Southeastern Washington and Adjacent Idaho. 75 
important member of the climax communities elsewhere, is not 
represented in the area under consideration. 
Symphoricarpos-O pulaster Associes 
A shrub community usually occupies the transition zone be- 
tween prairie and forest or at least is intimately associated with 
the earlier stages of development of the latter. I have already 
mentioned that on the most mesophytic hillsides a more or less 
well developed scrubby mictium may compete with the grasses. 
These shrubs are even better developed on the protected slopes 
of the canyons, where seepage from the rocks may form a very 
congenial habitat; and where, indeed, they often entirely replace 
the prairie. Likewise, the sheltered sides of the lower buttes 
and the more exposed slopes of those of greater elvation may be 
covered with a rather dense chaparral. The presence of such a 
shrubby growth indicates increased soil moisture. This at first 
may be due to slight unevenness in the topography such as a ridge 
over which snow may be blown and accumulate in sufficient 
quantity to furnish considerable additional moisture, or, it may be 
due to biotic factors such as the loosening of the soil by squirrels 
and other burrowing animals. The water holding capacity of 
such worked-over areas is considerably increased and results in 
an immediate dominance of various shrubs, especially roses, Sym- 
phoricarpos or Spiraea, which show a much poorer development 
in the drier, compact soils. The shrubs once present tend to 
make conditions more mesophytic, a phase which constitutes their 
most important role in succession. Not only do their fallen 
leaves but also their decaying roots and rhizomes add considerable 
humus to the soil, while their shade lowers the rate of evapora- 
tion from the soil and at the same time prevents the too rapid 
oxidation of the decaying organic matter. Such enriched soil 
forms an excellent seed bed for species of xerophytic trees which 
may invade the sodded prairies only with difficulty. Thus in the 
normal succession the shrubs may prepare the way for tree 
growth. The trees in their turn, because of their shade, espe- 
cially if they are in dense stands, cause the shrubs to disappear. 
The Symphoricarpos-Rosa Mictium.—The members of this 
75 
