146 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



South Dakota, only 4, including 2 cacti, are rooted in the surface 2 feet 

 of soil. Sixteen species, including 10 grasses and sedges, reach a depth 

 of 3 to 5 feet, while a similar number, among which are 3 grasses, 

 extend their roots well below the fifth foot and many to a depth of 7 to 

 9 feet, but none were found deeper than 13 feet. 



Of 45 species excavated in mixed prairie of the Colorado-Nebraska 

 sandhills, only 4, including 1 grass, have shallow root systems; 23 

 species, of which 8 are grasses, have roots reaching depths of 2 to 5 feet; 

 while 18, including 3 grass dominants, have root systems which extend 

 beyond a depth of 5 feet ; in fact, many are 8 to 10 feet deep. Thus the 

 root-depth of the sandhills species is very similar to that of the hard 

 lands. Of all the species examined in the two habitats, only 10 per 

 cent have roots confined to the surface 2 feet of soil, 48 per cent have 

 roots which are of moderate depth (2 to 5 feet) , and 42 per cent are deep- 

 rooted. Compared with species of true prairie, those of mixed prairie 

 are not as deep-seated. Among sandhill species especially it is quite 

 usual to find the branches most numerous and best developed in the 

 surface 3 or 4 feet of sand. Unlike true-prairie species, however, plants 

 in mixed prairie have usually developed a very efficient, widely spread- 

 ing absorbing system in the surface soil. Thus, in hard lands, 72 per 

 cent and in sandhills 82 per cent of the species are well provided with 

 wide-spreading surface roots. Of the remaining species none are 

 dominants and only a few are subdominants of considerable importance. 

 Those on hard lands are often found growing in places especially favor- 

 able for water penetration (cf. Shantz, 1911). This widely spreading, 

 surface-rooting habit is apparently a response to the relatively low 

 water-content of the deeper soil and to moisture in the surface soil being 

 frequently replenished by showers during the season of growth. In 

 sandhills, where it is often most pronounced, it may possibly be ac- 

 centuated by a low supply of certain soil nutrients. However, the habit 

 is of quite as wide occurrence among species of the short-grass plains. 



Among the 8 dominants and subdominants excavated in short-grass 

 plains of Colorado and western Kansas, only 2 are shallow-rooted, 

 while but one, Psoralea lanceolata, usually extends to a depth much 

 greater than 5 feet. The dominant grasses are all of intermediate 

 root-depth. All the species except Psoralea show a marked develop- 

 ment of the surface-spreading root-habit. The poor development of 

 root-layers, so marked in true prairie, is closely related to the much 

 lower water-content. Roots of Bulbilis or Psoralea extending below 4 

 or 5 feet suffer little or no competition for water which is undoubtedly 

 present, at least during wet phases of the climatic cycle. As a com- 

 munity, short-grass -plains species are more shallow-rooted than are 

 those of mixed prairie, while the latter are less deeply rooted than are 

 species of true prairie. While 65 per cent of true-prairie species reach 

 depths beyond 5 feet, this occurs among only 42 per cent of mixed- 



