140 MICHIGAN SURVEY, 1908. 



it tends to encroach npon the open places with a zone of Jack Pine,, 

 aspens, or White Birches. These areas are largely strips along the crests 

 of ridges or small park-like openings on rather level rock. In no case are 

 these single areas large, so that the habitat is only extensive in the 

 aggregate. With the presence of the open aspen and birch wQods, the 

 following society is likely to be characteristic : — Junco, Oven Bird, Eed- 

 eyed Vireo, Chipping Sparrow, White-throated Sparrow, Flicker, Cedar 

 Waxwing, Wilson's Thrush and the Chickadee. As the deciduous trees 

 are replaced by the open encroaching conifer forest, the Song Sparrow, 

 the Nashville, Myrtle and Black-throated Green Warblers and Wilson's 

 and Olive-backed Thrushes, which frequent the forest margins, increase 

 in abundance. The Oven Bird has an extensive northern range from 

 Labrador into the Yukon Valley and may well have been a very early 

 pioneer upon the island as the aspens and birches were probably the 

 first broad-leaved tree arrivals. From the above it is seen that this 

 means an extensive variety, but as the dominance of the climax forest 

 encroaches this number again becomes reduced. 



The composition of the society varies somewhat, depending upon 

 the surroundings, as proximity of the present shore or distance from 

 it. Many of these openings are continuous with the present beach. 

 It is not improbable that this was a prominent society whenever the 

 waters fell rapidly- from the island between, rather stationary levels. 

 This has been a society decidedly on the decline with the encroachment 

 of the forest. 



Probably this association varies considerably in its composition, and 

 has done so in the past; but its main features are fairly constant. 

 These variations seem likely, through the influence of openings pro- 

 duced by flres which, when extensive, may have caused a new equilib- 

 rium among those species frequenting openings. 



The Burned Area Association. 



This phase should perhaps be considered as supplementary to the 

 aspen-birch association just considered. A Are brings about a. reversal 

 of conditions through the destruction of the forest, and in some esses, 

 a part of the soil as well. As there are all degrees of extent and com- 

 pleteness in this process, there is a con-esponding variation in the 

 details of the resulting succession, at least in its early stages. It is 

 on'y when there is a very complete destruction of the vegetation that 

 the continuity with former occupancy is wholly broken. 



The easily inflammable character of these conifers, even when in a 

 green condition, makes it likely that natural causes, such as lightning 

 or marsh gas {cf. Penhallow, '07), may have been influential. The 

 proximity of the gas supply and the conifers is of interest as this may 

 influence their liability to flre and thus to this sort of reversal of con- 

 ditions. Thus liability to fires is I'ather characteristic of the region, 

 and man's influence has tended merely to reinforce rather than to intro- 

 duce this feature. Thus it seems probable that flres have been a factor 

 in supplementing the natural park-like openings. In. addition to the 

 burned areas found upon Isle Eoyale, other limited open areas are due 

 to cultivation and are kept open. 



The birds characteristic of the more open situations are the Sharp- 



