A synoptic survey of 49 landslides ranging in age from 1 to 155 years was conducted to compare revegetation patterns and forest productivity with surrounding logged and old-growth stands. The landslides left a chaotic mixture of logs, rocks, and soils of varied proportions, further complicated by additional materials eroded from the slide edges following initial failure. The upper portion of slides was partially scoured to bedrock or more rarely to a compact glacial till, whereas the lower portion mainly consisted of slide material deposited on or mixed with original soil. Effective soil depth on the upper two-thirds of the slides was significantly less than in the lower third or in the adjacent, non-slide surround. Bedrock exposure on slides amounted to 8, 10, and 2% for upper, middle, and lower slope positions, respectively, as compared with 1, 1, and 0% for non-slide terrain. For all slides and slope positions combined, exposed mineral soil and bare bedrock averaged 88% at 4 years after failure and 14% at 90 years. The reduction in bare bedrock and mineral soil exposure occurred more rapidly on the lower than middle or upper slopes.
Soils on non-slide areas were chiefly Orthic or Gleyed Ferro-Humic Podzols. Soils on slides were severely perturbated but, in addition to the Orthic Regosols, many were recognized as Dystric Brunisols and Gleyed and Orthic Ferro-Humic Podzols. Organic carbon contents in mid-slide soils were high (avg. = 4 to 8%). Vegetation development on these soils should be considered as secondary rather than primary in nature.
Two major trends in vegetative development on slide surfaces were observed, one dominated by red alder (Alnus rubra) and one dominated by conifers. The former was most common on the lower portion of slides, particularly those occurring in low elevations on material derived from fine-textured bedrock or those with a relatively high calcium content. Conifers tended to dominate on the middle and upper portions of slides, especially in high elevations on relatively coarse and acid soil material. Initial revegetation was dependent upon the availability of stable mineral soil microsites and islands of debris and other remnant organic material.
Vegetation on slides was classified into eight groups and on logged areas into three groups based on dominant plants. The groups particularly reflected stages of development, degree of red alder invasion, and type and stability of substrate.
Revegetation in logged areas differed from the adjacent slides largely as a result of a higher component of western hemlock (Tsuga heterophylla), lower stocking of red alder, and a greter proportion of stable microsites. Logged areas produced about 3 times the volume of wood than slides in the same (30- to 59-year) age class. A lack of older logged stands in the study precluded direct comparisons at later stages. However, based on appropriate yield tables it was established that the oldest group of slides, averaging 85 years of age, had produced about one-half the wood volume (conifer and red alder) of normal second-growth stands of the same age.
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Updated December 16, 2008