ECOLOGICAL SUCCESSION OF MOSSES 211 



aspect which represent stages in an endless chain of permutations, 

 the total result of which is that the forest as a whole remains the 

 same, although a given area is continually changing. Thus we 

 may find side by side an extremely dense group of young trees 

 with no undergrowth, an area with scattered large trees giving 

 moderate shade and with a thick growth of mosses, and a similar 

 area with Ground Hemlock (Taxus canadensis) instead of the 

 latter. Masses of wreckage resulting from windfalls are frequent 

 everywhere. 



Three elements of vegetation are concerned in these changes : 

 first, the trees, mainly balsam, fir, paper birch, and white spruce; 

 second; the undershrub ground hemlock; third, the mosses Cal- 

 liergon Schreberi, Hylocomium proliferum, and Hypnum crista- 

 castrensis. The interrelations of these elements will best be 

 understood by tracing the history of a single area. 



It will be most convenient to begin with the stage in which 

 the trees are all large and few and scattered. The shade is 

 moderate, and the ground is covered either by a thick carpet of 

 the three climax mosses or else by a dense growth of ground 

 hemlock, sometimes almost impenetrable. With passing years 

 the trees decrease in number and the shade becomes correspond- 

 ingly less ; and up to a certain point this is favorable to the mosses 

 or ground hemlock, both of which thrive best under rather light 

 shade. Sooner or later windfall is almost certain to take place 

 upon the area: that is, in some severe storm or succession of 

 storms a number of trees are blown down together. The effect 

 upon the undergrowth is frequently disastrous, though in small 

 windfall areas it may survive and even flourish afterward. 

 Ground hemlock sometimes retains possession, preventing for a 

 time the new crop of trees that normally follows. After the 

 mass of debris has partially disintegrated, conditions become 

 favorable for a fresh generation of trees, which usually start in 

 dense clumps, shading the ground completely and thus inhibiting 

 the establishment of undergrowth of any kind, including even 

 seedlings of their own species. Many trees die through the stress 

 of competition and the shade gradually lightens. Mosses come 



