42 BOTANICAL GAZETTE [JANUARY 
Since glacial times there has been a continual northward advance 
of the forest, with the conifers as the pioneers, closely followed by 
the hardwoods. The problem as to whether the extension of the 
latter is still going on might be studied to good advantage in such 
localities as the southwestern end of Isle Royale, and Michipicoten 
Island. The large size and thriftiness of the maple at its northern- 
most limit would seem to indicate that it has not reached its climatic 
limit (BELL 8). The manner and causes of ‘‘climatic successions,” 
or the invasion of one climax forest by another, are still to be worked 
out. 
SUMMARY.—THE CLIMAX FOREST 
I. The dominant forest of Isle Royale is composed of Abies 
balsamea, Betula alba var. papyrifera, and Picea canadensis, with a 
few other species occasionally present. Abies, all sizes and ages 
considered, is by far the most abundant, but the greater number of 
individuals are small. Betula, although conspicuous, is not abun- 
dant, and young trees are scarce. Picea is rare, though occasional 
specimens tower high above the other trees. Shrubs and herba- 
ceous growth are sparse except in partial openings. The most 
important element in the latter is the moss contingent, which is 
responsible for the formation of great amounts of humus. 
II. Studies of individual species gave the following results. 
Abies is preponderant in the young growth because (1) the seedlings 
make a successful start in almost any situation provided sufficient 
light be available; (2) the species reproduces abundantly by layer- 
ing. Its rapid decrease when greater size and age are considered 
is due to (x) competition because of abundant germination; (2) 
fungus attacks, and (3) brittleness of wood, both resulting in 
extreme liability to windfall. Its high birth-rate is balanced by @ 
high rate of mortality. Betula does not germinate abundantly in 
the forest, but, because it is not liable to disease and windfall, 
holds its own with Abies. Even when broken off by severe winds 
it has a means of recovery in its ability to produce stump sprouts. 
Its more rapid growth gives it an advantage in competition with 
Abies. Abundant light is necessary for successful reproduction. 
Low ee is compensated by a very low mortality. Picea is 
