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Forestry Sciences Laboratory - Moscow, Idaho
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Declining root strength in Douglas-fir after felling as a factor in slope stability

Burroughs, E.R., Jr.; Thomas, B.R. 1977. Declining root strength in Douglas-fir after felling as a factor in slope stability. Research Paper INT-190. USDA Forest Service. Intermountain Forest and Range Experiment Station. 27 p.

Keywords: root strength, slope stability

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Abstract: Numbers of roots per unit area of soil and the individual root tensile strength for two varieties of Douglas-fir -- Coast fir (Pseudotsuga menziesii) in western Oregon and Rocky Mountain fir (var. glauca) in central Idaho -- decreased rapidly with time after felling. Seventy-five percent of the Douglas-fir roots 1 cm or smaller in diameter were lost within 24 months after felling in the coastal variety, and within 60 months after felling in the Rocky Mountain variety.

Numbers of roots per unit area of soil were combined with tensile strength of individual roots to estimate total tensile strength per unit area of soil. Total tensile strength declined from about 1,700 kg/m2 to about 230 kg/m2 within the first 30 months after felling for roots 1 cm and smaller of the Coast Douglas-fir. Total tensile strength declined from about 850 kg/m2 to about 300 kg/m2 for the same time period and root size class of the Rocky Mountain Douglas-fir. Coast Douglas-fir roots are stronger than those of the Rocky Mountain variety, but decay faster. Live strength is a characteristic of the variety of fir, but differences in decay rates are probably a function of climate.

Moscow FSL publication no. 1977a