21 



There are two main lines of normal succession leading to the 

 spruce-birch-balsam forest as their ultimate or penultimate stage : 

 the rock shore series and the lake bog series. 



The rock shore succession is made up of the following 

 stages: 1, crustose lichen stage; 2, crevice plant-lichen 

 stage (Fig. 1) ; 3, Cladonia-Juniperus stage; 4, mature forest. 

 The abrupt transition from the comparatively youthful third 

 stage to the mature forest may perhaps be explained by the 

 former presence of intermediate societies which have been 

 "pinched out" by the advance of the forest toward the lower 

 limit of plant growth, at the present time practically stationary, 

 which is determined by wave action and ice. Suggestions of 

 a xerophytic transitional society, dominated by jack pine, were 

 found at a few points by Adams. 



The lake bog series develops in the valleys and depressions 

 which have been shut off from the main lake by the emerg- 

 lence of the island. The early stages are as follows: 1, aqua- 

 tic stage ; 2, bog sedge stage ; 3, bog shrub stage. From this 

 point two types of bogs develop. The first, in undrained de- 

 pressions, is characterized by abundance of sphagnum and heaths, 

 especially Labrador tea, followed by bog forest composed of 

 tamarack and black spruce. The second type (Fig. 2), de- 

 veloping in basins possessing active inlets and outlet, is charac- 

 terized by practical absence of sphagnum. The shrub zone 

 is made up of Chamaedaphne and alder, and the bog trees fol- 

 lowing are tamarack and arbor vitae. The condition and causes 

 of the two bog types are still to be worked out. Both types 

 develop into the spruce-birch-balsam forest, usually very soon 

 after the bog forest is mature. 



The changes brought about in the dominant forest by fires 

 are as follows. If the humus is completely burned from the 

 rocks, a succession is instituted which is essentially like the 

 rock shore succession. If more or less of the humus and 

 forest vegetation remain, certain species develop to the exclu- 

 sion of the rest, and dominate for a time. The balsams are 

 exterminated, while the birches increase enormously by sprout- 

 ing from the stump. The result is a birch forest, under the 



