1913] COOPER—ISLE ROYALE 2r 
upon their own root systems for sustenance, and there is evidence 
that a considerable number of them become independent through 
the decay of the connecting branch. The habit is so common in 
the Isle Royale forests that a large proportion of the apparent 
balsam seedlings may be accounted for in this way. 
The preponderance of balsam in the young tree growth being 
accounted for, it is now necessary to explain its rapid decrease 
when greater size and age are considered. Several causes combine 
to bring this about. Abundant germination is itself a disadvantage, 
since it results in severe competition, much of the stand undergoing 
suppression and finally death. The species is very susceptible 
to fungus attacks and to diseases of many kinds. Rotten-hearted 
trees are very common. Witches’ brooms caused by a rust (Peri- 
dermium) are familiar objects. According to Moore and RocGErs 
(41), the liability to fungus attack is greater in pure stands than 
where trees are scattered. The common group habit of the species 
is therefore a disadvantage in this respect. The prevalence of 
heart rot, together with the natural brittleness of the wood, cause 
extreme liability to windfall, and broken trunks are a common 
sight, while uprooted balsams are rare. It is not surprising, in 
view of these facts, that in spite of its prolific power of germination 
the balsam never reaches the position of dominance in the mature 
stand. In a word, its high birth-rate is balanced by a high rate of 
mortality. 
BETULA ALBA var. PAPYRIFERA (paper birch).—The prominence 
of this species in the mature stand and in the general aspect of the 
forest has been noted, and also its comparative scarcity in the young 
growth. It is certain that the germination of the birch in this 
region is far from prolific. Very young seedlings were frequently 
seen, and in situations almost as varied as those inhabited by the 
balsam, but never in abundance as in the case of that tree. I 
quite frequently found very small seedlings in dense shade, but they 
were never more than five or six years old, indicating that conditions 
(probably light supply for photosynthesis) were not favorable for 
continued growth. Opportunity for successful reproduction comes 
usually, as in the case of the balsam, after windfalls (fig. 8). On 
account of its much less prolific germination the birch is far less 
