344 A NATIONAL PLAN FOB AMERICAN FORESTRY 



single fire may remove this entire protective mantle. Heavy cutting 

 of the forest prevents its prompt replacement. Because hardwood 

 leaf litter blows from place to place, and may thereby be absent from 

 considerable areas, any cutting in mixed stands of pine and hardwoods 

 that reduces the proportion of pine encourages erosion. 



In the mountain province, in spite of heavy and often concentrated 

 precipitation and abuse of the forest by fire and cutting, the streams 

 from forested watersheds run extraordinarily clear. However, the 

 soils of the mountain area are easily eroded when exposed. Land 

 clearing for agricultural purposes, although of minor importance as 

 far as area is concerned, is a major factor in erosion. Indeed, so 

 rapidly does erosion take place that many fields are eroded and aban- 

 doned before the girdled trees have fallen. Geologists, foresters, and 

 agriculturists alike agree in ascribing most of the erosion in the moun- 

 tains to land clearing and to agriculture on steep slopes. 



Investigations by the Geological Survey 32 in the southern Applach- 

 ian region have shown that erosion takes place almost universally on 

 cleared slopes. Glenn estimates that slopes in excess of 15 percent 

 should not be cleared although as he points out, some slopes of 10 

 percent erode faster than those of 30 percent. What the safe gradient 

 should be in any case depends upon the erosive characteristics of the 

 soil. The soil of some fields is so impoverished by erosion that vegeta- 

 tion is unable to obtain a foothold and gullies continue to erode 

 actively long after the abandonment has taken place. In places, the 

 deeper gullies even eat back into the forest before the erosion is 

 finally checked. 



Measurements were made by the Appalachian Forest Experiment 

 Station of the dry weight of suspended matter found in streams of 

 western North Carolina following heavy rains in August, 1928. 

 These showed that as the percentage of cultivated area in the drainage 

 increased, a progressive increase occurred in the amount of silt carried 

 by the stream. This ranged from 4,370 parts per million on a water- 

 shed from 85 to 90 percent in cultivation to 1 1 parts on a watershed 

 where only 5 or 10 percent of the area was cleared. The material 

 obtained from the agricultural watersheds was fine sand, clay, and silt ; 

 from the forested watersheds, mostly organic matter. 



Grazing, although not important as a whole, is locally a cause of 

 serious erosion. Too intensive grazing use and pasturage of steep 

 slopes have been responsible for deep gullying. On some of the 

 " balds," cattle trails have resulted in erosion which, spreading rapidly 

 in the shallow soil, has exposed large areas of rock. 



Timber cutting is often of little consequence as a source of erosion 

 because new growth returns to the land quickly. Cutting, however, is 

 often followed by slash fires which result in understocked stands and 

 in these, erosion often continues for many years. In many cases, the 

 dragging of logs down mountain slopes starts small gullies, but these 

 are usually soon healed over. 



Erosion is not a great problem in most parts of the coastal plain. 

 Large areas near the coast are so close to tide level that exceptionally 

 heavy precipitation, or overflow from the streams, remain on the 

 surface of the ground for considerable periods before finally draining 

 away. At higher elevations percolation of rainfall into the sandier 



32 Glenn, L. O. Denudation and Erosion in the Southern Appalachian Region. U.S.Geol.Sur.Prof. 

 Paper 72. 1911. 



