PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 157 



unglaciated foothills is interesting, in that they are quite com- 

 parable to associations on the morainic soils. For instance, the 

 lower zone bears a close relation to the evergreen association in 

 the broad rocky ravine; the mixed forest is closely comparable 

 in many ways to the climax mesophytic forest, containing as it 

 does the most mesophytic of the deciduous trees of this region; 

 and the upper deciduous forest has a composition similar to that 

 of the drier slopes upon the moraines. The major factors in 

 determining the association in each of the instances are the 

 same, namely, the available soil moisture, and exposure to desic- 

 cating influences, especially the wind. 



The climax formation of western Pennsylvania is a mesophytic 

 deciduous forest of which Acer, Fagus, Castanea, and Quercus 

 are the dominant members, but always occur associated with 

 additional species. 



The composition of the climax formation is not directly refer- 

 able to physiographical conditions but to climatic ones; and in 

 accordance with the annual rainfall (41.7 in.), its relatively equal 

 distribution during the months of greatest vegetative activity, 

 and the average temperature for the period of growth (61°F.), 

 this type is to be expected. 



LITERATURE 



Christy, C. W. : Preliminary check list of the flora of Crawford County, Penn- 

 sylvania. 



Illick, J. S.: Pennsylvania Trees. Pa. Dept. Forestry, Bull. 11, 1916. 



Jennings, O. E. : Botanical survey of Presque Isle, Erie Co., Pa. Ann. Carnegie 

 Mus., 5: 289-421. 



Jennings, O. E.: A note on the ecological formations of Pittsburg and vicinity. 

 Sci. 27: 828-830, 1908. 



Jennings, O. E.: Notes on the distribution of certain plants in western Penn- 

 sylvania. Fern Bull. 18: 99-101, 1910. 



Shaper, J. A. : Plants of Allegheny County, Pennsylvania. Ann. Carnegie Mus. 1. 



