452 A NATIONAL PLAN FOR AMERICAN FORESTRY 



been seriously depleted. Studies by the Forest Service in southern 

 Idaho have shown that in the main these open timber stands, under 

 satisfactory management and with a good understory of a herbaceous 

 plants, are highly effective in controlling abnormal erosion that 

 would otherwise become serious. Where such conditions prevail, 

 these forests have been classified as of major watershed-protection 

 influence. In the volcanic pumice soils, especially in central Oregon 

 and part of eastern Washington, absorption of rain and melting snow 

 is normally so rapid, and the danger of abnormal erosion so slight, 

 that where these forests occur on such soils their watershed-protection 

 influence has been classified as moderate or slight. 



At intermediate elevations the forest is more dense, made up chiefly 

 of ponderosa pine, often with an intermixture of larch or fir. The 

 western white-pine type occurs as an unusually dense forest with a 

 heavy litter on the better soils, especially of northern Idaho. Usually 

 above these types but also intermixed with them in places are extensive 

 stands of lodgepole pine. Mixed with these several types and some- 

 times occurring as an individual type is Douglas fir. In these types 

 precipitation is somewhat higher than in the lower fringe types. It 

 ranges from as low as 20 inches in the ponderosa pine type at the 

 lower elevations to 50 inches or more at higher elevations. Summers 

 are normally dry; much of the precipitation comes in the form of 

 snow, which accumulates to depths of 5 or 10 feet or more. The 

 melting of this snow causes a high spring run-off and sustains reason- 

 ably well a low summer stream flow from underground seepage. An 

 adequate forest litter prevails which together with the timber and 

 understory vegetation ordinarily controls erosion and regulates 

 stream flow rather effectively. 



In northern Idaho and northeastern Washington these dense forests 

 have been classed as of moderate watershed-protective influence. In 

 this section water yield is normally high, the demand for water is 

 only moderate, and erosion is seldom serious, although the loess soils 

 characteristic of the region are eroded readily if exposed. After 

 destruction of the cover by fire a plate moss forms on the soil and holds 

 it in place until brush, herbs, and timber reproduction reclothe the 

 soil completely. Erosion of course can become serious if this reclo thing 

 is hindered by overgrazing. 



Types in which the timber and other growth is dense have a very 

 high watershed-protection value on the easily eroded granitic, clay, 

 or clay-loam soils and where delivery .of maximum quantities of 

 usable water is important, as in southern Idaho. On the pumice soils 

 of Oregon and Washington these dense forests exert a slight to mod- 

 erate moisture-conserving influence through delaying snow melt and 

 improving and protecting the soil with their litter. 



In the upper reaches of Clark Fork River, in western Montana, 

 forest types commonly in more open stands are of major watershed- 

 protective influence. The demand for water and the need of regular 

 stream flow are great. Erosion may become serious if fire, grazing, or 

 some other agency thins the coyer, since revegetation tends to take 

 more time than in the more humid area on the lower reaches where the 

 forest is classed as of moderate watershed-protection value. 



Excessive run-off and erosion occur on areas around Butte and 

 Anaconda, Mont., that have been rendered practically barren by 

 smelter fumes. The fumes have killed tree growth over an extensive 



