390 



MAN AS A CONQUEROR 



changes have resulted. When the earth's surface is covered with 

 trees, their roots make a soil-mat which holds water in the ground, 

 preventing a rapid run-off. The blanket of foliage above intercepts 

 the moisture and allows a more gradual passage of water into the 

 ground, while the soil under the trees, rich with an accumulation of 

 humus and dead leaves, holds the water, so that a forest floor is 

 estimated to absorb and hold back for some time a rainfall of four to 

 five inches. But let the forest cover be destroyed by fire or poor 

 lumbering, and what is the result? As soon as the forest cover is 

 gone, the first heavy rain washes off the soil, carrying it to places where 

 it will be of little value. The water lost by surface run-off in level 

 areas like the Great Plains region varies from 15 per cent after light 

 showers to 50 per cent during heavy storms. In sloping areas the 

 run-off naturally is much greater. It is estimated that the annual 

 wastage of soil in the country from erosion amounts to 1,500,000,000 

 tons, containing over 126,000,000,000 pounds of soluble mineral 

 material necessary for the life of plants. In dollars and cents this 

 wastage is estimated at $2,000,000,000 annually. Most of this loss 

 comes through man's carelessness with fire or downright rapacity in 

 denuding forests through cutting without proper provisions for 

 replanting. 



The relation of forest fires to erosion and floods is seen in the fol- 

 lowing table, which was made after the devastating flood near Los 

 Angeles, California, during the storm of December 30, 1933, to 

 January 1, 1934. This area had been burned over a few months pre- 

 viously, with the result that the cover of chaparral and small trees 

 was completely destroyed. 



A study by C. A. Connaughton, of the United States Forest Serv- 

 ice, on 3000 test plots of burned-over forest areas, comprising both 

 cut-over areas and virgin timber, showed in general that the degree 

 of subsequent erosion varied with the severity of the fires. Plots of 

 virgin timber burned over showed only about half as much erosion as 

 cut-over land having the same degree of forest fire. 



