PLANT SUCCESSION AND KANGE MANAGEMENT. 17 
develops a somewhat deeper root system when 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. 1 
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 nodosum), single-flowered heli- 
anthella (HeliantTiella uni flora) , 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 fully 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 
(Lupinus alpestrls), yellow brush, and the like, as well as certain 
surface-feeding plants, like single-flowered helianthella (Helian- 
thella umiflora), mountain squirrel tail {Hordeum nodosum), and 
blue foxglove {Pentstemon 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-grass tufts exists only when the maximum density of the 
bunch-grass stand has been reached and has become stabilized. 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. 
i Cannon, W. A. (Plant World, vol. 16, No. 12: 323-241, 1913), found that the root 
development 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 
