THE DECIDUOUS FOREST CLIMAX. 205 



procumhens. The heath arises on fossil beaches, secondary embryonic dunes, 

 or wherever the wind is relatively inactive and where the conditions are too 

 xerophytic for the development of a deciduous flora. Before long the heath 

 passes into a coniferous forest, in which Pinus banksiana, P. strobus, or P. 

 resinosa dominate. Coniferous forests also occur on sterile barrens and in 

 bottoms, where the conditions are also unfavorable for deciduous forests. A 

 slight change in the physical conditions may bring about the rejuvenation of 

 the coniferous dimes, because of their exposed situation. Rejuvenation com- 

 monly begins by the formation of a wind-sweep; the vegetation on either hand 

 is forced to succumb to sand-blast action and gravity. 



"The evergreen floras are more and more common northward, while to the 

 south there are developed forests in which Quercus coccinea tindoria prevails. 

 The oak forests are more common on inland dunes and on southern slopes. 

 The oaks may follow the pines, when the areas occupied by pines become 

 sufficiently protected from cold winds. The pines have a much wider range 

 of life conditions than the oaks, since they appear at lower levels, higher levels, 

 and on northern or windward slopes. The oaks flourish best on southern 

 slopes. The flora of the oak dunes is xerophytic, but of the desert type, while 

 that of the pine dunes is of the arctic xerophytic type. The pine dunes have 

 a northern flora, the oak dunes a southern flora." 



Kearney (1900, 1901) has indicated the general succession on dunes in his 

 study of Ocracoke Island, and of the Dismal Swamp. The sequence is illus- 

 trated by the grouping of the sand communities in the Dismal Swamp region: 



Sand Btrand formations. 



Beach and outermost dunes: AmmophUoTJniola association. 

 Middle (open) dunes. 



Dry sou — Myrica association. 



Wet soil, dime marshes — Juncus dichotomus association. 

 Inner (wooded) dimes. 



High dunes — Quercus virginiana association. 

 Strand pine woods. 

 Forest formations. 

 Mixed forest. 



Cowks (1901 : 156) has sketched the general course of succession on rock 

 hills, sand hills, clay hills and lake bluffs: 



The lichen pioneers of the first are followed by crevice mosses, especially 

 Ceratodon and Bryum, and these by herbs and grasses, such as Solidago nem- 

 oralis, Potentilla arguta, Poa compressa, etc. The deepening soil permits the 

 entrance of shrubs, such as Prunus, Physocarpus, Rhus, Ptelea, Pyrus, etc., 

 w;hich are finally replaced by mesophytic forest. The secondary sere of clay 

 hflls begins with xerophytic annuals and perennials. These are followed by 

 the development of a thicket of Populus, Pyrus, Crataegus, etc., the forerunner 

 of the oak-hickory forest, which may be finally succeeded by the maple-beech 

 climax. The dune sere has already been considered (p. 203) . The lake-bluff is 

 first covered with xerophytic herbs, Melilotus, Aster, Equisetum, and various 

 grasses. A xerophytic thicket follows, dominated by Juniperus, Salix, Cornus, 

 Shepherdia, and Rhus. The tree stage which ensues consists of Populus' 

 Ostrya, Pinus strobus, Juniperus, and Quercus, which finally yield to the meso- 

 phytic forest. 



Whitford (1901 : 295) has traced the general features of the development of 

 the forest of northern Michigan from initial areas of sand, rock, and water. 

 The best series of stages perhaps is shown by sand: 



