A simulation model of wood dynamics in Pacific Northwest streams

Conceptually, the dynamics of wood in streams can be viewed in terms of input and in-channel processes. Input processes are associated with both the riparian (tree fall, bank cutting, windthrow) and upsiope forests (mass failures). In-channel processes include log breakage, movement, and decomposition. A mechanistic view of these processes is presented and served as the basis for the development of a simulation model. STREAMWOOD is an individual based stochastic model that operates on an annual time step at the reach scale. Stream systems that can be simulated range from a single reach to a small basin. Stream wood dynamics considered are tree entry, breakage, movement, and decomposition. Riparian forest inputs are either from a simplified forest gap model built within STREAM WOOD or from a user-specified input file. The model is run under a Monte Carlo procedure and the results are reported as average conditions per reach. STREAM WOOD was developed for coniferous-forested streams of the Pacific Northwest fifth-order and smaller. Species considered include Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), western hemlock (Tsuga heterophylla (Raf.) Sarg.), western red cedar (Thuja plicata D. Don ex Lambert), and red alder (Alnus rubra Bong.). Single parameter sensitivity analysis of STREAM WOOD concluded that parameters affecting Redacted for privacy tree growth were the most important in the forest model and decomposition was found to
be the most important of the processes tested in the wood model. STRBAMWOOD was used to assess long-term implications of selected riparian management regimes on the standing crop of wood in channels. Total wood volume (volume of all pieces intersecting the channel) strongly related to the width of the non-harvested riparian forest. A riparian forest width of 30 m was associated with 90% of the maximum site potential, which was the standing crop associated with a 75-rn non-harvested forest. Plantation forests (clearcut at time intervals up to 120 years) had very little effect on total wood volume in rip arian forests with non-harvested forests at least 10 m wide. These results suggest that forest age and width of the non-harvested buffer was more important than the rotation age of plantation forests in providing long-term supplies of wood to the streams.