Landscape-scale Dynamics of Wood in Stream Networks of the Western Cascades, Oregon

This study develops a network and patchwork dynamics approach to predict wood in a stream network in order to understand wood dynamics in river networks and address riparian reserve design for managed forest landscapes. We examined the effect of four factors on wood sources and transport: (1) forest harvest, removal of wood from streams, and creation of young forest plantations; (2) roads adjacent to or crossing the stream; (3) debris flows in tributary channels; and (4) capacity of the stream to transport wood by fluvial processes (i.e. floods). Wood volumes and numbers of pieces were surveyed along 25 km of 3^{rd}- through 5^{th}-order stream reaches in summer 2002 in a steep forested basin in western Oregon, and related to land use (forest harvest and roads) and fluvial geomorphic processes (debris flows and floods) recorded over the period 1948- 2002. Overall 20,299 pieces of wood with a total volume of 17,688 m3 were measured (averaging 70 m3 per 100m or 80 pieces per 100 m of stream length). Large wood pieces anchored 85% of accumulations, accounting for only 8% of pieces (1468 pieces), but 66% of wood volume (9818 m3). Wood patterns reflect 50-year legacies of land use practices, especially the conversion of old-growth forest to young forest near the stream, and the construction of roads alongside, or crossing, streams. Controlling for stream order, stream segments with adjacent roads had 56%, stream segments with young forest plantations on one side had 34 to 56%, and stream segments with young forest plantations on both sides had 12% of the wood volume found in stream segments adjacent to mature or old-growth forest (150 to 500 yrs). In channels with low fluvial transport wood depletion was localized adjacent to harvest patches, but wood reductions were extensive in channels with high fluvial transport, or where debris flows have entered the mainstem. The configuration of harvest patches, road networks, and stream networks provide a landscape- scale basis to explain and predict patterns of wood in streams, and hence, the effectiveness of riparian reserves for sustaining wood in streams.