ECOLOGY OP ISLE ROYALE. 223 



bare islands was well illustrated in the case of one of the small islands 

 of this 'group. Its smooth, sloping surface was bare except for a few 

 small patches of crustaceous lichens and a single large procumbent 

 juniper. The juniper was growing in a crevice along which it had 

 reached for several feet in either direction, occasionally rooting along 

 the crevice which held it more securely in place. The spaces between its 

 dense sheltering branches were filled with a vigorous growth of moss 

 which doubtless started on the wind-blown material that had lodged 

 there. So solidly had the moss filled the spaces between the branches 

 where it was growing that in breaking oft' a portion of the juniper every- 

 thing was stripped off down to the bed-rock. As such a juniper patch 

 spreads, and the humus made by the moss increases, other plants come 

 to grow on the juniper patch, and an ever-increasing heath mat is formed. 

 Other similar crevices may, in like manner, spread to join this, and in a 

 comparatively short time the entire surface is carpeted with vegetation. 

 On other small islands crevice trees and shrubs have contributed shade 

 and partial protection from the wind, and the process has gone on even 

 more rapidly. Had there been no crevices to enable these higher pioneer 

 plants to secure a foothold the process of vegetative capture would have 

 gone on infinitely slower. What the possible steps are in such a case 

 may best be considered in connection with the rock shore-heath-forest 

 series to be referred to presently. 



To suggest the severe and varied conditions of the exposed rock shore 

 the following is cited. On a bare, gently sloping (10°-12°) portion of 

 the rock shore near Eock Harbor, there were, in an area approximately 

 40 feet square, over 100 fresh scars where the thin (1-6 to 1-8 inch") 

 patches of rock had recently been broken off. These patches varied in 

 size from 12 inches in diameter down : some were covered in part by 

 lichens (principally Parmelias), others were entirely bare. The intense 

 daily heating and expansion to which the immediate surface of the dark 

 colored, exposed rocks is subjected, together with the rapid cooling and 

 the resulting contraction at night, doubtless has much to do with weak- 

 ening the immediate surface, and starting the chipping. The freezing 

 of moisture in the rock surface may have been responsible for the final 

 breaking away and lifting. 



For a brief survey of the vegetation from the water's edge back through 

 the heath zone to the forest at the top of the gently sloping rock shore 

 area, V 2, (designated as "the heath zone and beach" of Siskowit Bay) 

 will be selected as a typical locality, and supplemented by additional 

 observations on similar places elsewhere along the southern shore. The 

 portion of rock shore to be considered has a rather uniform slope of 

 about 10° and a width of 200-250 feet from the water's edge back to the 

 forest at its summit. 



The first zone of no vegetation extends back about 20-25 feet from the 

 water's edge, although the winter waves doubtless reach far beyond this. 

 Back of this occur, in turn, the crustaceous and foliaceous lichen zones, 

 which meet in a somewhat irregular tension zone that can, nevertheless, 

 be distinguished by looking up or down the shore. The lichens of these 

 zones are included in the annotated list, and will not be enumerated here. 



Numerous crevice plants (as already listed under shore forms) make 

 their appearance in the crustaceous and foliaceous lichen zones, also 

 Thuja occidentalis and Picea canadensis, the former being the hardier 



