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

Shallow, rapid soil mass movements are common events and primarysources of sediment in steep terrain of the Pacific Northwest.Poorly vegetated debris deposits and scars resulting from landslidesremove land from the productive timber base, and are subject tocontinuing erosion. To examine the impact of these events on thetimber growth potential of forest land, height growth of Douglas—fir(Pseudotsuga menziesii (Mirb.) Franco) and stocking level of ancommercial conifer species on landslides in the western OregonCascades were compared with height growth and stocking level onadjacent clearcut units of similar aspect, elevation, ages, and slopeposition. Cumulative height growth of Douglas—fir trees 18 yearsold on the landslides was reduced 38% compared to trees on clearcuts,and the stocking level was reduced to 75% of the clearcut level.
One—third of the landslide area was estimated to be non—stockablebecause of unstable or impenetrable substrate. Calculations combiningheight growth and stocking level estimate a reduction of 40% in woodvolume grown on landslides compared to clearcuts, when trees are 18 years old.
The persistence of unvegetated landslide scars and depositswas examined by sampling established vegetation on a chronosequenceof landslides 6-28 years old in the western Oregon Cascades. Averagevegetation cover for all landslides was 51%, ranging from 7-88%.
No discernible trend in vegetation cover or species composition overtime was detected. Variations in landslide distribution and topo-graphy resulted in a wide range of plant microhabitats, with 140species identified. The combinations of wet—site, bare mineral soil,and droughty bedrock habitats disguised overall landslide vegetationrecovery trends.