PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 153 



podium lucidulum, Aspidium spinulosum, Viburnum alnifolium, 

 and Oxalis acetosella. Accompanying these may be found in 

 greater or lesser amount, Habenaria orbiculata, Tiarella cordi- 

 folia, Mitella diphylla, Maianthevium canadense, and Clintonia 

 borealis. 



The seedlings present in the undergrowth of this stage are 

 predominantly of Betula and Tsuga. Occasionaly those of 

 Fagus, Acer rubrum, or Magnolia occur; but these are unable to 

 develop to maturity except when an opening is afforded, as in a 

 windbreak. The birch and hemlock alone are well adapted to 

 this situation and they successfully retain it against the invasion 

 of outside forms. The seedlings of both of these species have 

 the peculiar habit of germinating on old stumps or fallen logs, 

 thus avoiding the necessity of contending with rival species for 

 a position during their early development. When the old stump 

 decays, the new member has already taken the position of the 

 old one; or when the log upon which such seedlings are develop- 

 ing falls upon the humus, the root system quite readily extends 

 itself into the soil and makes possible the further development 

 of the seedlings concerned. 



While the lowest portion of the slope is almost exclusively 

 represented by Tsuga, at the height of approximately 80 feet, 

 Pinus Strobus becomes sufficiently abundant to be conspicuous. 

 From this point of entrance it occurs associated with the hem- 

 lock to the top of the ridge, or may be replaced at a higher level 

 by a deciduous stage. 



The Mixed, Forest 



It is in the pine-hemlock zone just before the entrance of the 

 pioneer deciduous forms that the finest timber occurs; the 

 pines frequently attaining a height of 175 feet and a diameter of 

 5 to 6 feet, while the hemlock may attain an equal diameter and 

 a height of 150 feet. 



The mixed forest consists largely of evergreens, especially 

 in the lower portion of the zone. The conifers gradually de- 

 crease, however, with the increase of altitude; and there occurs a 



