550 
eve grass €-------+-: Andropogon furcatus 
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Y Andropogon scoparius Mixed Breas 
os association 
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2 Pioneer bunch-grass 
Re we” association 
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ge 
Xerophytic pasture 
Bouteloua etc. 
— 
Blowout association 
and blow-sand complex. 
Fic. 8. Diagram showing the, interrelation of the principal associations 
leading to prairie on sand. The broken lines represent retrogressive 
successions resulting from blowouts. The dotted lines represent suc- 
cessions resulting from grazing. See Plates LXXI-LXXYV. 
virginica, and Cenchrus carolinianus are more local in their distribution. 
Lastly, the “deposit association,” composed chiefly of such dune-forming 
species as Rhus canadensis, var. illinoensis, Ceanothus americanus, Te- 
phrosia virginiana, Panicum virgatum, and occasionally Calamovilfa longi- 
folia. To this list Vestal (27) proposes to add a fifth association, the 
“blowsand complex”, which arises from the continued growth of a large 
blowout or by the confluence of a number of blowouts. The vegetation 
is variable and is composed mainly of annuals as in the case of the blow- 
sand association. 
The meager vegetation of these five “‘associations” is of little conse- 
quence in the stabilization of the sand. The first permanent step in the 
development of vegetation on denuded areas is accomplished by the 
pioneer bunch-grasses which succeed in getting a foothold in any of these 
associations. Selaginella rupestris is also an important pioneer invader 
in these areas at Hanover and Savanna. The order of the entrance of 
the grasses is not definitely known, though, as Gleason has pointed out, 
it depends largely upon the nature of the neighboring areas of the 
