Effects of harvest and roads on in-stream wood abundance in the Blue River Basin, western Cascades, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/d217qs485

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  • Despite many studies of large wood in streams, few landscape scale studies have been conducted. Large-scale studies can reveal how the history of forest harvest and road building has influenced wood patterns in streams of the Pacific Northwest. This study examined the relationships between wood in streams, timber harvest, and road building at large (>50 km² ) spatial and long (>25 year) temporal scales, based on longitudinal surveys of 25.2 km of stream length in five sub-basins of the Blue River Basin, Cascade Range, Oregon. In-stream wood was surveyed in six sampling sites, ranging from 1.6 km to 14.1 km in length along 2nd to 5th -order channels in public forests. Wood volumes, numbers of pieces of large wood, numbers of accumulations and timing of emplacement were determined at 50-m intervals. Wood volumes and numbers of pieces were expressed per unit of channel area in order to account for variations in channel width. Wood volumes and piece numbers were related to spatial data on harvest and road building using GIS. Analysis of variance (using SAS) tested how wood volumes and piece numbers were related to the presence of a harvest or road unit within 40 m of the stream, sampling site, timing of harvest, distance harvest was from the channel, and effects of natural processes. P-values for pairwise comparisons were adjusted using a Bonfenoni procedure. Distributed patch clearcutting and road construction were concentrated in the 1950's and 1960's in the Lookout Creek basin and 1960's to the 1980's in the Upper Blue River basin. A total of 66% (Cook Creek), 55% (Mack Creek), 53% (Lower Lookout), 37% (McRae Creek and Upper Lookout), and 7% (Quentin Creek) of stream length had harvests and/or roads within 40 m of the stream. Approximately 80% of the wood volume and 85% of the number of large pieces occurred in accumulations. Wood volumes were lower in 5th-order compared to 3rd -order streams. Lower Lookout (the only 5th-order channel) had significantly lower wood volumes (109 m³/ha) than all other locations (200-378 m³/ha, p<O.O5) and significantly lower numbers of large pieces (23 large pieces/ha) compared to all other locations (39 vs. 64 large pieces/ha, p<O.008). All other locations had fairly similar wood volumes and numbers of large pieces For all study sites combined, 50-m stream segments without harvests or roads had significantly higher volume (356 m³/ha) and number of large pieces (57 pieces/ha), than stream segments with harvests or roads within 40 m (80 to 157 m³/ha, p<O.O3; 19 to 39 pieces/ha, p<O.O4). Stream segments without harvests or roads (32 pieces/ha) also had significantly higher number of accumulations compared to stream segments with roads (16 pieces/ha, p<O.004), or compared to stream segments with harvest on one side and roads (16 pieces/ha, p<O.004). Distance a harvest was from the stream and timing of harvest did not appear to affect wood volume or number of large pieces. Harvests and roads were associated with decreases in wood abundance 50 m upstream and 50 m downstream of harvest units and road crossings. The 50-m stream segments immediately upstream (171.8 m³/ha) and downstream (191.6 m³/ha) of a section of stream with harvest and roads had significantly lower volume than the 50-m stream segments without harvest or roads (427.7 m³/ha, p<O. 1). Natural processes such as floods, windthrow, near-stream toppling, and debris flows from the last 50 years had not obscured the effects of harvest and roads on wood volume and number of pieces in stream channels of Lookout Creek and Blue River. The legacy of harvest and road activities was still apparent in in-stream wood patterns up to 50 years later in this site, which consisted of old-growth forest subjected to dispersed patch clearcutting, road construction, and salvage logging conducted in the 1950's to the 1980s. However, harvest, road, and salvage practices on public lands have changed since that period. Therefore, these results should be extrapolated to other sites only with careful consideration of site history.
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