Impact of landslide erosion on tree growth and vegetation in the western Oregon Cascades

Graduation date: 1983Shallow, rapid soil mass movements are common events and primary sources\ud of sediment in steep terrain of the Pacific Northwest.\ud Poorly vegetated debris deposits and scars resulting from landslides\ud remove land from the productive timber base, and are subject to\ud continuing erosion. To examine the impact of these events on the\ud timber growth potential of forest land, height growth of Douglas-fir\ud (Pseudotsuga menziesii (Mirb.) Franco) and stocking level of all\ud commercial conifer species on landslides in the western Oregon\ud Cascades were compared with height growth and stocking level on\ud adjacent clearcut units of similar aspect, elevation, ages, and slope\ud position. Cumulative height growth of Douglas-fir trees 18 years\ud old on the landslides was reduced 387. compared to trees on clearcuts,\ud and the stocking level was reduced to 75% of the clearcut level.\ud One-third of the landslide area was estimated to be non-stockable\ud because of unstable or impenetrable substrate. Calculations combining\ud height growth and stocking level estimate a reduction of 40% in wood\ud volume grown on landslides compared to clearcuts, when trees are 18 years old.\ud The persistence of unvegetated landslide scars and deposits\ud was examined by sampling established vegetation on a chronosequence\ud of landslides 6-28 years old in the western Oregon Cascades. Average\ud vegetation cover for all landslides was 51%, ranging from 7-88%.\ud No discernible trend in vegetation cover or species composition over\ud time was detected. Variations in landslide distribution and topography resulted in a wide range of plant microhabitats, with 140\ud species identified. The combinations of wet-site, bare mineral soil,\ud and droughty bedrock habitats disguised overall landslide vegetation\ud recovery trends