351 



cessions this province maintains its dominance. The prairie can not 

 natnrally supersede it because the chmatic conditions are suitable for 

 the development of forests, and the prairie, as a unit, can not make 

 headway under shade. Conifers can not succeed the deciduous for- 

 est because they can not reproduce themselves in it. 



In view of these facts, it is plainly evident that, under the pres- 

 ent conditions of climate, the deciduous forest province is the domi- 

 nant one in this region, and if left to itself in nature would ulti- 

 mately occupy the entire region. 



Summary 



1. The Beach area is a strip of low sandy land bordering 

 Lake Michigan in northeastern Illinois and southeastern Wisconsin. 

 Its length is about 14 miles and its extreme width is a little over a 

 mile. Its maximum elevation above Lake Michigan is less than 30 

 feet. 



2. This region lies a little way south of the southern limits of 

 the Northeastern Conifer Province, within an area of competition 

 between the Prairie and Deciduous Forest provinces, in a climate 

 which is favorable to tree growth. 



3. During postglacial times the entire region was submerged, 

 and w^ithin the past eighty years the region has at times been virtu- 

 ally inundated. 



4. The region contains 55 plant associations, representing three 

 plant provinces : Northeastern Conifer, Prairie, and Deciduous For- 

 est. 



5. A study of the successions between the different plant asso- 

 ciations gives a very satisfactory understanding of plant dynamics. 



6. The two fundamental starting-points for genetic "series are 

 the open water of Lake Michigan and of the streams that flow into 

 it. The lines of succession commence with open water and proceed 

 through stages of progressively increasing dryness, which culminates, 

 in the Beach area proper, in the Ijlack oak association. The interme- 

 diate steps group themselves along several genetic lines. 



7. Commencing with Lake Michigan, one genetic line extends from 

 acjuatic algae through associations inhabiting progressively drier soil 

 in the depressions and swales between the ridges. Another line be- 

 gins with the plants that inhabit the open beach, where they are ex- 

 posed to extreme xerophtic conditions, because of a continual addition 

 to the food in the soil, and advances to associations of an increasin"- 

 number of species and a higher type of vegetative development. A 

 third line commences in the streams with plants which are whollv 



