428 A NATIONAL PLAN FOE AMERICAN FORESTRY 



CLASSIFICATION OF FOREST AREAS ACCORDING TO INFLUENCE 



Of the 29,780,000 acres of forested land within the California 

 drainages, about 21,056,000 acres has been classified as of major 

 influence in protecting watersheds, that is, in regulating run-off or 

 reducing soil erosion, or both. As pointed out in the foregoing, 

 this acreage of major influence occurs chiefly in the foothill and 

 mountain areas of the Sierra Nevada and along the Coast Kange. 

 The mixed conifer, woodland, and brush cover of the Sierra Nevada 

 and its higher foothills, the redwood and Douglas-fir mixtures and 

 dense brush types of the north Coast Range (with the exception 

 of a narrow strip near the coast north of San Francisco Bay), and 

 the redwood, Douglas fir, and other conifer cover and dense brush 

 of the central Coast Range, all are considered to exert a major water- 

 shed-protection influence. All the forests of southern California, 

 being chiefly chaparral and woodland on the lower slopes and mixed 

 conifers (Jeffrey pine, sugar pine, Coulter pine, white fir, incense 

 cedar, juniper, and pinon) on limited areas at elevations greater 

 than 4,000 or 5,000 feet, have likewise been classed as of major 

 influence. These forests differ widely in respect to rainfall, vege- 

 tative composition, soil, and underlying geological structure. 



Approximately 3,736,000 acres of forest area has been classified 

 as of moderate watershed-protective influence. This includes the 

 narrow strip of whitebark pine and Jeffrey pine forest and pinon- 

 juniper woodland on the abrupt east slope of the Sierra Nevada 

 facing Owens Valley. It is classed as of moderate rather than major 

 influence chiefly because of the scantiness of the rainfall and the 

 porous nature of the granitic soil. The other large area of moderate 

 influence is that along the coast north of San Francisco Bay occupied 

 by dense stands of redwood and Douglas fir. On this area the forest 

 effectively protects the soil against erosion and has a material in- 

 fluence in retarding the run-off of the heavy precipitation. After 

 logging or burning, forest cover is rapidly reestablished. Erosion 

 damage is ordinarily very much localized, and high water is seldom 

 serious. Furthermore, the water supply is adequate to meet all 

 demands. 



Approximately 4,988,000 acres in the northern part of California 

 and in the adjacent part of Oregon included within the California 

 drainages has been classed as of slight watershed-protective in- 

 fluence. This large area of forest is made up chiefly of ponderosa 

 pine, white fir, and other coniferous species growing upon volcanic 

 hills and ancient lava flows. Here the precipitation percolates so 

 promptly through the porous soil and lava into underground chan- 

 nels that very little surface run-off occurs and abnormal erosion is 

 almost negligible. Such rivers as the McCloud and other tribu- 

 taries of the Pitt River are fed by large springs, which are in fact a 

 bursting forth of underground streams of considerable size. 



WATERSHED-PROTECTION REQUIREMENTS AND HOW THEY 



ARE BEING MET 



The principal factors modifying or disturbing watershed-pro- 

 tective forest cover within the California drainages are fire, grazing, 

 and lumbering. The State watershed committee in its report 

 Forestry in the State-wide Water Plan pointed out that 



