A NATIONAL PLAN FOR AMERICAN FORESTRY 313 



equivalent of moisture absorbed by the litter normally accumulating 

 in this wide assortment of types ranges from a negligible quantity to 

 nearly an inch, and within a single region, according to Alway and 

 Harmer, 19 may be 9 times as much in a dense stand as in one opened 

 by cutting. The Red Plains Experiment Station near Guthrie, 

 Okla., found the litter on a post-oak area to have a water-absorption 

 capacity of 16.7 tons per acre. 



American data on the effect of a litter cover on evaporation from 

 forest soils are singularly lacking, but European comparisons, quoted 

 by Zon (op. cit.) of forests with and without litter, indicates that this 

 natural forest mulch may reduce evaporation by 40 to 60 percent. 



A litter cover materially retards both the rate and depth of freezing 

 of the soil beneath. MacKinney (op. cit.) found that under a 2-inch 

 litter in a Connecticut plantation of Norway and white pines, frost 

 in 1926-27 was a month later in penetrating the soil at all, and final 

 penetration only 60 percent as deep, as where the litter had been 

 removed. 



The character of the frozen soil was influenced markedly by the litter. The 

 soil on the bare plot froze solidly, and the air spaces were practically filled with ice. 

 On the other hand the frozen soil beneath the litter cover was porous and loose, 

 at no time being frozen too hard to allow the insertion of a shovel * * *. 

 During winter rains and thaws, the water soaked into the soil of the litter-covered 

 plot and percolated to lower depths. On the bare plot the water ran off at such 

 times due to the nonporous character of the frozen soil. 



SUMMARY OF INFLUENCES 



Because forest that conserves snow and reduces evaporation of soil 

 moisture must at the same time interrupt precipitation and transpire 

 water drawn from the soil, its final effect on run-off can only be deter- 

 mined by the balance between these opposing influences. Whether 

 this net effect is beneficial or harmful in any particular region is 

 probably determined in part by the total amount of precipitation, but 

 chiefly by the occurrence of precipitation as snow or rain, its distribu- 

 tion throughout the year or during only a part of it, and its arrival in 

 light or heavy storms. American research to date, backed by a large 

 body of observational evidence from all parts of the United States, 

 justifies a strong belief that the forests of the country practically 

 always benefit stream flow. A possible exception is the canyon- 

 bottom vegetation of the drier regions, earlier described; even this 

 may prove to have a net favorable effect in the checking of erosion. 

 There can be no doubt at all that the net effect of forest litter, al- 

 though it intercepts some precipitation and returns it into the air by 

 evaporation, is extremely beneficial, since it reduces surface run-off 

 and increases the water-storage capacity of the soil by increasing 

 percolation at the same time that it shelters the soil moisture from 

 evaporation. 



From a careful consideration of each main region, it appears fairly 

 evident that the climax forest that type of forest which is best adapt- 

 ed to the climate and soil, and which nature, in the absence of fires, 

 human interference, or epidemic of tree-killing insects and diseases, 

 always tends to produce was admirably adapted to promoting 

 stream-flow conditions favorable to mankind. For example, the dense 

 hardwood and hemlock forests of the southern Appalachian Moun- 



19 Alway, F. J., and Harmer, P. M., "Minnesota glacial soil studies: II. The forest floor on the Late 

 Wisconsin Drift." Soil Science, 1927. 



