Vegetation of Northern Cape Breton. 299 



forest matures, the white spruce tends to become relatively less 

 abundant, the balsam fir relatively more so. 



Contemporaneously with the ever-increasing amount of shade 

 produced by the canopy of foliage overhead, the vegetation of the 

 forest floor also changes. The cladonias of the heath stage are 

 largely superseded by bryophytes. Of the mosses, Hypmim 

 Schreberi is the pioneer forest species and often appears on the 

 heath mat well in advance of the forest itself. Along with this, 

 but much less common, may grow Rhytidiadelphns triquetrus. 

 As the shade and moisture conditions on the forest floor become 

 more favorable, two relatively mesophytic mosses. Hyloconihim 

 splendens and Ptilium crista-castrensis, together with the liver- 

 wort, Bazzania trilohata, come to play an important part in the 

 formation of the moss carpet, by which the ground sooner or 

 later becomes almost completely covered over.^ Of the shrubs 

 and herbaceous vascular plants which are characteristic of the 

 heath mat, certain species, such as Pteris aqiiilina, Camus cana- 

 densis, Epigaea repens, Vaccinium pennsylvanicum and V. 

 canadense, are equally characteristic of the coniferous forest, 

 particularly during the early phases of its development. 

 Coincident with the formation of the moss carpet, however, 

 other species begin to appear which, while they may have been 

 represented to some extent in the earlier stages of the succession, 

 are more typical of the forest. The forerunners include Maian- 

 themiim canadense, Aralia nudicaulis, Pyrola secunda, Trientalis 

 americana, Chiogenes Idspidula, and Linnaea borealis americana. 

 Later on, as the forest matures, these relatively xero-mesophytic 

 forms are followed by other species which are more truly meso- 

 phytic, such as Clintonia borealis, Coptis trifolia, Oxalis Aceto- 

 sella, Moneses uniflora, Pyrola minor, and Aster acuminatus. 



Very often, during the early development of a coniferous 

 forest there is a considerable period when the ground underneath 

 the trees is almost barren of a plant cover. The probable 

 explanation of this frequently observed phenomenon is suggested 

 later in connection with the discussion of succession in abandoned 

 pastures. 



The edaphic climax association-type. — Theoretically, at least, 



* Cooper ('11) has described a similar succession of lichens and mosses 

 as accompanying the development of the climax forest on Isle Royale. 



