38 BULLETIN 1105, U. S. DEPARTMENT OF AGRICULTURE. 



not succeed in sending their roots into the lower strata during the first 

 season usually die. This may also explain why yellow pine does 

 not occur on clay soils in this region where the annual precipitation 

 is below about 16 inches, and where, in consequence, the moisture 

 does not penetrate deeply. The average depth reached by the roots 

 of first-year seedlings at Fort Valley in 1919 8 was 12.4 inches in fine 

 clay, 8.7 inches in stony clay, and 7.1 inches in gravelly soils. In 

 fine clay the laterals are mostly near the lower end of the root, in 

 gravel and cinders near the surface, and in stony clay near the 

 middle third. It is evident that, excepting in the fine-clay soils, 

 first-year seedlings secure most of their moisture from the upper 6 

 inches of soil. 



Presumably the roots of a plant develop more or less in response 

 to its needs, and therefore the form of the root is an index to the 

 conditions prevailing in the soil. If this assumption is correct, the 

 absence of laterals in the upper strata of the fine clay indicates 

 a deficiency of available moisture in these strata. It also fol- 

 lows that the main root will penetrate deeply in order to follow the 

 rapidly subsiding moisture with the approach of a dry period, and 

 that the laterals will develop at a considerable depth where the 

 moisture supply is fairly constant. The above conception corres- 

 ponds with the known fact that the clay soils dry out rapidly at 

 the surface, but are very retentive of moisture at the lower depths. 

 Following the same course of reasoning, it is inferred that roots 

 penetrate less deeply in the stony clay and gravely soils, because 

 they find sufficient moisture in the upper layers, which also offer 

 advantages in aeration, temperature, and supply of organic matter. 



HERBACEOUS VEGETATION. 



Even the densest forests of yellow pine in the Southwest support 

 a large amount of herbaceous vegetation of high light requirement. 

 Rarely does the crown canopy of the trees cover more than 30 per 

 cent of the total ground area, and usually not over 25 per cent. The 

 ground directly under and for 20 to 30 feet north of the larger groups 

 of trees is usually almost bare of herbaceous growth, although some- 

 times a luxuriant cover of lupine, vetch, and other shade-enduring 

 plants may be found in such situations. On the Coconino and Tusa- 

 yan National Forests the openings between the trees, which in virgin 

 stands are often from 100 to 150 feet in diameter, are usually occu- 

 pied by grasses. Cutting enlarges the openings and thus favors the 

 development and extension of herbaceous cover. It is the forester's 

 aim, however, to stock with trees both the original large openings 

 and those made by cutting. 



8 Theso figures are higher than for average years. 



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