PLANT SUCCESSION AND RANGE MANAGEMENT. 67 
plants. The bunched wheat-grass areas, on the other hand, are 
seldom pure in stand, and plants other than grasses usually occupy 
the soil space between the tufts. The nongr asses occur in varying 
density, depending chiefly upon the available soil water content. 
7. The root-absorbing surface of the densely turfed wheat 
grasses is relatively superficial, the greater proportion of the roots 
being confined to the upper 8 inches of soil. The roots of the 
bunched species typically extend to a depth of 3 feet or more, hence 
their moisture supply is largely gathered well below the depth at 
which the turfed species obtain theirs. 
8. Precipitation percolates so slowly through the matlike surface 
of the turfed wheat-grass area that only a small portion of the rain- 
fall,' especially that which comes during the growing season, pene- 
trates beyond the lower depths of the sod. Accordingly, other plants, 
especially deep-rooted species, fail to become established in competi- 
tion with the turfed wheat grasses. 
9. Precipitation percolates deeply on the rather exposed soils of the 
bunch-grass areas, and as a consequence both deep-rooted and shal- 
low-rooted species, chiefly other than grasses, are commonly found on 
bunch wheat-grass areas. 
11. When a stand of bunch wheat grass is opened up there is an 
some similar factor but not so as seriously to decrease the fertility 
of the soil, the precipitation naturally percolates to a much greater 
depth than where the turf remains intact. There follows an invasion 
of certain deep-rooted species, the most typical and persistent of 
which is yellow brush (C hrysothmivnus lance olatus). As the wheat- 
grass cover closes in about the j^ellow-brush plants, however, regard- 
less of their luxuriance of growth and root development, they are 
killed. This is due to the desiccation of the soil below the shallow 
roots of the grass. 
11. When a stand of bunch wheat-grass is opened up there is an 
increase in the density of other plant species. This secondary cover 
is rather transitory both as to species and density. 
12. In terms of the amount of dry matter produced per unit of 
surface, the turfed wheat-grass areas rank first, but owing to small 
wheat grass (Agpojyyron dasystachyum) , the most conspicuous turfed 
species, maturing somewhat earlier than the bunch grasses and at 
the same time becoming less palatable when mature than the bunch 
grasses, the turfed wheat-grass areas afford little, if any, more forage 
than do the bunched wheat-grass areas. 
L3. The wheat-grass type is the most permanent of any and with- 
stands heavy grazing better than any other. Since the turfed 
wheat grasses typically form a pure stand, this type of vegetation is 
better suited for cattle and horses than for sheep. The virgin bunch- 
grass lands, with their " combination " or mixed forage cover, while 
