3i8 George E. Nichols, 



rotundifolia, Anaphalis margaritacea, and Centaur ea nigra. The 

 mosses, Racomitrium cancscens and Polytrichuni pilifcrum, fre- 

 quently form a loose, discontinuous ground cover in protected 

 spots; while species of Cladonia may also be present. Locally, 

 wherever the soil is fairly moist, the early vegetation may include 

 mesophytic species, such as Clematis virginiana, Thalictrnm poly- 

 gamum, and Heracleum lanatnm — forerunners of subsequent 

 stages in the succession. 



The pioneer tree association-type. — Although the vegetation 

 in the gravel bar stage of the succession is predominated by 

 shrubby and herbaceous species, trees may be present from the 

 outset. The balsam poplar, more than any other species, is 

 preeminently the distinctive pioneer tree of gravelly or sandy 

 flood plains, although it often shares this honor with the paper 

 birch and white spruce (Fig. 21). The balsam poplar owes its 

 prominence to its copious root system and exceptional ability to 

 maintain itself on shifting alluvial soils, its tendency to repro- 

 duce and spread by means of root suckers, and its rapid rate of 

 growth, which enables it to outstrip any chance competitors. In 

 these respects it resembles its southern relative, the cottonwood 

 {Populns deltoides) , of which it may be regarded as an ecological 

 counterpart. One frequently encounters on flood plains groves 

 of good-sized balsam poplars, beneath which the more character- 

 istic trees of the climax forest apparently are just beginning to 

 establish themselves. But any of the climax trees may appear 

 simultaneously with the poplar. On one small, treeless stretch 

 of gravelly flood plain, for example, the writer noted seedlings 

 of nearly every tree (all except white pine, hemlock, and red 

 oak), which has been cited earlier as growing in the climax 

 forest; also seedlings of bird cherry and choke cherry (Prunus 

 virginiana). For the reasons suggested above, however, the 

 poplar usually gains a temporary ascendancy over its competitors, 

 thereby giving rise to a more or less distinct phase in the succes- 

 sion. 



The edaphic climax forest. — Flood plain forests of the sort 

 ordinarily associated with old-age rivers have been developed 

 along some of the larger lowland streams, and in some cases the 

 physiographic history of the areas which these occupy has 

 probably been similar to that of flood plains as described in the 

 preceding paragraphs. But in the most typical instances 



