314 A NATIONAL PLAN FOR AMERICAN FORESTRY 



tains, where annual rainfall is heavy and floods result from a succes- 

 sion of storms rather than from a single very heavy storm, appear to 

 constitute just the forest type to intercept the maximum of precipita- 

 tion in the tree crowns and litter, and to reduce the flood crests by 

 keeping part of the rain or melted snow from ever reaching the soil. 

 Were such a forest capable of developing in southern California, where 

 the comparatively light precipitation supplies a vital human need in 

 the rich valleys, its draft on available moisture would be extremely 

 serious. But the forest actually present on the hillsides is a dwarfed 

 one, not transpiring appreciably at the season of heaviest rainfall, but 

 producing a leaf litter having a profoundly favorable effect on percola- 

 tion and water storage. 



The natural climax forest of a region, although a valuable guide to 

 what constitutes the best form and composition for a forest managed 

 chiefly for its influence on run-off and stream flow, is probably not 

 beyond improvement. When vastly greater research than has yet 

 been attempted yields a knowledge of how much a given change in the 

 character of a forest will improve the flow of streams, it is entirely 

 possible that appropriate management will produce a forest with an 

 even greater beneficial influence than the original. At the very least 

 man has it somewhat in his power to control many of the natural 

 catastrophes fire and killing insects, for example that ravaged some 

 of the American forests long before Columbus' arrival, and which, as 

 will be seen under a later heading, today have disastrous effects on 

 stream flow. 



HOW FOREST COVER INFLUENCES EROSION 



Erosion, the removal of soil by water and wind, is taking place on 

 all land areas. Where this occurs on land that has not been dis- 

 turbed by man and is subject to no extraordinary climatic conditions 

 the process may be termed "normal erosion." The intensity of 

 normal erosion is determined chiefly by topography, geology, soils, 

 climate (especially precipitation), and vegetative cover. Only rarely 

 does it assume an intensity that involves serious damage to soil or to 

 water flow or storage. In humid regions completely clothed with 

 vegetation, as in a dense forest, natural processes are forming soil as 

 rapidly as it is eroded and actual net loss, if any, is imperceptible. 

 Surface run-off is ordinarily negligible, and consequently what erosion 

 there may be is limited to light or dissolved particles of organic matter 

 and practically no mineral soil is removed. Where the forest and 

 other vegetative cover is definitely scant, as under semiarid conditions, 

 there is still enough vegetation or debris to catch eroding soil and 

 litter on slopes, retard run-off, and cause deposition of much of the 

 eroded material already in motion. Even a light vegetative covering, 

 if undisturbed, is sufficient to hold normal erosion to a negligible 

 quantity. Only where the soil is unstable and easily erosible, as in 

 the Badlands of the Dakotas, or on the outcrop of the Mancos and 

 other similar shales, is normal erosion sufficiently rapid as to be 

 perceptible. 



On the other hand, when natural conditions are disturbed and 

 nature's balance is upset by a reduction in the forest cover as a result 

 of fire, logging, or overgrazing, or by marked changes in climatic con- 

 ditions or other major causes, erosion in excess of normal is liable to 

 occur. Furthermore, abnormal erosion, where it does occur, is an 



