6 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



foot wide was removed and the grasses kept out. In case (4) the 

 competition was entirely removed and in (1) was complete. In (2) 

 the water competition was accentuated; and in (3) the light competi- 

 tion was removed. All seedlings did best under (4), that is, when all 

 of the competition of the grasses was eliminated. At the end of the 

 third season, trees only 18 to 24 inches high had taproots 5 to 7 feet 

 deep and surface roots almost as long. The mortality was much 

 higher in the clipped area, showing the effect of the root competition, 

 and still higher in the watered region, since watering promoted the 

 grasses and increased the shade. In this case, (2), and also in (1), 

 nearly all the seedlings succumbed, and those that did not were poorly 

 developed both above and below ground. 



Weaver and Kramer (228) studied the distribution of the roots of 

 Quercus macrocarpa in the prairie in relation to soil type, root com- 

 petition, and soil moisture content. They found that the taproot of 

 this species may go down 14 feet but most of the thirty-odd branches 

 of the taproot arose in the first 2 feet of soil. The main branches run 

 out for 20 to 60 feet. The weight of the roots equalled that of the 

 tops, and the volume of the roots was about 90 percent that of the 

 tops. Water, it was concluded, was the determining factor in the 

 form and extension of the root system. 



Biswell (16) shows the relation between soil conditions in Nebraska 

 and the root habits of seedlings and/or saplings of the following de- 

 ciduous forest trees: Gleditsia triacanthos L., Acer negundo L., Pla- 

 tanus occidentalis L., Juglans nigra, Aesculus glabra Willd., Quercus 

 macrocarpa, Hicoria ovaia, Acer saccharum Marsh., and Populus 

 sargentii Dode. Since the soils, moisture, etc., varied, no general 

 conclusions are permitted except as to the possibilities of growth 

 under these respective conditions. This paper, however, is one of 

 the most recent on the root systems of North American forest trees. 



The comparatively shallow nature of root systems and the compe- 

 tition offered by the herbaceous cover of the forest floor is further 

 evidenced by the work of Pearson (166) who found that in ponderosa 

 pine in Arizona roots seldom penetrated to a depth of 4 feet. Most 

 of the taproots of the saplings studied were under 30 inches in length 

 and the strongest laterals were within 1 foot of the surface. 



That trees may absorb from deeper layers, however, is shown by 

 the work of Shear and Stewart (190), who, in Illinois, found not only 

 that larch, white oak, and white pine removed more water from the 

 top 4 feet of soil during the growing season than did green ash and 

 silver maple, but also that the white oak affected the water content 

 of the soil to a depth of 10 feet, larch to 9 feet, silver maple and white 

 pine to 8 feet, and green ash to 7 feet. 



Among the other biotic factors influencing the absorption of water 

 by trees should be mentioned the mycorhiza, filamentous fungi that 

 attach themselves to the roots of trees. In some forms (endotrophic) 

 they penetrate the cells of the root, while in other forms (ecto trophic) 

 they are merely closely applied to the surface. Their relation to the 

 trees is thought to be mutualistic rather than parasitic; while they 

 doubtless use some of the elaborated foods stored in the root cells, 

 they also are believed to supply soil minerals and water to the roots 

 and thus function in the capacity of root hairs. These fungi, which 

 are found especially in forest soils rich in organic nutrients, seem, in 

 fact, to replace the root hairs in many forest species, and in the ab- 



