200 CLIMAX FORMATIONS OF NORTH AMERICA. 



Cooper (1912) has studied the r61e of mosses in the succession of vegetation 

 on Isle Royale. He finds that there is a definite sequence of mosses from the 

 beginning of the rocky-shore population to the establishment of a climax forest. 

 As will be expected, the mosses are relatively unimportant in some stages, 

 while in others they form the bulk of the vegetation. Their reaction is of the 

 greatest importance when they colonize bare rocks as pioneers and also in the 

 Sphagnum stage of bog. In his complete paper, Cooper (1913 : 229) has sum- 

 marized in a comprehensive but concise manner the results of his thorough 

 study of succession on Isle Royale in Lake Superior, and hence the summary 

 is given in full here : 



"THE HYDRARCH SUCCESSIONS. 

 "THE BOG SUCCESSION. 



"Physiographic development. — The depressions which now contain lakes or 

 bogs owe their origin to glacial modification of the preglacial topography; 

 sometimes to the cutting off of bays or channels by wave-built bars. 



"The physiographic history of the habitat in which the bog succession runs 

 its course comprises two stages: the channel-bay stage and the lake stage. 

 The lakes and harbors are tending toward extinction through the agencies of 

 down-cutting of outlets, sedimentation, and vegetation, of which the last is 

 the only one of importance at the present time. The lake stage ends when 

 vegetation, aided by the other agencies, has entirely eliminated the open water. 



"Vegetational development. — During the channel-bay stage aquatics first 

 appear and gradually increase with increasing shelter; the beginnings of the 

 sedge mat are occasionally present. 



"During the physiographic lake stage all the vegetational stages of the 

 succession appear in order: aquatics (usually already present), sedge mat, 

 Sphagnumrshruh, bog forest. All may have their beginnings at practically 

 the same time. The sedge mat is usually the most prominent feature at this 

 period. The sedges gain their first foothold in shallow water close to shore and 

 build a floating mat out over the water. They are by far the most important 

 agents in peat formation. 



" During the covered bog stage the plant societies are successively eliminated 

 by the centripetal encroachment of the various zones. 



"Two lines of succession are distinguished after the sedge mat stage. One is 

 characterized by Chamaedaphne, Andromeda, and Alnus incana in the shrub 

 stage, practical absence of Sphagnum, and by Larix and sometimes Thuja in 

 the bog forest; the other by Chamaedaphne and Andromeda followed by Ledum 

 in the shrub stage, abundance of Sphagnum accompanying the shrubs, and by 

 Larix and Picea mariana in the bog forest. Sphagnum is the critical plant 

 in the differentiation of the two series, since Ledum and Picea mariana appear 

 later, and only in cases where Sphagnum is abundant. The differences may be 

 related to differences in drainage, since those bogs containing little Sphagnum 

 are usually well drained, while in those with abundance of Sphagnum, as far 

 as observation has gone, drainage was very poor or lacking entirely. A con- 

 tributing factor is found in differences in the composition of the sedge mat 

 preceding the shrubs and Sphagnum. In the Sphagnum bogs Carex limosa is 

 the principal mat-forming species. Being low and soft, it offers no resistance 

 to the spread of the moss. In the bogs with little Sphagnum, Carex filiformis 

 is the iinportant mat-former. On account of its height and stiffness and dense 

 growth it produces unfavorable conditions for the §read of Sphagnum. The 

 reason for the differing distribution of the two carices is unknown; it may be 

 merely accidental. 



