Effect of Root Strength and Soil Saturation on Hillslope Stability in Forests with Natural Cedar Decline in Headwater Regions of SE Alaska

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Oxford & IPH Pub. Co. Ltd., New Delhi

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Effect of Root Strength, Root Strength, Soil Saturation, Hillslope stability, forests, natural cedar decline, headwater regions, SE Alaska


A natural decline in the population of yellow-cedar (Clla~mecyparis nmthtensis) is occurring in pristine southeast Alaska forests and may be the most S~Icant forest decline in the western United States. The frequency of landslides in cedar decline areas is three times larger than in areas of healthy forest. Three regions are investigated in order to determine the influence of yellowcedar decline on root strength, soil saturation, and consequently, slope stability. Nearly complete deterioration is observed in mots up to 25 mm thick on cedar trees that have been dead for approximately 14 years. Soil saturation is quantified with piezometers installed in 120 wells located in steep (25 to 45 degree) hillsides with forests of cedar decline, healthy cedar, and spruce/ hemlock. Piezometric measurements indicate that the frequency of soil saturation is not demonstrably greater in areas of cedar decline, suggesting that the increased slide frequency in these areas is due to loss of soil strength through root deterioration. This conclusion is supported by the observation that complete soil saturation occurs regularly in slide-prone areas of both healthy and dead cedar.

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