142- 



canadensis, var. illinocnsis is again the most important member of the 

 association. The perpetuation and vertical growth of the river dune 

 arc chiefly due to its efficiency as a sand-binder. The general height 

 of the dune thus held is from 15-30 feet (5-10 m.) above the general 

 level of the sand, but a maximum height of about 80 feet (25 m.) is 

 attained in the Oquawka area, or fully 100 feet (30 m.) above the 

 high-water level of the river. It is noteworthy that this highest point 

 is occupied by a loose patch of Rhus, evidently of great age. Asso- 

 ciated with Rhus on these dunes are similar dense patches of Ccano- 

 thus oz'atus and, occasionally, of Rhus Toxicodendron. This shrubby 

 habit of the last species was not observed elsewdiere in the region, 

 but is very common along the dunes of Lake Michigan, particularly 

 toward the north, and is reported from Lake Erie by Jennings 

 (ipop). There are also the usual bunches of Tephrosia virginiana, 

 Panicuni zirgatuni (PI. XX, Fig. i), and, more rarely, Bragrostis 

 trichodes and Sporobolus cryptandrus. Intervening spaces of open 

 sand are occupied by the usual members of the blowsand association. 



If the erosion b_y the river proceeds at such a rate that the crest 

 of the dunes remains relatively stable for some years, opportunity is 

 given for the development of a higher type of vegetation. The first 

 step in this succession depends upon the introduction of seeds by 

 wind or animals from the alluvial bottom-lands. The species most 

 frequently introduced in this way are Ulnius aincricajia and Fraxi- 

 nus pennsylvanica, var. hmccoJata, both of which have light winged 

 seeds. Pods of Glcditsia triacanfhos are blown up the slope from the 

 trees on the river bank lielow, and more rarely Juglans nigra devel- 

 ops from seeds probably carried by animals. Seeds of Acer saccha- 

 riiiuni were also found on the dunes, but they probably do not germi- 

 nate, since no young plants were seen. These trees are not numerous, 

 and never reach a large size, partly because of the unfavorable habi- 

 tat, but chiefly because of the general movement of the dune. All 

 the older trees have jiortions of their root systems exposed. One ash 

 tree, 8 inches (2 dm.) in diameter, had the base of its stem 3 feet 

 (i m.) above the surface and 15 feet (5 m.) behind the present crest 

 of the dunes (PI. XVIII, Fig.^ 2). the ash, which is by far the 

 most abundant of the trees, usually branches freely from the base, 

 forming a complex of stems. 



The trees offer a roosting place for l:»irds, which in turn serve as 

 agents in the dispersal of several shrubs and lianes. These at once 

 spring up beneath the trees, and develop the dune thicket associa- 

 tion. The mature thickets (PI. XIX, Fig. i) are exceedingly dense, 

 impenetrable tangles of shrubs and lianes, with an occasional tree, half 



