208 



CLIMAX FORMATIONS OF NORTH AMERICA. 



(1) Beach-sand-plain-heath-forest, (2) beach-sand-plain-thicket-forest, (3) 

 dune-thicket-forest. The sequence of the first is as follows: Cakile-Xanthium 

 formation, Artemisia-Panicum formation, Ardostaphylus-Juniperus formation, 

 Pinus strolrus formation, Quercus vdviina formation. The second differs 

 in the substitution of a Myrica thicket for the heath thicket, and of Prunus 

 serotina for Pinus strobus. The development on the dunes begins with 

 Popvlus, Ammophila, Andropogon, or Prunus pumila, and then passes into 

 heath or thicket, followed by Prunus or Pinus, and finally Quercus. 



Coons (1911 : 37, 55) has distinguished the following successional series of 

 habitats on the sand beaches of Saginaw Bay, Michigan: 



1. Open beach through drying and wind action becomes — 



2. Embryonic dime or upper beach, which through sand binders becomes — 



3. Established dune; through heaths, shrubs, and trees, this becomes — 



4. Dry ridge, and — 



5. Jack-pine barrens; humus formation and cessation of fire change this to — 



6. Mixed pine woods, which through hiuius formation become — 



7. Mesoph3rtic woods. 



The sequence of formations and associations is shown herewith: 



Snow (1913 : 45) has repeated her observations upon the dune sere of the 

 Delaware coast after an interval of 10 years, noting surprisingly little change 

 in view of the extreme instability of dune topography. The developmental 

 changes are summarized as follows: 



"1. Ten years is too brief a period in which to show any considerable change 

 in plant associations, even in so active a region as that of the dunes. 



"2. Progressive changes were observed: (a) advancement of the Hudsonia 

 complex toward the heath; (6) advancement of the heath toward the thicket; 

 (c) drying of isolated swamps and lakes; (d) fixation of the 'canal dune.' 



"3. Retrogressive changes were noted: (a) recession of the coast, resulting 

 in more frequent flooding of certain regions, thus continuing or increasing their 

 hydrophytic character; (b) movement of active dunes over the forest. 



"4. The character and position of the sand-clay ledge suggests that it may 

 have been an ancient sea-cliff. 



" 5. The presence of a ledge of peat in the beach indicates the location of a 

 former swamp. 



"6. This swamp was probably due to the presence of an ancient bar 

 inclosing a lagoon. 



"7. The present erosion of the coast indicates that this bar with its dunes 

 was moved inland over the swamp, while portions of the beach material were 

 carried northward to form the hook. 



"8. This process of erosion is still in progress." 



