In the mountainous Pacific Northwest, variability of the structure and composition of riparian vegetation in time and space is an integral part of the sustained diversity and productivity of aquatic ecosystems, including anadromous fish. Disturbance processes such as flooding, sediment deposition, debris flows, and wildfire help maintain diversity of riparian vegetation. Spatial patterns of disturbance regimes and vegetation development correspond to climate and geology at the broad scale, and factors such as landform at the finer scale. We propose to use understanding of the spatial distribution of climate, geology, disturbance regimes, and landforms to divide riparian networks into segments with similar potential disturbance and vegetation development characteristics. The classified riparian network forms a template for evaluating historical variability of ecosystems in the absence of overt human management, providing a useful context for planning and management. Comparison of current conditions to historical, reference conditions will provide a transparent, logical pathway for planning management of riparian vegetation on federal forest lands in northwest Oregon.
We identified three ecoregions in northwest Oregon with important differences in geology and climate (Coast Range—dissected landscape dominated by sandstone, rain-dominated hydrology; Western Cascades—dissected landscape dominated by volcanic rocks, rain- and snow-dominated hydrology; High Cascades—gentle landscape dominated by volcanic rocks, snow-dominated hydrology). Within the High Cascades, we identified 12 stream-reach types, varying in channel width, predominant disturbance process (fluvial, mixed-severity fire, stand-replacement fire), and potential presence of hardwood trees. We delineated riparian zones associated with each stream reach based on floodplain width and the area within which trees can deliver wood to stream channels. We used a regionally-comprehensive vegetation layer based on remote sensing, environmental data, and inventory plots. For historical reference conditions, we took advantage of the large extent of relatively unmanaged areas (wilderness and roadless) within the High Cascades. In addition to calculating mean representation of various vegetation states by reach type, we used bootstrapping to estimate variability for reference conditions. We compared the range of reference vegetation conditions to current conditions for an upcoming planning area on the Willamette National Forest. We identified several vegetation states for which current conditions are outside of reference conditions and so may present appropriate management objectives for restoration.