PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 



105 



to and including that occupied by the shrubs. This condition 

 however has passed and the soil become more compact before 

 the development of the forest stage upon it. 



The pioneer trees like their predecessors the shrubs, advance, 

 occupying positions upon the hummocks, while the depressions 

 m.ay remain quite unoccupied for a time. These, however, are 

 eventually reclaimed, the first occupation being made by her- 

 baceous species. 



Among the earliest of the tree entrants, Fraxinus nigra, 



Fig. 2. A late stage in the reclamation of an upland depression, which was 

 formerly a pond. It is occupied here by a pure stand of Quercus bicolor. 



Beiula lenta, and Acer rubrum are most conspicuous. Fraxinus 

 develops abundantly while young but when well matured, it 

 almost invariably dies, giving a characteristic appearance to this 

 stage by the scattered trunks of the dead and dying trees. The 

 explanation of this behavior is quite uncertain, but it apparently 

 lies in the relation of the root system to the wet swamp soil. 

 So long as the roots of the seedlings are confined to the hummocks 

 the development is quite rapid, but when once the stem has 

 developed so far that the root system must extend itself into the 

 wet boggy humus of the depressions, there generally occurs a 



