1 



DISCUSSION AND CONCLUSIONS 



Mass failure of road fills is an important component of the overall erosion pro- 

 cess on slopes in the Idaho Batholith. A large body of studies elsewhere indicate that 

 deep-rooted vegetation, including trees and shrubs, should increase the mass stability 

 of fill slopes. Ponderosa pine possesses a number of attributes that makes it particu- 

 larly attractive for this purpose. In this study, the intrinsic value of ponderosa pine 

 for increasing the mass stability of fill slopes was accepted as fact and two logical 

 followup questions were investigated: (1) how well does ponderosa pine survive and grow 

 on roadfills ?nd (2) how well does ponderosa pine control surface erosion on road fills? 



Performance of planted and seeded ponderosa pine and a seeded grass mixture was 

 evaluated. Survival of the planted pine was high, averaging about 97 percent after 

 four growing seasons. Growth and vigor of the planted trees far exceeded that of the 

 seeded trees and grass. Poor grass responses were atypical, probably the result of a 

 severe grasshopper infestation during 1971 and 1972. Fertilizer increased the annual 

 growth of planted pine by an average of about 95 percent during the year of peak effect. 

 There was some indication that fertilization effects tended to decrease after 3 or 4 

 years; however, growth still a^/eraged about 45 percent greater than that of trees on 

 unfertilized plots. 



Wider spacing of planted trees also tended to increase growth, but not consistently. 

 There was an overall decrease in growth over time that was probably caused by intertree 

 competition. Increasing tree spacing from 1.5 by 1.5 to 2.5 by 2.5 feet did not in- 

 crease erosion. Considering growth responses, planting costs, and comparable erosion, 

 the 2.5- by 2. 5- foot spacing is preferable. Actually, 3- by 3- foot or even 4- by 4- 

 foot spacing should reduce the possibility of growth stagnation and probably would still 

 provide effective erosion control. 



Straw mulch held in place by netting was by far the most effective means of 

 reducing surface erosion. Effects were similar on both the tree and grass plots. 

 Mulch effects tended to diminish slightly over time, but even after 3 years, mulch 

 reduced erosion an average of 92 percent. Mulches provided additional benefit by 

 increasing tree growth on many plots. Planted trees alone provided surprisingly large 

 decreases in annual erosion rates, ranging from 32 to 51 percent. 



In conclusion, planting ponderosa pine at 3- by 3- to 4- by 4-foot spacings is 

 recommended as an erosion control measure on granitic road fills. If at all possible, 

 trees should be fertilized to accelerate growth, especially root growth. Trees alone 

 should reduce surface erosion by approximately one-third; additions of straw mulch, 

 held in place by erosion net, will reduce surface erosion by about 95 percent. 



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