Association between Cedar Decline and Hillslope Stability in Mountainous Regions of Southeast Alaska. Johnson, A. C. & Wilcock, P. 46(1-2):129–142.
Association between Cedar Decline and Hillslope Stability in Mountainous Regions of Southeast Alaska [link]Paper  doi  abstract   bibtex   
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 $<$0.7 m, and the loss of root strength has an important and possibly dominant influence on slope instability. In soils deeper than 1 m, changes in pore pressure have a proportionately larger influence on slope stability. Landslides appear most likely when cedar decline reaches snag class IV (approximately 50 years after tree death), when most of the cedar root strength is lost and root strength from secondary growth has yet to develop. [Excerpt: Conclusion] Cedar decline in SE Alaska may contribute to increased frequency in landsliding through an increase in soil saturation or through the loss of soil strength as roots decay. Observations of peak water table heights in 90 peizometers suggest that peak saturation levels on steep hillslopes do not differ significantly among areas of cedar decline and healthy cedar or spruce/hemlock forests. Although cedar-decline sites are not generally more saturated than surrounding ” healthy” forests, cedar-decline roots within shallower soils ($<$0.7 m) tend to remain saturated longer than roots in healthy cedar sites. Preliminary data suggest that some cedar-decline sites, with smaller contributing areas, may obtain levels of saturation found at healthy forest sites with larger contributing drainage areas. In addition, since anecdotal field observations indicate that declining cedar trees tend not to grow on elevated substrate located within areas of periodic blowdown, their roots may be exposed to higher levels of soil saturation and lower nutrient availability than roots within healthy forests of cedar and spruce/hemlock. [\n] A survey of root deterioration indicates that 70\,% to 90\,% of roots with diameters up to 30 mm are decayed when cedar decline reaches snag class II and that most of the root mass has decayed when decline reaches snag class IV. Because the abundance of understory vegetation is likely to increase at cedar-decline snag class V, we speculate that the minimum root strength available to resist landslides may occur at snag class IV. This supposition is supported by more landslides occurring in areas where the majority of snags are cedar-decline class IV. [\n] Model results indicate that the stability of shallow soils at the Prince of Wales and Mitkof sites was influenced more by root deterioration than by soil saturation. In the Baranof area, where deep deposits of volcanic ash were common, slope stability appeared to be reduced more by elevated pore pressure than by loss of cohesion due to root strength. Because the majority of soils in steep, headwater regions of the Tongass National Forest are not formed on deep soils of volcanic ash, but rather on slopes with shallow soils, root cohesion may be a major factor contributing to landslide initiation in areas of cedar decline. In addition, where forests are clearcut, the loss of root cohesion in shallow soils on steep slopes may have pronounced influence on reduced hillslope stability.
@article{johnsonAssociationCedarDecline2002,
  title = {Association between Cedar Decline and Hillslope Stability in Mountainous Regions of Southeast {{Alaska}}},
  author = {Johnson, A. C. and Wilcock, P.},
  date = {2002-07},
  journaltitle = {Geomorphology},
  volume = {46},
  pages = {129--142},
  issn = {0169-555X},
  doi = {10.1016/s0169-555x(02)00059-4},
  url = {https://doi.org/10.1016/s0169-555x(02)00059-4},
  abstract = {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.

[Excerpt: Conclusion]

Cedar decline in SE Alaska may contribute to increased frequency in landsliding through an increase in soil saturation or through the loss of soil strength as roots decay. Observations of peak water table heights in 90 peizometers suggest that peak saturation levels on steep hillslopes do not differ significantly among areas of cedar decline and healthy cedar or spruce/hemlock forests. Although cedar-decline sites are not generally more saturated than surrounding ” healthy” forests, cedar-decline roots within shallower soils ({$<$}0.7 m) tend to remain saturated longer than roots in healthy cedar sites. Preliminary data suggest that some cedar-decline sites, with smaller contributing areas, may obtain levels of saturation found at healthy forest sites with larger contributing drainage areas. In addition, since anecdotal field observations indicate that declining cedar trees tend not to grow on elevated substrate located within areas of periodic blowdown, their roots may be exposed to higher levels of soil saturation and lower nutrient availability than roots within healthy forests of cedar and spruce/hemlock.

[\textbackslash n] A survey of root deterioration indicates that 70\,\% to 90\,\% of roots with diameters up to 30 mm are decayed when cedar decline reaches snag class II and that most of the root mass has decayed when decline reaches snag class IV. Because the abundance of understory vegetation is likely to increase at cedar-decline snag class V, we speculate that the minimum root strength available to resist landslides may occur at snag class IV. This supposition is supported by more landslides occurring in areas where the majority of snags are cedar-decline class IV.

[\textbackslash n] Model results indicate that the stability of shallow soils at the Prince of Wales and Mitkof sites was influenced more by root deterioration than by soil saturation. In the Baranof area, where deep deposits of volcanic ash were common, slope stability appeared to be reduced more by elevated pore pressure than by loss of cohesion due to root strength. Because the majority of soils in steep, headwater regions of the Tongass National Forest are not formed on deep soils of volcanic ash, but rather on slopes with shallow soils, root cohesion may be a major factor contributing to landslide initiation in areas of cedar decline. In addition, where forests are clearcut, the loss of root cohesion in shallow soils on steep slopes may have pronounced influence on reduced hillslope stability.},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-13682103,~to-add-doi-URL,alaska,chamaecyparis-nootkatensis,clear-cutting,forest-resources,landslides,root-deterioration,soil-resources},
  number = {1-2}
}
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