1920] CLAY BERG—UP LAND SOCIETIES 41 



hundreds of young trees and saplings. There is thus left a natural 

 glade to be closed by regenerative succession. 



Competition and parasitism are the main causes of death. 

 Destruction of branches at the synfolium base by lack of light is 

 due partly to slower growth, but primarily to disadvantageous 

 position. In old trees the most serious causes of death are boring 

 insects, fungus rot, loss of foliage and branches, and (possibly) 

 decreased vascular efficiency. 



The parasites present are mainly insects and fungi. Neither 

 show prominently in the forest, remaining more or less hidden 

 except for fungus sporophores and many adult insects. Forest 

 floor pileate forms are characteristically present, but individually 

 not very abundant. Coons (20) points out that fungi may also 

 be grouped in formations, certain species being characteristic of 

 each type of habitat. Conditions in the climax forest, especially of 

 the lower levels, favor fungus growth by the relative twilight, more 

 equable temperature, and higher humidity prevailing. 



Tunneling bark beetles are present, and, because Tilia ameri- 

 cana L. and Fraxinus nigra Marsh, seem more often attacked, the 

 insects may aid in keeping maple and beech dominant. These 

 beetles, being cambium eaters, would seem more destructive than 

 the duramen eaters, such as Tremex columba of maple and beech. 



Leaf parasites (23) seem rather few. Rhytisma acerinum forms 

 black blotches on maple and oak leaves. A similar fungus causes 

 scarlet patches. Mites causing bag formation on the upper 

 surface of maple leaves, and plant lice occur persistently; woolly 

 aphids {Schizoneura) blight the alder, but rarely injure the hard- 

 woods ; several sorts of leaf-eating Microlepidoptera are found that 

 are worst on the birches, while the tent caterpillars (Clisiocampa) 

 confine their attention almost exclusively to rosaceous trees. Thus 

 the maple and beech would seem to enjoy relative immunity from 

 the more serious pests, which may aid in their retaining dominance. 

 The debris includes leaves, twigs, branches, trunks, and stumps, 

 most being found on the ground. Arrest is rare for very light 

 objects (leaves and twigs) and for heavy large ones (trees), but for 

 different reasons. The numbers of the different sorts of debris vary 

 inversely with their size. The leaf layer at the ground surface, 



