44^ George E. Nichols, 



Successional relations. — Assuming for the purpose of illustra- 

 tion a nearly level or gently sloping rock floor, approximately 

 flat as a whole but in detail with a more or less irregular surface, 

 with slight elevations and depressions but with no basins capable 

 of retaining any appreciable body of water, the successive steps 

 in the evolution of a raised bog may now be outlined. On an 

 uneven rock surface of the sort under consideration the pioneer 

 aspect of the vegetation varies locally. In the higher, drier 

 situations it is essentially xerophytic. Commonly the vegetation 

 here is that of the sedge-grass heath association-type, as 

 described in connection with xerarch successions : the ground is 

 covered by a carpet of cladonias and Racomitrium, and sup- 

 ports a more or less luxuriant growth of Scirpus caespitosus and 

 Calamagrostis Pickeringii, with a scattering of low shrubs. In 

 the lower situations the vegetation may be quite similar, but here, 

 owing to the generally more favorable moisture relations, the 

 sphagnums commonly establish themselves, either coming in at 

 the outset or later on replacing the cladonias and Racomitrium. 

 Subsequent changes in the nature of the substratum and in the 

 ecological aspect of the surface vegetation depend very largely 

 on the sphagnums, not merely on their presence or absence but 

 on the species which come to predominate. Where conditions 

 are such that none of the sphagnums are able to establish them- 

 selves in force, any further changes will probably conform closely 

 with what has been described earlier in connection with xerarch 

 successions. Where conditions are such as to favor the growth 

 of the sphagnums and these assert themselves as one of the pre- 

 dominating elements of the plant cover, further changes depend 

 very largely on which particular group of sphagnums gains con- 

 trol over the situation. 



For the sake of simplicity there will be described a hypothetical 

 example of what may be regarded as the logical sequence of 

 association-types : a series in which the pioneer stage gives way 

 to a bog meadow, which becomes superseded by a wet bog, which 

 in turn gives way to a dry bog; and, in this connection, various 



buries the lower branches, these put out new roots, turn upwards at 

 their tips, and grow as independent stems. This growth probably, how- 

 ever, does not go on indefinitely, since the trees are ultimately over- 

 whelmed and destroyed by the moss." 



