Effects of Harvest and Roads on In-stream Woody Material in Blue River Basin, Cascade Range, Oregon

Despite many studies of large wood in streams, few landscape-scale studies have been conducted, and yet the history of forest harvest and road building might be expected to have left a signal on wood patterns in streams. This study examined this question 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. Six sites, ranging from 1.6 km to 14.1 km in length, were surveyed along 2nd to 5th-order channels on public forest land. Wood volumes, numbers of pieces of large wood, numbers of accumulations and timing of emplacement were determined in the surveys. Survey data were matched with spatial data on harvest and road building practices using GIS. Study streams have undergone distributed patch clearcutting and road construction concentrated during the 1950s and 1960s. The proportions of surveyed stream lengths with harvest and road activities within 40 m was 66% (Cook Creek), 55% (Mack Creek), 53% (Lower Lookout), 37% (McRae Creek and Upper Lookout), and 7% (Quentin Creek). For all study locations combined, 50-m channel segments when there is no harvest or road influence had an average of 356 m³/ha, whereas harvested and roaded areas had from 80 to 157 m³/ha (Bonferroni adjusted p<0.03). Approximately 80% of the wood volume occurred in accumulations. The 5th-order channel (Lower Lookout) had significantly lower wood volumes (109 m³/ha) than all other locations (200-378 m³/ha, Bonferroni adjusted p < 0.05) and significantly lower numbers of large pieces (23 large pieces/ha) compared to all other locations (39 vs. 64 large pieces/ha) (Bonferroni adjusted p<0.008). Thus, the legacy of harvest and road activities conducted in the 1950s and 1960s was still apparent in in-stream wood patterns nearly 40 years later.