FACTORS INFLUENCING REPRODUCTION 833 



of ail association is the distribution of the age classes, for on this de- 

 pends to a large extent the method of cutting. Hitherto this feature 

 has been too often overlooked, even by foresters. The spruce asso- 

 ciation on Mt. Desert Island is even-aged in groups, the size of the 

 groups varying from one or two trees to half an acre or more. A 

 small opening, created by the death of one or two large trees, will fill 

 up with reproduction and develop into a small group ; a large opening, 

 made by windfall or some other agency destroying a considerable num- 

 ber of trees, will form a larger group. When the stand has been de- 

 stroyed by fire, the first generation (after the birch and aspen stages) 

 is rather open. Eventually the open spaces fill in, giving a two-aged or 

 sometimes uneven-aged forest. Later on, as the stand becomes older, 

 the differences in age become less apparent and we have again, for all 

 practical purposes, an even-aged stand. This is true not only of red 

 spruce in the east, but also of Engelmann spruce in the Rocky Moun- 

 tains. 



Reproduction. — Reproduction is abundant in all openings. In small 

 openings spruce predominates, though generally accompanied with a 

 little fir. There are spruce seedlings even under the main canopy, 

 where there is very little light ; and they persist for a long while, mak- 

 ing imperceptible growth, but keeping alive. If the stand is opened up 

 they will grow, unless the opening permits the ground to dry to such 

 an extent that they are killed. In moderate-sized openings other spe- 

 cies often outnumber the spruce. Fir seems to be the first tree to take 

 advantage of such openings and often predominates. In larger open- 

 ings white pine, and sometimes even red pine, may come in, but as the 

 canopy closes again the spruce seems to be the only one which con- 

 tinues to reproduce. Hence it will eventually regain the mastery on 

 sites favorable to it. 



White-pine Association 



Composition. — The white-pine association at its best, in central New 

 England, Pennsylvania, and the Lake States, is composed of nearly 

 pure white pine. There is little of this on Mt. Desert Island, although 

 considerable areas are covered with a forest in which 50 per cent or 

 more of the trees are white pine {Pinus strobus). It seems better to 

 class these forests as white pine than to make associations for every 

 local variation. Most of the white pine contains a large proportion of 

 red spruce and considerable amount of white cedar. The chief de- 

 ciduous tree in mixture is gray birch {Betula populifolia) ; paper birch 

 (Betula papyrifera) is found only in the cooler situations where white 



