1913] COOPER—ISLE ROYALE 127 
C. alpesiris being about equally abundant. There is frequently 
considerable young growth of Picea mariana beneath the pines. 
The evidence derived from the examination of numerous localities 
indicates that such a type of forest will gradually become more 
mesophytic in character as the vegetation of the forest floor 
increases in amount and in water-holding capacity. Changes 
occur in the character as well as in the amount of the undergrowth. 
Mosses of the climax forest invade the areas dominated by the 
cladonias, growing around and over the masses of the lichens, 
Fic. 21.—Ground cover in the jack pine-black spruce forest: the three cladonias 
of the heath mat (right), C. rangiferina, C. sylvatica, C. alpestris, invaded by the 
climax forest moss Calliergon Schreberi (left). 
finally smothering them to death. Calliergon Schrebert, which 
among the forest mosses endures the driest conditions, is the most 
important of these (fig. 21). This species remains the most im- 
portant element in the herbaceous vegetation throughout the 
intermediate forest stages, and frequently after the establishment 
of the climax type. 
Picea mariana gradually increases, and at the same time the 
climax trees begin to be of importance. Here too the phenomenon 
of telescoping is to be seen, for we seldom find an area of jack 
