CHAPTER XIX 

 TYPICAL EXA^IPLES OF PLANT SUCCESSION 



Having discussed plant succession in general in the preceding 

 chapter we will describe in the present chapter some specific seres. 

 Lack of space will not permit us to discuss the many dilTerent suc- 

 cessions that are progressing in different parts of the world. We will 

 therefore describe one typical sere for each of four general types of 

 bare areas, sand, clay-gravel, rock, and water, and make brief 

 mention of only a few of the better known modifications. 



135. Plant Succession on Sand.— Sand areas are found in North 

 America along the shores of the Atlantic and Pacific Oceans, along 

 the shore of Lake Michigan, in parts of the desert region of the south- 

 west, in the sand-hill region of Nebraska, and in several other limited 

 regions in the Mississippi Valley. We shall discuss the development 

 of the beech-maple-hemlock association along the shore of Lake 

 Michigan in Northern Indiana since this has been more thoroughly 

 studied than any other sand succession. 



An area of sand that is unoccupied by plants is exceedingly 

 unstable. Sand which is thoroughly wet is not readily moved by 

 the wind but after each rain the superficial layer quickly dries out 

 and is then subject to being blown about by the wind. Any small 

 object or an unevenness of the surface may serve as a slight check 

 upon the wind and cause it to deposit the sand in a mound. This 

 mound increases in size and becomes what is called a sand dune. 

 Often the sand is more or less constantly being blown away from one 

 side of such a dune and deposited on the opposite side so that the 

 dune as a whole slowly moves with the wind and is spoken of as an 

 active dune. Such a dune, while very dry at the surface, usually 

 contains an adequate amount of moisture a short distance beneath 

 the surface. There is usually an almost total lack of organic matter 

 and the essential mineral elements may also be practically lacking or 

 present only in unavailable forms. 



Under these unfavorable conditions there are but few plants that 

 can gain a foothold. In the xerarch succession the most successful 

 pioneer is the beach grass (Ammophila arenaria) (Fig. 98). This 

 grass and the wormwood {Artemisia) (Fig. 99) seem to be the only 

 plants that can grow to full maturity in pure dune sand. They have 

 (220) 



