PLANT STJCCESSIOK AND RANGE MANAGEMENT. 17 



develops a somewhat deeper root system wlien the specimens are 

 isolated than when the plants grow in a well-established sod. This 

 increased development is doubtless accounted for by the fact that 

 the water content of the soil immediately below the deeper roots 

 of the isolated bunched wheat grasses is appreciably higher than 

 in the soil below the deeper roots of the sodded' stand. Since all 

 stands of small wheat grass become matted when permitted to de- 

 velop normally in well-disintegrated soils, it is evident that the 

 increased development in depth of the roots of isolated specimens, 

 as compared with that of the turfed plants, is purely temporary, 

 and probably of little or no economic significance.^ 



In contrast with the turfed wheat-grass type, the conditions that 

 obtain in a normal, fully developed bunch wheat-grass type are such 

 as to permit the presence of other plants of both deep-rooted and 

 shallow-rooted species (fig. 7). The shallow-rooted species, such 

 as mountain squirrel tail {Hordeum nodosvmh) , single-flowered heli- 

 anthella {H eliantheUa unifiora) , and others, feed chiefly in the upper 

 foot of soil, but the density of the cover as a whole is never such as 

 to prevent a comparatively rapid percolation of water to a depth of 

 several feet. In general, a large part of the rainfall is absorbed on 

 lands where bunch wheat grass is iullj developed, so that serious 

 erosion seldom occurs so long as the natural cover remains unim- 

 paired. Because of the high power of absorption of the soil and 

 the relatively high percentage of available moisture in the lower soil 

 depths, a few deep-rooted species, like wild bean or alpine lupine 

 {Lwpinus alpestris)^ yellow brush, and the like, as well as certain 

 surface-feeding plants, like single-flowered helianthella {Helian- 

 tJiella tmiflora), mountain squirrel tail {Hordeum twdosimh), and 

 blue foxglove {Pentsterrwn procerus) ^ occupy the space between the 

 grass bunches where the spacing is fairly wide and the intervening 

 soil not fully occupied by grass roots. Therefore, where the bunch 

 wheat grass stand is opened up by grazing or by other adverse factors, 

 a good balance both of deep and of shallow rooted species, chiefly 

 other than grasses, follows, one set of species predominating at one 

 time and another set at another time. Accordingly, a reasonable 

 state of equilibrium in the vegetation occupying the space between 

 the bunch-gTass tufts exists only when the maximum density of the 

 bunch-grass stand has been reached and has become stabilizred. This 

 stabilization of the rather transitory type of vegetation may be ac- 

 counted for by the comparative equality in the utilization of the 

 available water content of the soil by the wheat grasses. 



1 Cannon, W. A. (Plant World, vol. 16, No. 12: 323-241, 1913), found that the root 

 deyelopment of desert plants varies widely in soils of different texture and depth. These 

 variations were observed to hold regardless of whether the plant was grown under natu- 

 ral conditions, in garden soils, or in artificial cultures. 



112655°— Bull. 791—19 2 



