ECOLOGICAL SUCCESSION OF MOSSES 199 



Succession mosses take little part, and it will therefore be elimi- 

 nated from the present discussion. In the course of the Bog 

 Succession lake basins become filled through the centripetal 

 encroachment of vegetation. The stages upon Isle Royale are 

 as follows: aquatics; sedge mat; shrub or shrub-sphagnum; bog 

 forest; climax forest. 



A " secondary succession" should be mentioned, which follows 

 the burning of the forest and leads back to the climax state. 

 In this mosses play a subordinate part. 



I. THE ROCK SHORE SUCCESSION 



The rock shores of Isle Royale may be classified in various 

 ways: according to slope, degree of exposure to waves and wind, 

 and material. There are vertical cliffs, particularly character- 

 istic of the northwest coast of the island, and gently sloping 

 shores, corresponding to the dip of the strata, found mainly upon 

 the southeast coast. Along the land-bound harbors are shores 

 that are thoroughly protected from wind and waves, with forest 

 to the water's edge; while along the exposed southeast coast 

 there is a broad belt of bare rock supporting a sparse vegetation 

 or none at all. The material of which the shore is composed is 

 important in its physical rather than in its chemical character. 

 The manner and degree of weathering, with resulting rough or 

 smooth surfaces, abundance or paucity of crevices, are the varia- 

 able conditions that affect plant life. Upon all these types of 

 coasts the course of the succession is in general the same, but we 

 find that the stages are most fully represented and most plainly 

 marked upon the gently sloping shores about midway in the 

 scale of exposure. In fully sheltered situations the preliminary 

 stages of the succession have all been passed through and the 

 climax forest extends to the water's edge, while upon the most 

 severely exposed shores some of the intermediate stages are apt 

 to be missing, " pinched out" by the advance of the forest against 

 the impregnable barrier of waves and ice. 



At a variable height above the lake level, according to degree 

 of exposure, the three subsuccessions have their beginnings. 



