549 
vegetation of the more xerophytic sand areas is more representative of 
the sand-hill vegetation of Kansas and Nebraska than of the Great 
Plains or the Prairie. 
With the establishment of a pioneer association sufficient to pre- 
vent the erosive action of the wind, conditions become less severe and 
further growth of a less xerophytic type is favored. As the vegetation 
encroaches and establishes itself upon the sand, the accumulation of or- 
ganic matter from decaying roots and tops of successive associations 
results in physical changes in the soil which slowly approach conditions 
existing on the more mesophytic prairies. The more important of these 
changes is a decrease in the extremes of temperature and an increase 
in humus with a consequent increase in soil stability and soil water. 
Gleason (10) has shown that the evaporation rate of the air also gradually 
decreases from one association to another. Each association thus im- 
proves conditions and paves the way for its successor, ultimately leading 
to the climax association of the region. 
Observations were made on the sand areas at Hanover, between 
Savanna and Albany, at Oregon, and in the northwestern part of Bureau 
county. The succession of the principal associations leading to prairie 
in these areas is given in Figure 8. 
The Panicum pseudopubescens association is most frequently found 
in the vicinity of blowouts, where it is a stage in the retrogressive suc- 
cession from the bunch-grass association to the blowout association. Plate 
LXXI represents such a stage with the blowout just beginning, and shows 
the relative amount of the sand that may be occupied by this species. 
The secondary species are relics of the bunch-grass associations previously 
destroyed by the wind. 
With the further development of the blowout Panicum pseudopubes- 
cens is replaced by the blowout association, which Gleason further sub- 
divides into four distinct “associations.” First, the basin of the blow- 
out, where the sand is being removed by the wind. The vegetation of 
this “association” is very meager and is composed of a few scattered 
perennials persisting from the Panicum pseudopubescens , association. 
Chief among these are Acerates viridiflora and its varieties lanceolata 
and linearis, Lithospermum Gmelini, Euphorbia corollata, and Lespedeza 
capitata. Second, the windward slope, where the sand is being removed 
by both wind and gravity. The vegetation of this “association” consists 
of relics of the Panicum pseudopubescens and bunch-grass associations 
which have been undermined at the top and have gained a foothold while 
sliding down the slope. This association is limited to perennials, among 
the most important of which are Panicum pseudopubescens and Selaginella 
rupestris. Third, the lee slope, or “blowsand association,” over which 
the sand is being sprayed by the wind. The vegetation of this association 
consists mainly of annuals characteristic of the bunch-grass association. 
Gleason lists 31 species for this association, of which Aristida tuberculosa 
is the pioneer. This species is common to all the areas studied. Other 
important species such as Diodia teres, Paspalum setaceum, Commelina 
