A HATIONAL PLAK FOR AMERICAN FORESTRY 441 



years, high evaporation, and soils which lose fertility readily through 

 their tendency to be eroded easily, nature's balance for maintaining 

 he plant cover is delicate. 



Over extensive woodland areas the loss in plant cover has averaged 

 one half to three quarters, as shown by the erosion survey of the 

 watershed above Elephant Butte Dam. Such destruction is largely 

 the result of overgrazing since the drainage was settled by whites, 

 although locally, extreme changes have resulted from timber cutting. 

 In some instances an increase in tree reproduction has failed to offset 

 declines in grasses and weeds. Sheet erosion is widespread, and 

 wherever this has reached an advanced stage gullying also is severe. 

 Rapid soil wastage is attested by remains of grass clumps, sagebrush, 

 and tree reproduction on soil pedestals often a foot or more in height, 

 exposure of large tree roots, and the formation of straight-sided gullies 

 even on slopes of low gradient. The loss of fertile top soil and of its 

 moisture-holding capacity has intensified the deficiency of soil mois- 

 ture, which at best severely limits the density of vegetation. 



Such conditions prevail on most of the woodland areas in the unre- 

 served public domain and on far too many private holdings, including 

 many of the large Spanish land grants. On the national forests, 

 efforts to improve conditions through regulating grazing and timber 

 cutting have been in progress for 20 to 25 years; on many of the wood- 

 land areas, however, destruction of vegetation and soil had reached 

 such a serious degree, particularly on readily erosible clay soils, that 

 improvement of plant cover has been extremely slow and has not yet 

 stopped the abnormal erosion. 



Woodland areas have been classed as of major watershed-protective 

 influence if erosion resulting from depletion of their cover would 

 endanger irrigation or other values in valleys below. Most areas in 

 this type where erosion would chiefly affect the productivity of the 

 forest soil, and have little influence on other values, have been classed 

 as of moderate watershed-protective influence. 



Within the forest types above the woodland, watershed conditions 

 are in general reasonably good. The greater part of the water supply 

 for irrigation and for municipal use in this drainage comes from these 

 forested mountain lands, largely as stream flow from immediate sur- 

 face run-off of snow water and from springs fed by percolated snow 

 water. The forest types which produce lumber, the ponderosa pine 

 at intermediate elevations and the spruce at higher elevations, exert 

 a major watershed-protective influence through retarding snow melt 

 and run-off of snow and rain water, aiding in absorption of moisture, 

 and protecting the soil against erosion. In the ponderosa pine type 

 the tree stand is rather open but the litter cover and undergrowth of 

 grasses, other herbs, and occasional shrubs, where not depleted, is 

 normally sufficient to afford good watershed protection. In the uncut 

 spruce forests the stand of timber is generally rather dense and, with 

 its heavy duff, serves admirably in watershed protection. In these 

 types lumbering, overgrazing, and fire usually decrease the watershed- 

 protective values of the forest cover. 



Most of the commercial timberland is within the national forests. 

 Here deterioration of the protective cover has been or is being checked 

 in most instances. Marks of past erosion still remain, but numerous 

 eroded areas have been restored to cover conditions capable, under 

 effective regulation, of arresting abnormal erosion. For example, on 



