Southeastern Washington and Adjacent Idaho. 75 



important member of the climax communities elsewhere, is not 

 represented in the area under consideration. 



Symphoricarpos-Opulaster 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 



