Figure 4. — Ponderosa pine tree {Pinus ponderosa Laws) buttressing 

 and restraining a slope in a road cut in granitic soil of the Idaho 

 batholith. Note massive buttress roots in right foreground. 



Figure 5. — Slope buttressing by ponderosa pine. Unbuttressed part of 

 slope on left has failed. Mendocino National Forest. California. 



CONSEQUENCES OF REMOVAL 



One way of ascertaining the significance or contribution of 

 forest vegetation to the stability of slopes is to examine the 

 consequences of its removal. The concensus of investigators 

 on this question is clear, namely, indiscriminate removal 

 weakens soils and destabilizes slopes. This concensus applies 

 not only to the Idaho batholith but to other forested slopes as 

 well (Bishop and Stevens 1964; Rice and Krammes 1970; 

 Swanston 1 974; Swanson and Dyrness 1 975: 0'Loughlin 1 974; 

 Swanston and Swanson 1976; and Wu 1976). 



The landslide inventory conducted by Megahan and others 

 (1978) in the Boise and Clearwater National Forests revealed 

 significant contributions of vegetation roots to stability of steep 

 mountain slopes. Landslide hazard was observed to increase in 

 direct proportion to the amount of vegetation removed because 

 of root decay. The amount of residual vegetation, including both 

 trees and shrubs, was an important factor regulating the in- 

 crease in landslide hazards following timber cutting or forest 

 fire. The rate of root decay relative to the rate of new root growth 

 appeared to determine the time of occurrence of maximum 

 landslide hazard following vegetation removal. On the average, 

 landslide hazards were greatest 4 years after timber removal 

 and remained high for about 6 years. By the end of 20 years, 

 landslide hazards had returned to their predisturbance levels. 

 These slide-vegetation relationships are shown graphically in 

 figures 6 and 7. 



10 20 30 40 10 20 30 40 



Trees - Percent Slide Shrubs - Percent Slide 



Occurrence Occurrence 



Figure 6. — Landslide occurrence by vegetation crown cover, western 

 and central Northern Rocky Mountain province (from Megahan and 

 others 1978). 



Years Since Logging 



Figure 7. — Landslide occurrence by years since logging, western and 

 central Northern Rocky Mountain province (from Megahan and 

 others 1978). 



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