312 A NATIONAL PLAN FOR AMERICAN FORESTRY 



days, the rate at which clear water percolated through columns of 

 soil, he introduced sediment of less than 2 percent by weight into the 

 water; within 6 hours the rate of percolation fell to 10 percent of 

 what it had been. Moreover, the effect was permanent, as a return 

 to the use of clear water did not restore the original rate. A good 

 forest litter keeps the rain from becoming muddied when it hits the 

 earth and so decreases run-off ; in the absence of litter, surface run-off 

 is enormously increased. When Lowdermilk applied artificial rain, 

 at an average rate of 1 inch an hour, for several periods of % to 8 

 hours, to sloping tanks filled with typical California soils, he found 

 that the surface run-off was from 3 to 16 times as great from bare 

 soils from which the litter was burned as from those on which a litter 

 cover was present. 



An effect similar to that of litter cover in aiding percolation and 

 in lessening the proportion of surface run-off is produced by low, 

 permanent vegetation. Duley and Miller (op. cit.) state that 

 whereas only 11.5 percent of 6 years' rainfall ran off over a sloping 

 surface protected by a permanent grass sod, kept clipped, 49 percent 

 ran off bare soil. Even where tilth of the bare soil was maintained 

 by annual cultivation nearly 30 percent of the precipitation was 

 carried off over the surface. 



The superior physical condition and consequent permeability of 

 forest soils has been demonstrated for Ohio Valley conditions by 

 Auten. 17 Samples of the upper 9 inches of soil under several old- 

 growth stands in oak-hickory and other hardwood types were found 

 to be 13 percent lighter at oven dryness than equal volumes of soil 

 from adjacent cultivated fields and a few pastures indicating more 



Eore space and better tilth. Although this difference in weight was 

 iter found to be confined to the upper 6 inches, the forest soil was 

 still distinctly the more pervious to moisture at a depth of 8 inches. 

 At a 3-inch depth 14 times as much water was absorbed per minute 

 by the forest as the field soil, and at a 1-inch depth, over 50 times as 

 much. 



Auten points out that the favorable effect of the forest on the soils 

 he studied is not entirely due to the litter, but is increased by the 

 roots, which upon their death decay and leave the soil interpenetrated 

 with tube-like cavities; also by the activities of burrowing worms, 

 insects, and animals, which make the soil porous. A litter cover 

 promotes these activities. Lowdermilk reported that earthworms 

 appeared under the litter during the second year of his comparison 

 of run-off from bare and litter-covered plots. 



Secondary but sometimes important effects of litter upon run-off 

 are produced by its absorption of moisture, its retardation of evapo- 

 ration from the soil beneath, and its prevention of deep freezing. 

 Investigations by the Forest Service and other agencies 18 have 

 shown that litter from both the conifer and oak-chaparral types of 

 California absorb about 1.8 times their own dry weight of moisture; 

 freshly fallen pine litter in both the Lake States and the South, 1.5 

 times; conifer litter in the southern Appalachian Mountains, 3.4 

 times and hardwood litter, 4.6 times; and spruce-northern hardwood 

 litter in New England up to 9 times its dry weight. The rainfall 



i ? Auten, John T., "Porosity and water absorption of forest soils." Accepted for publication by Jour. 

 Agr. Res., 1933. 



is "Relation of forestry to the control of floods in the Mississippi Valley." House of Rep. Doc. No. 573, 

 70tb Cong., 2d sess., 1929, 



