Association between cedar decline and hillslope stability in mountainous regions of southeast Alaska

Old-growth forests experiencing widespread decline of yellow-cedar (Chamaecyparis nootkatensis) in southeast Alaska have a 3.8-fold increase in the frequency of landslides. We report here on an investigation of the cause of this increased slope instability. Time since death of cedar was assessed using surveys around landslide sites. Root decay on dead trees was used to estimate the decline in the apparent soil strength provided by roots. Changes in soil hydrology were measured with 120 piezometers located in areas of healthy cedar, healthy spruce/hemlock, and sites with cedar decline. Relative influences on slope stability by changes in soil moisture and root strength were evaluated with a simple stability model. At most sites, soil depth is <0.7 m, and the loss of root strength has an important and possibly dominant influence on slope instability. In soils deeper than 1 m, changes in pore pressure have a proportionately larger influence on slope stability. Landslides appear most likely when cedar decline reaches snag class IV (approximately 50 years after tree death), when most of the cedar root strength is lost and root strength from secondary growth has yet to develop.