distance below the surface. The herbaceous and shrub vegetation of 

 the previous associations persists with few additions. The dominant 

 and almost the only tree species (Populus deltoides) gives its name to 

 the association, but is occasionally replaced by the closely allied Populus 

 balsamifera. The germination and development of these trees are 

 dependent upon such moist areas as pannes and the margins of ponds. 

 (See Fuller, Trans. 111. Acad. Sci. 5:137-143. 1912.) 



The Cottonwood owes its successful establishment and permanence 

 upon the moving dunes to its great powers of vegetative activity which 

 is seen in the production of adventitious roots from the buried trunk 

 and branches and also in the development of adventitious shoots from 

 its uncovered roots. 



4. The pine dune association. — Soon after the fixation of the dunes 

 there develops an association succeeding the cottonwood and charac- 

 terized by evergreens. Among the dominant plants are species of pine, 

 juniper, and arbor vitae, together with an undergrowth in which the 

 heath family is well represented in species of Arctostaphylos, Pyrola, 

 GauUheria, etc. The herbaceous and shrub vegetation becomes much 

 more abundant and is composed of species quite different from those in 

 the former association. Humus formation proceeds, but the water con- 

 tent of the soil is less than in the previous association on account of the 

 greater demands made upon it by the increasing amount of vegetation. 

 The evaporating power of the air is, however, much less, and hence per- 

 mits the development of seedlings of oak and other trees which dominate 

 the succeeding association. The pine association is short-lived, but the 

 undergrowth finally shows very decidedly mesophytic forms, especially 

 upon the north face of slopes. This association was formerly well 

 developed over much of the area between Buffington and Gary, Ind., 

 where a few remnants are still to be seen. It occurs between Miller and 

 Dune Park, Ind., and in a modified form at Lake Bluff, 111., and Sawyer, 

 Mich. 



5. The black oak dune association. — This forest association consists 

 of a somewhat open and often almost pure stand of Quercus velutina with 

 an undergrowth of rather xerophytic herbs and shrubs. The evergreen 

 forms of the previous association disappear and are replaced by such 

 shrubs as species of Vaccinium, Rhus, Prunus, Sassafras, Viburnum, 

 etc., while noticeable among the herbs are species of Solidago, Asclepias, 

 Opuntia, Smilacina, etc. Soil moisture and evaporation conditions 

 differ but little from those of the previous association, and the increase 

 of humus in the soil is very slow. The association apparently persists 

 for a much longer time than any of its predecessors, and is seen exten- 



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