102 STABILIZATION AND CLIMAX, 



the mat habit as a rule and resembles the moss cushion in many respects. 

 It is quite different in character from the forms of herb and grass which grow 

 in the rock clefts. These belong essentially to the next stage, as they actually 

 grow in soil and are only apparent rock plants. With the appearance of grasses 

 and herbs the later life-forms of the rock sere become the same as in the water 

 sere. 



The sequence of life-forms in secondary seres is essentially the same as in 

 primary ones. A characteristic exception, however, is furnished by the fact 

 that the pioneer life-forms are perhaps never the same. The approach is 

 sometimes very close, as, for example, when mosses appear after a bum. 

 In practically all such cases flowering plants develop the same year, and the 

 mosses, as well as possible algae and lichens, never form a characteristic 

 stage which persists for several or many years. In fact, the very nature of 

 secondary succession as a course of development less complete than the pri- 

 mary one precludes its beginning with the original initial stage. 



Reasons why plants disappear. — Stages are obviously the result of the dis- 

 appearance of occupants and the appearance of invaders. The causes of the 

 disappearance of plants are thus in large part the explanation of the stages 

 themselves. Most species disappear wholly, though some persist through 

 more than one stage, usually in this case becoming subordinate. Others are 

 reduced to a small or insignificant nimiber of individuals, which may persist as 

 relicts for a long time. Plants disappear for one or more of the following 

 reasons: (1) unfavorable conditions due to reaction; (2) competition; (3) 

 unfavorable conditions or actual destruction due to parasites, animals, or 

 man; (4) old age. The first two are the universal causes of disappearance, 

 and while reaction is much the most important, its effect is distinguished 

 with difficulty from that of competition. Complete, or nearly complete, 

 destruction of a community results in secondary succession. It is only 

 when the destruction operates upon the dominant or dominants alone that 

 a change of stage may occur without clearly producing a secondary sere. 

 This may occur in the selective lumbering of a mixed forest, and in grazing 

 when not too close, but there is a question in both cases whether this is not 

 really imperfect secondary succession. The influence of old age in the dis- 

 appearance of dominants is far from evident. It seems important in deciding 

 the competition between short-lived trees, such as aspens and birch, and long- 

 lived conifers, and in the resulting dominance of the latter. But it is quite 

 possible that this is really due to differences in growth and especially in height. 

 In the case of pioneers with radial growth, such as lichens, cushion herbs, and 

 grass, the death of the central portions seems due to what may well be called 

 old age. This process sometimes extends throughout the whole mat, and is 

 apparently a factor of some importance in the disappearance of the mat 

 pioneers of alpine gravel-slides, as in that of rock lichens. 



Eeasons why plants appear at certain stages. — Migrules are carried into an 

 area more or less continually during the course of its development. This is 

 doubtless true of permobile seeds, such as those of the aspen. As a rule, 

 however, species reach the area concerned at different times, the time of 

 appearance depending chiefly upon mobility and distance. As a conse- 

 quence, migration determines in some degree when certain stages will appear. 

 The real control, however, is exerted by the factors of the habitat, since these 



