Roots and the stability of forested slopes

Root decay after timber cutting can lead to slope failure. In situ measurements of soil with tree roots showed that soil strength increased linearly as root biomass increased. Forests clear-felled 3 years earlier contained about one-third of the root biomass of oldgrowth forests. Nearly all of the roots < 2 mm in diameter were gone from 7-year-old logged areas while about 30 percent of the < 17 mm fraction was found. Extensive brushfields occupied areas logged 12 to 24 years earlier. The biomass of brushfield roots < 2 mm in diameter was 80 percent of that in the uncut forest, and fewer large roots were found there than in the forest. Roots < 17 mm in diameter in the brushfield accounted for 30 percent of that found in the forest, and for total root biomass, only 10 percent. Individual, live brush roots were twice as strong as conifer roots of the same size. This difference may partially compensate for reduced root biomass in brushfields. Net strength of the soil-root matrix in brushfields was about 70 percent of that in uncut forests. If soils are barely stable with a forest cover, the loss of root strength following clear-felling can seriously affect slope stability. Racines et la stabilite des pentes bois6es R6sum6. La Pourriture des racines apr6s coupe peut aboutir ZL des glissements de terrain. La mesure faite in situ du sol contenant des racines