Hill: Penobscot Vegetation 351 



Development of the regional climax. Wherever soil con- 

 ditions and topographic factors are favorable, deciduous 

 trees soon begin to make their appearance, first among 

 which is the yellow birch. Because of its shallow root sys- 

 tem, well adapted to a comparatively thin soil, and its abil- 

 it}' to germinate in the moss carpet, this tree easily becomes 

 established in openings in the forest. As a consequence of 

 this ability to grow both in the climax forest and a spruce 

 forest as well, the yellow birch is one of the most widely dis- 

 tributed trees in the region. Owing to its tolerance it 

 thrives in either situation. The beech follows the birch and 

 like it does not require an extremely deep soil. These two 

 trees are the only deciduous members of the regional climax 

 forest which are at all frequent on the islands and the coast- 

 al portion of the mainland, where they occur as scattered 

 individuals or occasionally form small stands in locally fa- 

 vorable situations. 



As the leaf litter from these trees accumulates and the 

 amount of humus increases, a seed bed is furnished for the 

 sugar maple and hemlock. In localities where all the physio- 

 graphic factors are favorable for their existence seedlings of 

 these latter trees soon become frequent. Since they are very 

 tolerant they grow rapidly in the shade of the other species 

 and, when present in sufficient numbers, soon win out in 

 competition with any conifers which may be present. The 

 continual increase in leaf litter is proportionately unfa- 

 vorable for the moss carpet, and this tends to disappear as 

 the hardwoods increase, together with the herbaceous forms 

 especially characteristic of such a habitat. The final stage 

 in the succession on glacial till is a forest of the regional 

 climax association-type. Figures 13 and 14 show such a 

 mixed deciduous coniferous forest at Craigs Pond, in Or- 

 land. Theoretically such a forest should develop in all 

 cases. Practically, however, the influence of various biotic 

 factors has so altered the course of events that in only a few 

 cases is the climax actually attained. Elsewhere secondary 



