Corbett and Rice (1966) found a similar situation in southern California. Mass 

 movement was five times greater on brushlands converted to grass than on unconverted 

 brushlands. Apparently, the deep-rooted brush provided more stability than that 

 maintained by the grass after conversion. Bishop and Stevens (1964) and Swanston (1967) 

 stated that mass erosion was reduced by tree roots on shallow soils in southeast Alaska. 

 Gray (1969) summarized the literature and showed that trees tend to reduce mass erosion 

 by mechanical reinforcement from roots, soil moisture depletion from transpiration, and 

 providing a surcharge from the weight of the trees. Trees can have a destabilizing 

 effect because of windthrowing and root wedging; however, he concluded that the net 

 effect of tree cover is to increase mass stability. 



In summary, a major problem associated with road construction on steep granitic 

 lands is that of stabilizing road fills with regard to hoth surface and mass erosion. 

 Research has shown that surface erosion on road fills can be greatly reduced and 

 areas of mass erosion can be stabilized by deep-rooted vegetation. Thus, the problem 

 is to find a way of establishing vegetation that will reduce both types of erosion. 

 Shrub planting presents possibilities; however, at the outset of this study, tree plant- 

 ing was the most logical solution. Tree planting is used extensively throughout the 

 Southeast to successfully control surface erosion (USDA Agricultural Research Service 

 1967) . The question arises as to which species is best suited for this purpose on Idaho 

 Batholith lands. Preferably, the species used should be a native that (1) has a wide 

 habitat range in the Idaho Batholith; (2) is well adapted to the mineral soils found 

 in harsh, steep sites; (3) has a rapidly growing and extensive root system with a taproot 

 if possible; and (4) is readily available as planting stock or seed, or both. 



Ponderosa pine {Firms pondevosa Laws.) is a species that meets most of the above 

 requirements. This native species is naturally distributed over most of the Idaho 

 Batholith (Munns 1938) . It is found as a climax species on warm, dry sites or, more 

 commonly, as a serai species on cooler, raoister sites. 



Road fills on granitic soils constitute harsh sites for vegetative growth and 

 survival of most species. Coarse-textured surfaces can be hot during the summer because 

 of the lack of plant cover and can dry rapidly, at least on the surface, during rainless 

 periods. Curtis and Lynch (1957) showed that ponderosa pine survival and growth are 

 most dependent on available soil moisture. They found that seedlings possessed the 

 ability to withstand prolonged drought and high surface temperatures. Drought resist- 

 ance was primarily due to rapid growth of the root system, especially the taproot. Tree 

 growth was more rapid when water use by competing vegetation was minimized and when 

 sunlight was maximized. 



Road fills necessarily exceed the depth of the original soil, often on the order 

 of two to five times or more. Summarizing edaphic requirements of ponderosa pine, 

 Curtis and Lynch (1957) stated, the species probably reaches its best development on 

 well-drained, deep, sandy gravel and clay loams. Olson and others (n.d.) found the 

 most important soil-growth relationship for ponderosa pine to be a direct increase in 

 growth with soil depth for three different kinds of soils. Similar results were found 

 by Cox and others (1960) . 



The rooting characteristics of ponderosa pine are probably a causal factor of 

 drought resistance and growth response. Ponderosa pine develops a strong taproot, that 

 is considered to be deep (Kramer 1949) . Curtis and Lynch (1957) stated that root growth 

 is rapid and continues after cessation of top growth. They reported average lengths of 

 22.4 inches for roots on 1-year-old natural seedlings and 60.7 inches for 4-year-old 

 seedlings on severely burned sites with southerly aspects on coarse granitic soils in 

 Idaho. Similar results were found by Boldt and Singh (1964), who reported root depths 

 on planted 2:1 ponderosa pine stock of 2, 4,9, and 6.4 feet, and 1, 2, and 4 years, 

 respectively, after planting in Nebraska. 



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