212 WILLIAM S. COOPER 



in, or else ground hemlock, and increase as conditions become 

 more favorable; and the cycle is now complete. 



The factors which determine whether mosses or ground hem- 

 lock shall become dominant are not certain. One fact may be 

 mentioned however that may suggest at least a partial explana- 

 tion. Mosses are decidedly more abundant in parts of the forest 

 which are much exposed to the wind, such as the outer row of 

 islands bounding Rock Harbor on the southeast, where windfalls 

 are consequently very frequent and extensive. Ground hem- 

 lock is more abundant ■ in sheltered situations where windfalls 

 are comparatively rare — where the trees drop one by one without 

 seriously disturbing the lower vegetation. The explanation here 

 suggested is that the development of an abundant moss vegetation 

 requires a comparatively short time, and may thus be accom- 

 plished in the brief intervals between windfalls characteristic 

 of exposed situations; while for the ground hemlock to establish 

 itself in sufficient abundance and luxuriance to obtain and main- 

 tain dominance requires a long time and comparatively stable 

 conditions, obtainable only in very sheltered localities. 



Equilibrium is thus maintained in the forest as a whole, and 

 it is to be considered as the permanent climax society in spite of 

 the state of continual change existing within it. 



Of the three elements the mosses are the weakest in the com- 

 petition. The tree element is able to exclude both the ground 

 hemlock and the mosses; the ground hemlock when abundant 

 excludes mosses and holds its own against the trees; while the 

 mosses are able to exclude neither. It is nevertheless true that 

 the mosses, because they quickly establish themselves whenever 

 conditions permit, and through their accumulation of humus and 

 conservation of moisture, are immensely important to the main- 

 tenance of the climax forest. 



SUMMARY 



There is upon Isle Royale a definite succession of mosses, 

 continuous in the Rock Shore Succession, from the very begin- 

 ning to the establishment of the climax forest; and in the Bog 



