cial means of attaclinient to keep tlieiii from being 

 l>lo\vn away by the wind, such as strong claws or 

 cement glands on the feet, long bristle-like threads to 

 entangle among the moss filaments ; stickers or spines 

 covering the eggs. Since wet periods are often too 

 short to permit complete development, all stages must 

 he tolerant of desiccation, at which time activities 

 and growth are largely suspended. 



.Animal life in general and land snails in particu- 

 lar are usually more abundant in vegetation (grass- 

 land and forests) established on calcareous soils 

 derived from limestone than in the vegetation estab- 

 lished on soils derived from sandstone, granitic, or 

 volcanic rock. Calcium carbonate is a mineral essen- 

 tial to the metabolism of most animals and for build- 

 ing such skeletal structures as bones and shells. 

 Snails are less numerous in the grass stage than in 

 the later, moister forest communities that develoji in 

 tiie succession. 



SAND SERE 



Plant communities 



Sand is the product of mechanical pulverization 

 of various rocks. It is deposited by wind and water. 

 Where extensive areas of sand occur, strong winds 

 pile the sand into shifting dunes. These dunes have 

 a characteristic shape as the sand grains are blown 

 up a long, rather gentle windward slope and swept 

 over the crest onto a steep lee slope. Moving dunes 

 may engulf whole forests ; they eventually move on, 

 leaving the denuded trunks of trees that they have 

 smothered. The dunes continue to move until they 

 reach the shelter of some other dune, get beyond the 

 full force of the wind, or until invading vegetation 

 covers the surface and anchors them down. The most 

 successful sand-binding plants are the grasses Am- 

 mophila. Calamoznlfa, and Agropyron, willows, sand 

 cherry, and cottonwoods. Willows and cottonwoods 

 will survive even when almost buried. Each succeeding 

 stage ties the sand down more firmly, but any break 

 in the vegetation occasioned by a blowdown of trees 

 or disturbance by man may invite the wind to start 

 moving the exposed sand, and change the partially 

 anchored dune again into a moving one. Only when 

 the pine stage or the black oak stage is reached does 

 the dune become relatively secure from the wind. 



The plant sere on the south shore of Lake Michi- 

 gan consists essentially of the following stages 

 (Cowles 1899) : 



Lower beach: \\'ashed by summer storms and 



devoid of vegetation. 

 Middle beach: Washed only by severe winter 



storms : comparatively dry in summer ; upper 



limit marked by driftwood and debris. Scat- 

 tered annual plants present. 



Calamagrostis-Andropogon associes (upi)er 

 beach) : This is where the dunes begin to form. 

 In this early developmental stage (associes) 

 grasses are dominant, particularly Calama- 

 (jrostis lonc/ijolia. Amiropogon scoparius, 

 Agropyrum dasystachymn, Amniophila are- 

 iiaria. Elymiis canadensis; various biennial and 

 perennial herbs make their appearance. The 

 sandbur grass occurs extensively in some areas. 



Prunus-Cornus associes : The coinmoner .shrubs 

 are sand cherry, chokecherry, red-osier dog- 

 wood, creeping juniper, and the frost grape 

 vine. Shrubs may invade the grass directly but 

 become more common in the following tree 

 stages. 



Populus-Populus associes: The first tree stage 

 in the southern portion is made up principally 

 of the eastern cottonwood, and in the northern 

 portion, of the balsam poplar. The trees com- 

 monly occur in open stands with grasses and 

 shrubs forming the lower strata. The habitat is 

 essentially forest-edge. The shrub and cotton- 

 wood stages are often missing so that the sere 

 progresses from the grass directly to the pine 

 or black oak stage. 



Pinus-Pinus associes: Jack pine, red pine, and 

 eastern white pine may invade one after an- 

 other, commonly forming mixed stands. North- 

 ern white-cedar and eastern redcedar also oc- 

 cur ; the former, more commonly northward. 

 Succession to this stage is mainly contingent on 

 stabilization of sand in dunes, and more efficient 

 utilization of water resources. For succeeding 

 stages to emerge, soil must develop by deposi- 

 tion of humus. The floor of pine forest is cov- 

 ered with a carpet of needles, although patches 

 of bare sand still occur. As the sere advances, 

 all bare areas become covered with a layer of 

 humus. 



Quercus velutina consocies: Black oak often 

 forms a nearly homogeneous stand that may 

 persist for a long time. 



Quercus-Carya associes: Black, white, and, to 

 a lesser extent, red oaks are commonly mixed 

 with shagbark and bitternut hickories and, in 

 moist habitats, American basswood. 



Fagus-Acer association: When soil humus and 

 moisture become sufficient, American beech and 

 sugar maple invade the sand to form the final 

 climax stage. 



In other localities, the taxonomic composition of 

 the communities, especially the later stages, differs 

 considerably. The character of the climax varies ac- 

 cording as climate and geography, but perhaps the 



Rock, sand, and clay 105 



