1913] COOPER—ISLE ROYALE 135 
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the nearly bare rock surfaces on which the early lichens and crevice 
_ plants are almost the only vegetation, to the climax forest in its 
full development. Intermediate stages are often entirely lacking. 
The causes that bring about this abrupt transition are as follows. 
On account of the immediate proximity of the open lake the limit 
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se Juniperus communis d Jd 
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Picea ean ensis FLEA Rens 5 Shore-li — and crevice plants 
Betula alba meets 5 é Humus with forest vegetation 
: Fic. 28.—Quadrat 8: located on island illustrated in i. 27; for a uname of 
symbols see fig. 6. 
of possible forest extension is very high. Above this line invasion 
has been sufficiently rapid to bring about the development of the 
climax forest upon the whole of the forestable territory. The inter- 
_ mediate stages have been “‘pinched out,” as it were, by the advance 
of the climax forest against the ee barrier of the waves 
: and i ice. 
