242 TH E STUDY OF PLANT COMMUNITIES * Chapter X 



lichens, which, after mat formation, lead to a heath mat stage. In 

 the western part of the range, this is followed by the xerophytic 

 jack pine, or black spruce to form a subclimax, but eastward white 

 spruce-balsam fir may come in directly. Jack pine also occupies 

 extensive areas of sand plains and gravelly soils. 



Fig. 114. Typical stand of jack pine (Pinus banksiana) on sand or gravel 

 soils in northern Michigan.— U. S. Forest Service. 



Bog succession is everywhere apparent in the many lakes that 

 are filling up. The usual submerged and floating-leaved aquatics 

 are commonly followed by sedges and grasses, which may form 

 a floating mat upon which a bog-shrub stage develops. This may 

 include Chamaedaphne calycidata, Andromeda polifolia, Almis in- 

 cana, Ledum groenlandicmn, and Vaccinium spp. Larch is the 

 commonest tree to come in after shrubs, followed by black spruce 

 or, in less acid bogs, sometimes Thuja occidentalis. Any of these 

 species may maintain their dominance for long periods, but they 

 can be superseded by climax. 



Secondary succession is usually caused by fire. If the burn is so 

 severe that all humus is consumed, leaving bare rock, primary suc- 

 cession may be repeated. If a dry peat bog burns, it usually fills 



