PRIMARY AND SECONDARY AREAS. 61 



or no viable ones other than those of pioneers. Secondary areas, on the con- 

 trary, such as burns, fallow fields, draiaed areas, etc., contain a large number 

 of germules, often representing several successive stages. In some cases it 

 seems that the seeds and fruits for the dominants of all stages, including the 

 climax, are present at the time of initiation. The steriUty of the soil of a pri- 

 mary area is due chiefly to the relatively long period of its formation, and to 

 the effect of excessive water-content or drouth upon migrating germules. In 

 aU cases it arises in a measure also from the impossible conditions for the ecesis 

 of aU plants except pioneers. In these points most secondary soils offer a 

 sharp contrast. The method of origin permits the persistence of seeds or 

 perennial parts or both, and its suddenness usually allows the immediate 

 entrance of many migrants. The soil affords favorable conditions for the 

 preservation of seeds and fruits, often for many years, as of course for ready 

 ecesis (plate 17, a, b). 



This contrast between primary and secondary areas is seen most strikingly 

 in the case of land-shps, where the slip exposes rock on the mountain side and 

 produces a mass of soil and vegetation at the bottom (plate 12, a, b). This is 

 sometimes true also of the fragmentation of cliffs by gravity and of erosion 

 and deposit due to torrential rains or other agents which act suddenly. 



Denudation. — Secondary areas are the result of denudation, with or without 

 the disturbance of the soil. Their nature is dependent upon the process of 

 denudation and upon the degree to which it acts. The latter is ordinarily 

 much the more important. It determines in the first place whether the result 

 will be merely a change in the existing community or the production of a bare 

 area. In the case of the complete removal of vegetation, as by fire, the soil 

 may be disturbed so little that it offers essentially the same conditions as before 

 denudation, and initiates a sere correspondingly brief and simple. On the 

 other hand, the disturbance of the soil may operate to various depths and pro- 

 duce correspondingly extreme conditions up to the final extremes, water and 

 rock, which constitute new areas. The production of new areas by denudation 

 and soil disturbance is relatively infrequent, however. 



Methods of denudation. — ^Denuding forces operate normally by the destruc- 

 tion of vegetation, accompanied by the disturbance or removal of the soil. 

 Destruction may, however, be a consequence of flooding or deposit. Apart 

 from the destruction of the existing population, it is the depth of removal or 

 deposit of soil which is critical. The rate of removal or deposit often plays an 

 important part also, though it is usually expressed in depth. In burns there 

 is practically no disturbance of the soil at aU, though its composition may be 

 materially affected. Cultivation disturbs soil, changing its texture and water- 

 content in different degrees. Construction and engineering operations effect 

 removal and deposition of the soil in varying degree. Because of its action 

 in destroying vegetation, water must be considered in this connection also, 

 especially in the case of flooding. Climatic initial causes produce denudation 

 alone, while topographic ones exhibit the same wide range of effect shown by 

 biotic causes. 



Depth of removal or deposit.— The reaction of plants upon the soil is confined 

 wholly or chiefly to the layer in which the roots grow. This depth estabUshes 

 the limit to which removal may ordinarily go without changing soil conditions 

 essentially. In the early stages of very loose soils, such as the sand of bars 



