76 A Study of the Vegetation of 



community although present generally throughout the less xero- 

 phytic portion of the prairie region seldom come into dominance 

 except on moist sheltered slopes or ravines. In such situations, 

 dense tangles of shrubby growth consisting of the following 

 species may occur: 



Rosa nutkana Spiraea corymbosa 



Rosa gymnocarpa V actinium caespitosum 



Symphoricarpos racemosus Prunus demissa 



Here, as along sheltered canyon slopes, they may dominate local 

 areas almost to the exclusion of grasses and herbaceous plants. 

 They are frequently accompanied, especially in the latter situa- 

 tions, by dwarfed specimens of Crataegus brevispina, Amelan- 

 chier spp., or Populus tremuloides. Where pines occur the pre- 

 ceding trees and shrubs are invariably accompanied by Opulaster 

 pauciflorus, the latter usually becoming the dominant shrub. 



The roses are especially conspicuous in May, when their large 

 showy blossoms fill the air with a delicate perfume. Spiraea is 

 always less conspicuous, while Symphoricarpos is much more 

 noticeable in the fall when it is covered with its large white fruits, 

 than throughout its flowering period. Vaccinium caespitosum 

 often covers large areas and while its reddish leaves may add tone 

 to the vernal aspect, later, because of its low stature, it is more or 

 less hidden by other vegetation. 



Opulaster Consocies. As already indicated Opulaster pauci- 

 florus is the shrub par excellence of the scrub community. It 

 often covers large areas in almost pure growth 'especially in the 

 deeper canyons and on the outlying buttes. Its well developed rhi- 

 zones enable it to invade the grass land which soon gives way be- 

 cause of the dense shade, if the shrubs are well developed. In 

 most situations, the balance between chaparral and prairie is so 

 nicely adjusted that for a long time the shrubs occupy places in 

 swales, ravines, or other irregularities of the surface, while 

 tongues of grass land extend down on the slightly higher eleva- 

 tions (Fig. 23). Among the last herbaceous plants to give way 

 are those of the more mesophytic type such as Geranium, Dry- 



