PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 149 



It is at once seen that although the floodplain forest includes 

 the larger number of the upland mesophytic species, the domi- 

 nant members are different. That is, Acer saccharum, Gary a 

 ovata, Uhnus americana, Juglans cinerea, Platanus occidentalis, 

 Quercus bicolor, and Acer saccharinum, are dominant and char- 

 acteristic members in the former, while those most representa- 

 tive of the latter are Fagus grandijolia, Acer ruhrum, Magnolia 

 acuminata, Castanea dentata, Quercus alba, Liriodendron tulipi- 

 fera and Quercus rubra. 



THE UPLAND FORESTS 



The upland forests of western Pennsylvania may be divided 

 into two types with reference to the topography and soils upon 

 which they occur: (a) The morainic forests, or the type occur- 

 ing as a climax upon the glaciated area of the western most part ; 

 and (b) The forests upon the unglaciated clays. 



Upon Morainic Soils 



If the chemical composition of the soil is the chief factor in 

 determining the type of vegetation it shall bear, one might reason 

 that the morainic clays would be characterized by a flora of 

 closely smilar composition over wide areas; for these soils are 

 relatively uniform in composition, and at the same time are 

 usually well represented by all the essential elements. This is 

 due to the fact that they are composed of a mixture of soils 

 brought together by the glacier from widely separated points. 

 There is no such uniformity, however, but various topographical 

 features with the same soils, regularly show a change in com- 

 position. For instance, that occurring upon exposed upland 

 slopes has a very different makeup from that characteristic of 

 wooded depressions, and that of level uplands has many features 

 uncommon* to either. 



It is the forest occupying gently rolling areas which shows the 

 climax development of the region ; for those occurring in other 

 situations are advanced to this type eventually, as the topog- 

 raphy changes. The climax forest is deciduous and contains 



