122 THE ECOLOGICAL RELATIONS OF ROOTS. 



X. SUMMARY. 



This study was undertaken to determine the root habits of dominant 

 and subdominant plants growing under a wide range of climatic and 

 edaphic conditions; to find the root rektions of the plant communities 

 as units of vegetation; and to determine the root distribution and root 

 competition of the individual species in their relation to other species 

 in the community. Other aims were to determine the relation between 

 the root habits of plants in various communities and their successional 

 sequence, and to obtain a more definite knowledge of the indicator 

 value and the significance of various species used in classifying lands 

 for grazing or for agriculture, as well as to aid the forester in selecting 

 sites for afforestation or reforestation. 



The investigation extended over four years, during which time 

 more than 1,150 individual root systems of about 140 species of shrubs, 

 grasses, and other herbs were excavated and studied. These species 

 were members of 8 different plant communities widely separated geo- 

 graphically and growing under distinctly different climatic or edaphic 

 conditions. The communities studied were the prairies of eastern 

 Nebraska, the chaparral of southeastern Nebraska, prairies of south- 

 eastern Washington, the plains association and sandhill subclimax of 

 Colorado, and the gravel-slide, half-gravel-sUde, and forest communities 

 of the Rocky Mountains of Colorado. 



The trench or pit method was used in excavating the roots, the root 

 system being removed with appropriate apparatus from the face of the 

 trench. In case of roots extending to depths beyond 15 feet, two 

 working levels were maintained. The quadrat-bisect method has been 

 employed to show the root systems of communities in place and thus 

 exhibit their interrelations in detail. 



The root systems of 33 species were examined in the prairies of 

 eastern Nebraska; 13 of these were grasses. All of the dicotyledonous 

 plants were found to extend well beyond a depth of 2 feet, while the 

 roots of 6 of the grasses were confined to the surface 2 feet of soil. 

 Four of the grasses and 5 other species were found to penetrate well 

 below the second foot of soil, but seldom deeper than 5 feet. More 

 than half of the plants studied, including three dominant grasses, have 

 roots which reach depths greater than 5 feet — indeed, most of them 

 penetrate to distances of from 7 to 9 feet and a few to a maximum 

 depth of from 13 to 20 feet. 



These prairie species grow under a mean annual precipitation 

 of about 28 inches, over 20 inches of which fall during the growing- 

 season. The soil is of the type commonly called loess, much of which, 

 however, is confounded with glacial drift. Soil-moisture determina- 

 tions for more than two seasons show that the water-content is some- 

 times reduced to such a degree that no moisture to a depth of 4 or 5 



