A NATIONAL PLAN FOR AMERICAN FORESTRY 



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accelerating process. Its least conspicuous form is as sheet erosion, 

 recognizable in the exposure of root crowns and roots of plants, in the 

 lowered productivity of the site, in the accumulation of soil on slopes 

 immediately above obstructions, and in the final disappearance of the 

 mellow black surface soil. As the mellow surface soil is washed away, 

 a more compact subsurface soil is generally exposed, less capable of 

 absorbing rainfall and less resistant to erosion. Furthermore, de- 

 creased productivity may render it incapable of supporting as dense 

 a stand of vegetation as formerly. There is thus less obstruction to 

 run-off, less binding power in the soil, and less possibility of rebuilding 

 the organic content. As sheet erosion becomes advanced the more 

 rapid surface run-off concentrates and tends to cut small gullies. 

 After hard rains on soils inadequately protected by vegetation the 

 entire area may be cut by lines from the size of a pencil to gullies 

 several inches deep. The smaller lines may be readily obliterated by 

 creep of soil as it dries, but their original presence indicates a rapid 

 soil wastage. 



Finally, abnormal erosion assumes its most spectacular form as 

 deep gullies on slopes and large straight-sided channels cut through 

 flood plains. Its final effects are heavily soil-laden streams, especially 

 when in flood; silted channels and reservoirs; mud rock flows from 

 mountain stream courses ; and deposits of inert sands and gravels on 

 fertile bottom lands. 



The control of erosion through retarding run-off is largely a function 

 of the forest cover. This cover is responsible for improvement of 

 soil structure, protection of the surface soil from beating rains, and, 

 by intercepting run-off, reduction of the velocity and carrying power 

 of the surface water. Lowdermilk has pointed out in his studies 

 under controlled conditions that the run-off from soil surfaces pro- 

 tected by a litter cover alone was nearly clear. A summary of 

 Lowdermilk's findings in table 6 shows strikingly the value of forest 

 litter in reducing erosion on three important soil types. The weights 

 of soil eroded from these plots, all of which had been litter covered for 

 a year before half of them were burned bare, indicate that, on the 

 average, removing the litter caused, respectively, 73, 160, and 1,196 

 times as much sediment to be eroded as was carried off from the pro- 

 tected soils. Studies in Arizona by C. K. Cooperrider, of the South- 

 western Forest and Range Experiment Station, show that a vigorous 

 herbaceous and shrub understory in the woodland type exerts a some- 

 what similar influence. 



TABLE 6. Weights of soil eroded from plots protected by litter cover and plots burned 

 bare on slopes of equal gradient and soils of three important types, under equal 

 quantity and duration of artificial rainfall 1 



1 10 runs made for each combination of time and condition. 



