PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 99 



is 31.18°. While the average summer temperature is favorable 

 for rich vegetative growth, it is at once evident from the average 

 of the colder months, that conditions are unfavorable for other 

 than a deciduous type of flora. 



In accordance with the annual precipitation, its relatively 

 equal distribution through the months of greatest growth, and 

 the favorable temperature during these same periods, there is 

 developed a climax mesophytic forest for this region. It is 

 also typically deciduous; an adaptation suitable to the low 

 average temperature for the five months of dormancy. 



It will not be within the scope of this paper to treat exhaustively 

 the various exceptions occasioned by pecuhar combinations of 

 physical factors, and the corresponding features in the vegeta- 

 tion. An attempt will be made, rather, to correlate the vegeta- 

 tion of the most important physiographic features to their 

 habitats; to define the generic composition; and to observe the 

 sequence of development. 



The specific forms enumerated from time to time as components 

 of particular associations in the various series are not exhaustive, 

 but include in each instance the forms which dominate or are of 

 considerable ecological importance. 



THE SWAMP SERIES 

 LOWLAND SWAMPS 



The distribution of swamps in western Pennsylvania is limited 

 largely to the glaciated region. Here the irregular, rolling 

 topography frequently includes small glacial lakes and ponds. 

 The smaller of these have been wholly or partially reclaimed by 

 the natural development of vegetation, so that various stages 

 may be seen from the open swamp to the dense swamp forest. 



In Mercer County the area occupied by these is relatively 

 great. The land lies at an altitude ranging from 1100 by 1350 

 feet and the numerous swampy depressions serve as headwaters 

 for the small streams which flow south, east, and west into the 

 Shenango and Allegheny Rivers. 



Many of these included lakes and ponds have long since de- 

 veloped beyond the early stages of reclamation, but are pre- 



