Riparian vegetation and abundance of woody debris in streams of southwestern Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wm117r08b

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  • Riparian vegetation (trees > 10 cm dbh), woody debris (> 10 cm diam, > 1 m long), channel, and landform characteristics were inventoried in mature (80-150 yrs) and old-growth riparian areas (>250 yrs) selected from small tributaries of the Coos and Coquille River basins in southwestern Oregon. Basal area of riparian trees ranged from 39-94 m2/ha in old-growth stands, and from 28-75 m2/ha in mature aged stands; tree density was 192 stems/ha in both oldgrowth and mature stands. In active channels of old-growth forests, debris density was 58 pieces per 100 m of channel, and volume was 382 m3/ha of channel area; in mature sites, density and volume of woody debris were 41 pieces per 100 m, and 190 m3/ha, respectively. Basal area of riparian trees and quantities of woody debris associated with the stream channel and on the forest floor were significantly greater in old-growth than in mature-aged stands (p < 0.10). Except for mean piece volume, piece dimensions and mean tree diameter were similar between stand age-classes (p>0.10); very large pieces of debris (> 60 cm diam, > 15 m long) were found in significantly greater quantities in old-growth stands than in mature stands (p < 0.10). Density (#/ha) and basal area of riparian trees were directly related to density and basal area of woody debris associated with the stream channel. Basal area and density of conifer trees were inversely related to basal area and density of hardwood trees. For all sites combined, basal area of coniferous trees was significantly greater on hillslopes than on valley floor surfaces; on hillslopes, basal area of coniferous trees was significantly greater than basal area of deciduous trees. Species composition of woody debris associated with the stream channel and on the riparian forest floor reflected the species composition of surrounding vegetation. Conifer trees dominated both riparian vegetation and woody debris. Relative abundance and basal area of coniferous debris increased significantly with relative abundance and basal area of coniferous vegetation. However, conifer always constituted > 50 % of the total debris density and > 60 % of the total debris basal area, even at sites where a very small portion of riparian trees was conifer. Conifer debris is likely to predominate in forest stands where a large component of the vegetation is hardwood for several reasons: 1) a large proportion of debris in mature forests may be remnant from previous old-growth stands, 2) decay rates of coniferous species are lower than decay rates of deciduous species, and 3) delivery rates of coniferous and deciduous trees differ. Frequencies of debris in accumulations of three or more pieces increased with increasing debris density (#/100 m) and active channel width. Woody debris was most often associated with pools and cascades. Streams in clearcuts with riparian bufferstrips had similar quantities of woody debris in and above the channel to that found in unharvested forests. Clearcut sites with narrow bufferstrips (< 30 m slope distance between the channel and harvest area) had abundant slash on the riparian forest floor that was smaller in length and diameter than natural debris. Implementation of Oregon Forest Practice Rules on state and private lands would result in a 70-80 % decrease in conifer tree density and 95 % decrease in conifer tree basal area from levels that are found in natural old-growth and mature riparian areas. Loss in conifer vegetation from old-growth and mature riparian forests will result in a decreased supply of conifer debris to streams.
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