ig2o] CLAYBERG— UPLAND SOCIETIES , 49 



followed by maple-beech, although the regenerating climax forest 

 can enter at any stage of the upland secondary series. One of the 

 evidences suggesting equivalency of the thorn, fern, and milkweed 

 societies is that sumac can follow any one, and that any of the three 

 can succeed the fireweed flora. 



Xerarch tree society 



1 he aspen-white birch-pin cherry society varies much in general 

 form and specific content , so three types (consocies) are found. 

 The dominant trees are Populus tremulcrides Michx., P. grandi- 

 dentata Michx., Betula alba papyri/era, and Prunus pennsylvanica L. 



Pin cherry-birch. — The birch may be absent. Pure pin cherry 

 stands in particular occur in upland and middle level clearings fol- 

 lowing the thorn society. They can spring up suddenly. In spite 

 of good light the lower branches remain slender and die early. 

 The mode of growth is the arboreal expression of the clearing 

 society type; all are also soft wooded. The pin cherry is eco- 

 logically peculiar in being -strongly excurrent, with elongate form 

 and filiform type of branches, wasting the minimum of tissue on 

 laterals and trunk diameter. This gives it great power of vertical 

 elongation, an aid in competition for place in clearings, but makes it 



must 



this 



Aspen-pin cherry. — This is a dry open xerarch type found 

 along shore, especially on the ridge back of the Nipissing cliff. 



remai 



between the two species. Herbage is scant or none, and dead 

 twigs and branches are thick below. Such a stand is far opener than 



m 



in number of dead trees. 



Aspen-Pteris. — This is found more on dry levels inland. The 



dominates 



the large- 



prominent; and some birch may be found. Being 

 tierbaee below the bracken svnfolium suggests a 



high type of primary forest. The following 



Eat. 



proc 



podh 



The 



