198 WILLIAM S. COOPER 



at the southwestern end — an outpost of the Eastern Deciduous 

 Forest. The upland forest, which covers the island completely 

 with the exception of narrow strips along the exposed shores and 

 the bog areas in the valleys, is composed mainly of three trees, 

 in order of abundance: Balsam Fir (Abies balsamea), Paper Birch 

 (Betula alba var. papyrifera), and White Spruce (Picea cana- 

 densis) . In the general paper mentioned above I have shown that 

 this combination is the climax forest of Isle Royale, and prob- 

 ably of most or all of the Northeastern Conifer Region. 



The plant successions leading up to the establishment of the 

 climax forest are conveniently classified in two groups: the Xer- 

 arch Successions, having their origin in xerophytic habitats; 

 and the Hydrarch Successions, originating in hydrophytic hab- 

 itats. 



Under the first head there are two, the Rock Shore Succes- 

 sion and the Beach Succession, the latter not extensively devel- 

 oped upon Isle Royale. The course of the Rock Shore Succes- 

 sion is briefly as follows. The pioneers upon the rock shores 

 obtain their first foothold in three sorts of situations, thus initiat- 

 ing three separate lines of advance which may be conveniently 

 termed "Subsuccessions." In the Rock Surface Subsuccession a 

 crustose lichen stage is followed successively by foliose and fruti- 

 cose lichen stages, large cladonias being dominant in the last. 

 The Crevice Subsuccession is initiated by crevice herbs and these 

 are followed by trailing shrubs, and even trees frequently appear. 

 The Rock Pool Subsuccession has its origin in water-filled depres- 

 sions, and advances through the building of a mat after the 

 manner of the Bog Succession. These separate lines unite in 

 the formation of the Heath Mat, in which the elements of the 

 rock surface and pool vegetation are bound together by the trail- 

 ing shrubs from the crevices. A xerophytic forest stage, charac- 

 terized by Jack Pine (Pinus Banksiana) and Black Spruce (Picea 

 mariana), but seldom found pure, may here intervene, or the 

 climax forest may follow directly the establishment of the heath 

 mat. 



The Hydrarch Successions are also two in number, and of these 

 the Bog Succession is the more important. In the Delta Swamp 



