FACTORS INFLUENCING REPRODUCTION 831 



of jack pine (Pinus divaricata). On Schoodic Point the tree is abun- 

 dant and is reproducing itself, though whether it is spreading or not is 

 difficult to say for certain ; on Mt. Desert two trees were found grow- 

 ing on a granite ridge swept bare of other trees by fire. 



The island also contains surviving colonies of Arctic flora, probably 

 left stranded there after the retreat of the ice. 



The first characteristic of the vegetation to strike the casual observer 

 is the dominance of spruce (Picea ruhcns). The tree not only forms 

 a nearly pure spruce association, but occurs in all the other iypes in 

 greater or less abundance. It is the universal matrix of the forest 

 cover. 



Red spruce is not the only spruce native to the island ; the white 

 spruce {Picea canadensis) is also common. The latter is, however, 

 seldom found in the natural forest, occurring chiefly on the better soils 

 near the sea along the northern part of the island. The factors favor- 

 ing the white spruce are hard to determine on account of human inter- 

 ference on most of its site. 



The forest associations or types on the island are rather difficult to 

 portray because of the vagueness of the line of demarkation between 

 them. One association merges insensibly into the next. Exceptions 

 are the pitch pine (Pinus rigida) and the gray birch-aspen associations, 

 both of which are quite distinct and are sharply separated from all 

 others. The chief associations, both from the point of view of forest 

 value and area covered, are the spruce, the white pine, and the cedar. 

 Each of these trees forms a distinct association, with of course other 

 trees in mixture on certain distinct sites. But over a large part of the 

 island the three trees mingle in varying proportions in a rather discon- 

 certing manner. The complexity is probably attributable, in part at 

 least, to the struggle between the vegetation of the northern spruce re- 

 gion and that of the white-pine region. For rough descriptive purposes 

 it would probably suffice to recognize a spruce association and a mixed 

 conifer association, the latter composed of a variable mixture of spruce, 

 white pine, and cedar. But a better conception of the factors underly- 

 ing the forests will be secured by dividing the mixed conifer association 

 into the white pine and cedar associations which, together with the 

 spruce association, are the three distinctive units which so frequently 

 merge and tend to obscure the true character of the forests of the 

 island. 



Spruce Association 



Composition. — The characteristic spruce association is composed of 

 nearly pure spruce, but any forest containing 6 per cent or more of 



