328 Annals of the Carnegie Museum. 



" 53 P^r <^6"t of the individuals on the slope above the 760-foot con- 

 tour line being black oaks." "' Cowles finds, near Chicago, the black 

 oak predominating on the south slopes of the established sand-dunes and 

 on the higher sandy ridges and beaches of glacial origin. ^^ Cowles 

 further savs : "The future of the vegetation on the established dunes 

 and beaches is somewhat problematical. From analogy with other 

 plant-societies in this region, and from established dunes in Michigan, 

 we should expect a mesophytic forest, probably of the white oak-red 

 oak-hickory type at first and then followed by a beech-maple forest." 



The predominance of the black oak, rather than other hard woods 

 of the region, as a successor to the white pine is, perhaps, partly to 

 be explained by the fact that the black oak is of a more xerophytic 

 habit and, partly, by the fact that ''The percentage of ash in the 

 wood of such trees as form the principal covering of a dune region is 

 relatively small. As seen in the analyses reported in the volume of 

 the Tenth Census on the forest trees of North America, the pines have 

 an average range of .19 to .23 per cent. The two most common 

 oaks, Q. velutina and Q. coccinea, have .28 and .19 per cent, respec- 

 tively."*^ The white oak, burr oak, beech, sugar maple, basswood, 

 and hemlock have a considerably higher per cent, and are less common 

 or wholly absent in such sandy areas. 



Livingston's conclusions with regard to the relations of soils to 

 vegetation in certain portions of Michigan would apply also to the 

 distribution of the plant-formations on Presque Isle. Livingston's 

 conclusions are briefly that "the main factor in determining the 

 distribution of forests on the uplands of this region is that of the size 

 of the soil particles."** This factor determines directly the amount 

 of air in the soil, and thus indirectly the extent of formation of humus, 

 nitrates, and other soluble salts. 



The Beach-Sand Plain-Thicket-Forest Succession. 



The Beach-Sand Plain-Thicket-Forest Succession is very closely 

 related to the Beach-Sand Plain-Heath -Forest Succession just de- 



si Brown, F. B. H. "A Botanical Survey of the Huron River Valley, III." 

 Bot. Gaz.^a,o: 274-275, October, 1905. 



e''' Cowles, H. C. /. c. Bot. Gaz., 31 : 174-177, March, 1901. 



** Hill, E. J. "Flora of the White Lake Region, Michigan, and its Ecological 

 Relations." Bot. Gaz., 29: 434, June, 1900. 



** Livingston, B. E. /. c, p. 40. 



