62 THE PLANT SOCIETIES OF 



tation commonly gets its first foothold at the base of lee slopes about 

 the outer margin of the complex, because of soil moisture and protec- 

 tion from the wind. The plants tend to creep up the slopes by vege- 

 tative propagation. Antecedent and subsequent vegetation work 

 together toward the common end. Where there is no antecedent vege- 

 tation, Ammophilaand other herbs first appear, and then a dense shrub 

 growth of Cornus, Salix, Vitis cordifolia (Frost grape), and Prunus Vir- 

 giniana (Chokecherry). Capture may also begin within the complex, 

 especially in protected depressions, where Salix longifolia is often 

 abundant. 



D. The established dunes. — No order of succession in this entire 

 region is so hard to decipher as is that of the established dunes. 

 There are at least three types of these dunes so far as the vegetation is 

 concerned, and it is not yet possible to figure out their relationships. 

 The continuation of the conditions as outlined in the preceding para- 

 graph results in a forest society on the lee slope, in which is found the 

 basswood, together with a most remarkable collection of mesophytic 

 trees, shrubs, and climbers, which have developed xer'ophytic struc- 

 tures. These dunes are evidently but recently established, as is shown 

 by the absence of a- vegetation carpet; furthermore, the slopes are 

 almost always steep. 



Again, there are forest societies in wliich the pines dominate, either 

 Pinus Banksiana or P. Strobus (White pine). These arise from a heath, 

 composed in the main of Arctostaphylos (Bearberry) and Juniperus. 

 The heath appears to originate on fossil beaches or on secondary 

 embryonic dunes or other places where the danger of burial is 

 not great. It will be noted that both the heath and the pine forest are 

 dominated by evergreens. These Societies commonly occur near the 

 lake or on lakeward slopes, which are northern slopes as well. On 

 these coniferous dune slopes there»is to be found another notable col- 

 lection of northern plants, resembling ecologically the peat bog plants 

 already mentioned. Heaths and coniferous forests also occur on sterile 

 barren sand and in depressions where the conditions are unfavorable 

 for deciduous forests. A slight change in the physical conditions may 

 bring about the rejuvenation of the coniferous dunes, because of their 

 exposed situation. This rejuvenation commonly begins by the forma- 

 tion of a wind sweep, and the vegetation, on either hand is forced to 

 succumb to sand-blast action and gravity. 



A third type of established dune is that in which the oaks pre- 



