Snags and Down Wood in the Interior Columbia Basin Ecosystem Management Project 1

Snags and down wood are major ecosystem and wildlife habitat components of the Interior Columbia Basin ecosystem. Their natural abundance and distribution have been altered by decades of land conversion, fire suppression, and timber and firewood harvest, but the degree and pattern of change have been open to question. Consequently, an analysis of the changes that have occurred was a required element of the Interior Columbia Basin Environmental Impact Statement. Members of the Science Advisory Group of the Interior Columbia Basin Ecosystem Management Project modeled snag and down wood abundance at the landscape level for the project analysis area. We constructed 10 density tables for combinations of 3 Potential Vegetation Groups, 2 temporal regimes, 2 fire regimes, and 2 public land management characterization classes. Snags and down wood were divided into two size classes, small and large, with a 53 cm DBH break point, and a 13 cm minimum DBH. Forest stands were placed into one of four categories: early-seral, mid-seral, late-seral single-strata, and late-seral multistrata. We defined five categories of snag and down wood abundance: none, rare, uncommon, common, and abundant. Each size class/abundance category combination, such as large snag/common, small down wood/rare, etc. had its own unique density range. We combined computer simulations using the Columbia River Basin Succession Model with geographic information system (GIS) data layers to project historical (pre-settlement) and current snag and down wood densities. We also made long-term (100 year) projections of snag and down wood trends based on the three public land management alternatives described in the Interior Columbia Basin Supplemental Draft Environmental Impact Statement. Our models for the Basin as a whole showed small and large snag abundances currently below the historical levels, and small and large down wood amounts currently greater than historical levels. The 100 year simulations projected small snag amounts returning to historical levels, large snag amounts increasing above current levels but not returning to historical amounts, and the small and large down wood amounts generally decreasing from current levels but remaining above historical levels.