Recruitment and abundance of large woody debris in an Oregon coastal stream system

approved: '/c '4z,i.q, / Henry A. Froehflch Research was conducted in the Oregon Coast Range to address the concern that conversion of large diameter old-growth forests to smaLl diameter second-growth forests wouLd lead to reduction of large woody debris in adjacent stream channels. The objective of the study was to quantify spatial trends in large woody debris recruitment and abundance in a stream system bordered by a secondgrowth forest. Big Creek in Lincoln County was selected for the study. The watershed was clearcut between 1922 and 1935, and subsequent1y burned by wildfire in 1936. A large woody debris inventory was conducted in firstthrough fourth-order stream channe1s. Comparisons were made between pre-disturbance (wood in place during logging and fire) and post-disturbance (contributions from new forest) woody debris types within each stream order. Approximately 5200 pieces of large woody debris (greater than 0.1 m diameter and 1 m length) were measured in 11.5 kilometers of channel. Total volume per square meter and number of pieces varied considerably among stream orders. Second-order channeIs had the heaviest debris loading (0.0422 1i13/m2), followed by first(0.0308 m3/m2), third(0.0242 m3/ni2), and fourth-order (0.0201 m3/m2) channels. Piece numbers ranged from 54.8 to 35.6 per 100 meters of channel, with a basin average of 45.1 pieces per 100 meters. Pre-disturbance debris pieces constituted 63 to 70 percent of the total number of pieces and 86 to 89 percent of the total volume within all stream orders. Species composition within the post-disturbance group varied significantly among stream orders. Thirdand fourth-order channels contained mostly hardwood post-disturbance debris, whereas firstand second-order channels contained a greater proportion of conifer post-disturbance debris. Riparian stand density and basal area per hectare were positively correlated with the recruitment of post-disturbance woody debris in some channel segments. Flotation, windthrow, and logging were the most common delivery mechanisms for pre-disturbance debris. Fifty-two percent of pre-disturbance debris pieces were located in channels or on channel banks, and 17 percent influenced pool habitat formation. Bank cutting was the predominant delivery mechanism for post-disturbance debris. Thirty-two percent of postdisturbance pieces were in channels or on channel banks, and seven percent formed pools. Most post-disturbance pieces were suspended above channels or on side terraces. Fifty-two percent of the total debris pieces were found in debris accumulations. The largest debris accumulations commonly were located at tributary junctions. RECRUITMENT AND ABUNDANCE OF LARGE WOODY DEBRIS IN AN OREGON COASTAL STREAM SYSTEM