating their 

 special bearing upon the nature and course of succession. The use of the term 

 is restricted to its proper sense of movement. Migration is regarded as a 

 process distinct from establishment or ecesis. The two are most intimately 

 related in the general process of invasion, which comprises movement into a 

 habitat and establishment there. Migration begins when a germule leaves 

 the parent area and ends when it reaches its final resting-place. It may con- 

 sist of a single movement, or the number of movements between the two places 

 may be many, as in the repeated flights of pappose and winged fruits. The 

 entrance of a species into a new area or region will often result from repeated 

 invasions, each consisting of a single period of migration and ecesis. 



Mobility.— Mobility is the ability of a species to move out of the parent 

 area. Among terrestrial plants, it is indicated chiefly by the size, weight, and 



