1913] COOPER—ISLE ROYALE 227 
them were scattered black spruces, few more than 1 m. high, also 
even-aged, and averaging more than a year older than the pines. 
In extensive fires the patches of bog forest occupying the depres- 
sions often escape entirely because of abundant moisture. When 
they are burned over the coniferous element is destroyed, and the 
birches if present sprout from the stump. The underground por- 
tions of the two species of Alnus survive and renew growth, and the 
result is frequently a dense alder-birch thicket, which probably 
passes directly into the climax type. When the scattered trees 
growing upon an open bog are killed by fire the sphagnum, being 
usually saturated with water, seldom suffers severely. New bog 
trees begin growth and the succession goes on as before. 
SUMMARY.—THE SUCCESSIONS 
Primary successions 
The xerarch successions 
Every part of Isle Royale has at some point of its subaerial 
history been shore. 
The present coast of the island is made up of rock shores and 
beaches, the former being much the more extensive. Each type 
possesses its characteristic series of successional stages, the ROCK 
SHORE SUCCESSION and the BEACH SUCCESSION, both resulting 
finally in the establishment of the climax forest. 
With regard to area vegetated through its instrumentality, the 
rock shore succession is by far the most important of all the suc- 
cessions of Isle Royale. 
The full series of the rock shore succession includes in its early 
stages three subsuccessions which later unite into a single series. 
The rock surface subsuccession advances through crustose lichen 
and foliose lichen stages to a condition in which the large cladonias 
are the most important element. The process of invasion along this 
line alone is very slow. 
The pioneers of the crevice subsuccession are certain herbs, 
notably Potentilla tridentata, whose principal réle is the formation 
of humus. Trailing shrubs succeed them, the most important 
being J uniperus horizontalis, J. communis vat. depressa, and 
Arctostaphylos Uva-ursi. These spread over the rocks from the 
