PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 107 



Evergreen Swamp Forest. The deciduous type of swamp forest 

 is replaced by one which is predominantly evergreen. It. 

 occupies the outer borders of the swamp and constitutes an 

 association which is more persistent than those already described. 

 It is common to encounter a few scattered individuals of Fagus 

 grandifolia, Castanea dentata, and Prunus virginiana here, but 

 these are limited to elevations and represent entrants from the 

 morainic forests without. 



The coniferous forest persists as long as does the swamp 

 itself, and cannot be replaced naturally until the true swamp 

 conditions have passed; therefore it may be considered as the 

 climax stage of the swamp series. If succeeded eventually by 

 another stage, it is the deciduous climax forest. 



There are two essential tree members of the evergreen forest, 

 Tsuga canadensis, and Pinus Strobus. Tsuga is of much greater 

 abundance than Pinus, and follows the Betula-Fraxinus-Acer 

 stage more commonly than the Larix. It begins as seedlings 

 upon the hummocks. These it eventually broadens and greatly 

 enlarges, thus affording more ample space for the development of 

 herbaceous species which are not adapted to the heavy wet soil 

 of the depressions. Pinus is most abundant in Sphagnum bogs 

 where it regularly succeeds Larix. It likewise is associated with 

 hummock development. 



In contrast to the deciduous swamp forest the evergreen stage 

 is readily penetrable, so far as undergrowth is concerned, since 

 the close stand excludes almost all direct light; thus forbidding 

 the luxurious development of the lower forms. 



The undergrowth is likewise essentially evergreen, or largely 

 so. Especially is this true when one considers that which is 

 associated with the hummocks. Here the chief species include 

 Taxus canadensis, Coptis trifolia, Lycopodium lucidulum, Lyco- 

 podium ohscurum, Aspidium spinulosum, Porella, Mitchella 

 repens, Gaultheria procumbens, Sphagnum, Trientalis, americana, 

 Cypripedium acaule. Viburnum alnifolium, Osmunda cinnamo- 

 mea. Viburnum acerifolium, Epipactus repens, Trillium erectum, 

 Maianthemum canadense, etc. 



The depressions at* this stage are frequently carpeted with 



