A NATIONAL PLAN" FOR AMERICAN FORESTRY 305 



DEFINITION OF FOREST COVER 



Forest, as the term is used here and elsewhere in this report, includes 

 both trees proper and tall brush. In the West this embraces com- 

 mercial timber, woodland (pinon and juniper, for example), and 

 chaparral all areas except sagebrush, grassland, and other com- 

 parable prairie or desert types of vegetation. In the East it covers 

 such degenerate forest types as scrub oak. In calculating the per- 

 centage of a drainage region which is forested, small openings within 

 the forest have been included with the forest, but large meadows, 

 grassland, or other non-tree-producing openings within the forest have 

 been excluded. Windbreaks, shelter-belts, and other tree plantings 

 carried in the census records of the Plains region as woodlots are 

 admitted to the forest area. Cleared lands within the forest belt 

 which have been permanently abandoned or are in process of abandon- 

 ment by agriculture have been classed only as potential forest land. 



Forest cover in its relation to watershed protection is considered to 

 include: (1) The trees and tall brush; (2) the herbs and shrubs growing 

 thereunder or in openings in the forest or brush fields; (3) the litter, 

 or fallen leaves, branches, down trees, and other vegetative material 

 on the forest floor; and (4) the rich humus of partly decayed vegetable 

 matter at the surface and in the top layer of the soil. Thus in this 

 section it is the influence of the ^ en tire forest cover upon run-off, 

 erosion, or other watershed-protection values which is given consider- 

 ation, rather than the influence of the trees alone. 



There is a rather widespread popular acceptance in this country of 

 the idea that forests and associated or related vegetation exert a 

 favorable influence not only on streams but on climate. This prob- 

 ably had its origin in European experience, and has been fortified by 

 general observation, such as the muddiness of streams flowing from 

 cleared land compared with the clarity of those flowing from woodland, 

 and extreme fluctuations in the rate of streamflow from deforested or 

 denuded land. Only of late has a body of information based on careful 

 observation and experimentation begun to accumulate behind the 

 popular concept of the forest as a regulator of stream flow and preven- 

 tive of erosion. 



This concept of the beneficial influence of forests has been em- 

 bodied from the first in the administration of the national forests 

 created from the public domain, and was long the sole basis for pur- 

 chase of national forests in the East. The act of 1897, which first 

 provided for administration of the original ''forest reserves", named 

 as a major purpose " securing favorable conditions of water flows". 

 Certain of the national forests of the West among them the Tonto in 

 Arizona and the Angeles in California have been created in whole 

 or in large part principally for the protection of irrigation projects 

 or municipal water supplies. The Weeks Law of 1911 provided for 

 Federal cooperation with the States "for the protection of the water- 

 sheds of navigable streams", and for Federal "acquisition of lands 

 for the purpose of conserving the navigability of navigable rivers." 

 The Clarke-McNary Law of 1924 continued the cooperation "with 

 a view to the protection of forest and water resources", and directed 

 that in further purchases "due consideration" be given both to water- 

 sheds of navigable streams and those ' ' from which water is secured for 

 domestic use or irrigation." Nearly 60 percent of the Federal pur- 



