Distribution of Willows Along Streambanks of Publicly Managed Streams in the Interior Pacific Northwest

Abstract

Willows are an important component of western riparian zones as their roots stabilize streambanks, their overstory mediates stream temperature, and aquatic biota use their stems and leaves as forage and building material. We evaluated 1894 stream reaches in the Inland Pacific Northwest for the presence of willows and detected one or more willow species at 1 247 (66%) of those reaches. Most stream reaches where willow was present had two or more willow species. We identified 23 unique species, of which 11 were found in 50 or more stream reaches. Across all sites, stream reaches with willow were more likely to be in the southern high-elevation portion of the study area. Distribution models constructed using random forests included a measure of spatial location (latitude, longitude, elevation) as one of the top predictors for 10 of the 11 common species. Only one species included a stream reach-scale attribute (bankfull stream width) as a top predictor, although two species had gradient or percent of the reach forested as a secondary predictor. While spatial location within the study area was a key factor in determining species presence, differences among species resulted in willows being found across the range of landscape and stream reach conditions except where forest cover in the reach was high (> 75%). Given that willows share characteristics such as rapid root growth, rhizomes, flexible stems, and vegetative reproduction, in many situations different species may respond as functional equivalents when it comes to protecting streambanks, providing habitat for fish and wildlife, and promoting stream heterogeneity. Ongoing threats to willows near streams on public lands include grazing, water diversion, mining, and climate change. Each of these threats can be at least partially addressed by identifying stream reaches inhabited by willow and establishing reach-scale management objectives for this taxon.