566 



THE COMMUNITY 



to the regular changes that take place in a 

 flask of hay infusion. The diflEerence is that 

 the natural community usually has its biotic 

 changes augmented by the slower physio- 

 graphic changes, so that its study becomes 

 both quantitatively and quahtatively more 

 diflBcult; then, too, it changes much more 

 slowly. 



In sequences of natural communities, one 

 of the best documented is that of the forest- 

 on-sand sere, embracing the sand dune area 

 of northern Indiana and southwestern 

 Michigan. This area includes a series of 



TIME IN MONTH 



Fig. 203. Progressive decrease in amounts of 

 ultraviolet from pioneer to climax community in 

 the forest-on-sand sere of northern Indiana. 

 Compare with Figure 183. (After Strohecker.) 



communities that begins on the shores of 

 Lake Michigan and continues inland several 

 miles. Besides its documentation, the value 

 of this example is enhanced by its geologi- 

 cal background, knowledge of which has 

 made possible a reconstruction of the major 

 features of the succession of communities in 

 terms of post-Pleistocene history. 



Briefly, as the water accumulated in front 

 of the retreating Wisconsin ice sheet, Lake 

 Chicago was formed behind the morainic 

 barriers. As this lake drained off through 

 the Sag outlet, the water level gradually 



fell, exposing the Chicago plain and forming 

 the present, smaller Lake Michigan. The 

 gradually receding water exposed a series 

 of progressively younger beaches, so that 

 the present sequence of communities, from 

 the shore inland, represents horizontally 

 what presumably occurred vertically as the 

 lake retreated through the centuries (Fig. 

 202), exposing progressively lower, terres- 

 trially uncolonized areas. 



The open beach (1), including the beach 

 drift biocoenose discussed previously (p. 

 534), is the initial community; it is often 

 divided into lower, middle, and upper 

 beach. This is followed (2) by the fore- 

 dune community of sand-binding grasses 

 and sand cherry. The first b'ee-bearing 

 stage in this sere (3) is the cotton wood 

 community, a loosely organized pioneer 

 stage on shifting sands just back of the 

 foredune, and exposed to the wind-swept 

 lake front. At times the cottonwood stage 

 is thought to precede the foredune stage. 

 As one goes inland, the jack pine— juniper- 

 bearberry community (4) is followed by 

 (5) an oak forest community, or a subseries 

 of oak stages. This latter, in turn, is suc- 

 ceeded by a sugar maple-beech forest (6), 

 which has been able to stabilize its metab- 

 olism sufficiently to endure under present 

 chmatic conditions; that is, the sixth type 

 of community listed is regarded as the cH- 

 max community of this particular sere, and 

 is analogous to the Vorticella stage in the 

 hay infusion sequence. The particular serai 

 sequence given here is subject to consider- 

 able variation and modification, as in the 

 hay infusion. 



Each of the intermediate forest-on-sand 

 stages, other things remaining favorable, 

 was in past time preceded by a more primi- 

 tive or a more pioneer community or 

 associes. Or, from another point of con- 

 templation, each will give way to a 

 more advanced serai stage in the series. 

 That is, in time, if unmolested by man's 

 activities, the present coniferous stands will 

 give way to the oak woodlands, and these 

 oak stands will give way to the climax of 

 the sere. The rate of change may vary 

 from stage to stage, or from locality to 

 locality, and edaphic conditions may inter- 

 fere, but the general accuracy of this pic- 

 ture is established. 



For example, the physiographic and 

 geologic background was reported by Salis- 



