282 



this region are sancl-clunes well developed. This is because the pre- 

 vailing winds are westerly, while the lake, from which the sand 

 must come, is to the eastward of the beach. The largest dunes are 

 about four meters high. They are protected from westerly wnnds by 

 woods of pine or oak. Towards the northern and southern parts of 

 the area, where there is no protection from winds, the dunes are sel- 

 dom more than four decimeters in hight. x\ll but one of the dunes 

 in the area are fixed dunes, either permanently or for a season only. 

 Traveling dunes, such as occur along the southern and eastern sides 

 of Lake Alichigan, are absent because the prevailing westerly winds 

 merely take away any loose sand and carry it back into the lake. The 

 one traveling dune is nine meters high, and is protected from westerly 

 winds by oak woods. In order to have any permanent dunes what- 

 soever the sand must be fixed by vegetation. 



The Dune Associations 



The different dune-forming plants give a more or less character- 

 istic apearance to the dunes on which they occur. The dune-former 

 is the all-important plant in the dune associations. Only a very few 

 other species are capable of withstanding such a severe habitat, and 

 as a consequence the dune associations are poor in species. As soon 

 as the pioneer species begin to accumulate humus, invaders appear 

 and assume possession, while the pioneers advance onward, in gen- 

 eral, towards the lake. The process is, however, very slow, and is 

 greatly hindered by severe wind storms and tidal waves. 



Dune associations are usually independent of one another, and the 

 dune complexes are built up in part by the growth of individual 

 dunes. When this occurs, succession takes place which leads to the 

 formation of the climax dune vegetation, as the juniper dunes may 

 be designated. 



THE CAEAMOVILFA DUNE ASSOCIATION 



The sand-binding grass, Calaiiwz'ilfa longifoUa, plays the most 

 important part in initiating new dunes on the upper beaches. This 

 grass is a most efficient sand-binder, and it will commence its growth 

 under more adverse conditions in this region than will any of the 

 others. The root system is extensive and forms a very dense tangle, 

 as shown in Figure i, Plate XLVIII. This plant always grows in 

 tufts, and as soon as the leaves appear sand begins to be caught 

 around the stems and lower leaves. The dune soon takes the shape 

 shown in Figure 2, Plate XLVII. From the windward side the dune 



