PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 155 



The Deciduous Forest 



As previously stated, the conifers are generally replaced on the 

 higher slopes by a stage which is typically deciduous. It should 

 be noted, however, that Tsuga and Pinus persist through the 

 mesophytic stages; so that the upper deciduous forests are quite 

 xerophytic in comparison with those of the preceding zones. 

 The humus is not so well estabUshed, nor is the available supply 

 of soil water comparable to that of the areas just described. 



The dominating species here are usually Quercus alba and 

 Castanea dentata. Associated species occurring in greater or 

 lesser abundance are Quercus velutina, Quercus rubra, Magnolia 

 acuminata, Fagus grandifolia, Prunus serotina, Amalanchier 

 canadensis, Fraxinus americana. Sassafras variifolia, Cornus 

 florida, Carpinus caroliniana, Ostrya mrginiana, etc. The seedling 

 representation suggests a relative permanency of this stage, 

 since it is quite the same as that of the adult assemblage. 



The undergrowth of shrubs and herbs includes among other 

 species, the following: Clintonia umbellulata, Aralia nudicaulis, 

 Aspidium marginale, Aspidium spinulosum, Cornus canadensis, 

 Fragaria virginiana, Gaidtheria procumbens, Hieracium venosum, 

 Lonicera americana, Maianthemum canadense, Medeola virginiana, 

 Mitchella repens, Polygonatum biflorum, Polystichum acrosti- 

 choides, Prenanthes alba, Pteris aquilina, Ribes Cynosbati, Rubus 

 allegheniensis, Smilacina racemosa, Solidago ccesia, Uvidaria 

 sessilifolia, and Vaccinium vascillans. 



SUMMARY 



The plant associations of western Pennsylvania bear a close 

 relationship to the topographical features of the region; so that 

 a particular type of vegetation may be expected in similar 

 environments, although widely separated. 



The fact that distinct types of vegetation and flora are thus 

 associated with any given contour feature is referable to its 

 close relationship to the factors which bear directly upon plant 

 activities. For instance, the environment to which growing 

 plants are subjected in the ravine is closely similar, whether the 



