THE DECIDUOUS FOREST CLIMAX. 203 



Transeau (1913 : 189) has made a detailed study of the habitat and the 

 development of the littoral successions of Cold Spring Harbor. The serai 

 sequences and relations may be best siunmarized by figure 5. 



Nichols (1915 : 176) has sketched the hydrosere of lakes and swamps in 

 Connecticut. The serai zones are as follows: 



(1) pondweed zone {Potamogeton, Naias, Vallisneria, etc.); (2) water-Hly 

 zone {Castalia, Nymphaea); (3) bulrush-pickerelweed-cattail zone (Sdrpus, 

 Pontederia, Typha) ; (4) sedge aone {Car ex strida, Heleocharis, Glyceria, Juncus) ; 

 (5) shrub zone {Alnus, Cephalanthus, Comus, etc.); (6) swamp forest {Acer 

 riibrum, Ulmus americana, Betula lutea, Fraxinvs nigra). The formation of a 

 floating mat by sedges, and by shrubs, especially Decodon, is described, and the 

 relation to bogs indicated. The development of bog in Connecticut begins in 

 the floating mat with the appearance of Sphagnum. The bog stages are the 

 following: (1) the sedge {Car ex-Sphagnum); (2) the shrub {Andromeda, 

 Chamaedaphne, Kalmia, Ledum, etc., Sphagnum); (3) the tree {Picea nigra, 

 Larix laridna). They are often much fragmented and mixed, the sedge stage 

 occupying the hollows, the shrubs growing on hummocks, and the trees gradu- 

 ally invading the latter. 



XEROSERE. 



Cowles's study (1899 : 95) of the dunes of Lake Michigan is the classic 

 American work Ln this field, and is perhaps too well known to require abstract- 

 ing. Because of its value, however, the author's conclusions are repeated for 

 the benefit of those who may not have ready access to them: 



"The xerophytic beaches are subdivided into three zones: the lower beach 

 which is washed by summer waves and is essentially devoid of life; the middle 

 beach which is washed by winter waves and is inhabited only by succulent 

 annuals ; the upper beach which is beyond present wave action and is inhabited 

 also by biennials and perennials. There are also fossil beaches and gravel 

 terraces with a flora resembling that of the upper beach, but less xerophytic. 



"Perennial plants are necessary for any extensive dime formation on the 

 beach, since they alone furnish growing obstacles. Such plants must be pro- 

 nounced xerophytes and be able to endure covering or imcovering. The most 

 successful dune-formers are Ammophila arundinacea, Agropyrum dasystaehyum, 

 Elymus canadensis, Salix glaucophylla and S. adenophylla, Prunus pumila, 

 Populus monilifera. Ammophila and Agropyrum form low dunes that have a 

 large area, because of their extensive rhizome propagation. The Elymvs dunes 

 do not increase in area since rhizome propagation is absent. The Salix dunes 

 increase both in area and height, because of extensive horizontal and vertical 

 growth. The Populus dunes are the highest and steepest, since the cotton- 

 woods grow quite tall, but do not spread horizontally. 



" Small dunes are formed in more protected places by plants that are imable 

 to exist on the beach, or where there is rapid dune formation. Among these 

 secondary dune-formers are Andropogon, Ardostaphylus, Juniperus. Primary 

 embryonic dunes may pass gradually into this second type, as this latter 

 passes into the heath. 



"The stationary embryonic dunes on the beach begin to wander as soon as 

 the conditions become too severe for the dune-forming plants. The first 

 result of this change is seen in the reshaping of the dune to correspond with 

 the contour of a purely wind-made form. The rapidity of this process is 

 largely determined by the success or failure of the dune-formers as dune- 

 holders. The best dune-holders are Calamagrostis, Ammophila, and Prunus. 



