304 George E. Nichols, 



rocks dislodged from above, the trees remain scattered, and, at 

 this stage, paper birch commonly is the most conspicuous tree. 

 The reason for this frequently observed predominance of paper 

 birch over conifers at this time, as pointed out by Cooper ('13, 

 pp. 218, 219), is undoubtedly due to the ability of the former to 

 sprout from the stump and thereby recover from the injuries 

 inflicted by falling boulders. Ultimately, a coniferous forest of 

 the type already described may become established, in which the 

 predominant trees include the balsam fir and white spruce, 

 the paper birch and yellow birch, the white pine, the black 

 spruce and mountain ash. On north-facing slopes, coniferous 

 forests, while attaining a high degree of mesophytism, frequently 

 represent the culminating phase of the succession : in other 

 words, they constitute an edaphic climax. But, under favorable 

 conditions, the regional climax association-type is capable of 

 attainment on boulder talus, as on the glacial drift. 



The association-types of gravel slides. — In extreme cases, as, 

 for example, on gypsum slides" (Fig. 15), the instability of the 

 rock mass may be so great that plant life is almost excluded. 

 Largely on account of this instability, lichens and mosses usually 

 play but little part in gravel slide successions : only plants with 

 roots are capable of maintaining a foothold here. The most 

 important pioneers are xerophytic ferns and seed plants, 

 especially herbaceous forms which perennate by means of roots 

 and rhizomes : such species, for example, as Pteris aquilina, 

 Dicksonia piinctilohula, Danthonia spicata, Campanula rotundi- 



^" In this connection it is worthy of note that floristically the vegetation 

 of gypsum outcrops commonly differs to a marked degree from that of 

 other rock outcrops which may be physically similar. On the gypsum the 

 vegetation includes a pronounced calciphilous element which elsewhere 

 is mostly absent. Prominent among the seed plants are Carcx ehurnca, 

 Shepherdia canadensis, Cornus circinata, and Erigcron hyssopif alius. The 

 bryophytes include Swartsia inclinata, Gymtiostomuni rupestre, Tortnla 

 mucronifolia, Encalypta contorta, Myurella Careyana, and Thuidium 

 abietinum. Generally speaking, however, while there are frequent other 

 evidences throughout this region of a similar correlation between the 

 chemical nature of the underlying rock and the character of the vegetation, 

 the writer has been unable to distinguish any broad relationships of 

 general ecological significance. Aside from the influence of topography, 

 the general aspect of the vegetation appears to be correlated more with 

 the physical character of the substratum than with its chemical character. 



