Willow (Salix spp.) and associated riparian shrub communities provide habitat to a wide variety of wildlife species. Because of high between-observer variability and a lack of standardized protocols, ground-based monitoring of willow abundance has proven difficult. The objective of this study was to evaluate variability associated with collection and analysis of field data for a photo-based monitoring technique for willow communities. We evaluated variation in data collection by photographing 5 willow clumps, 10 times each, and comparing profile-area estimates within clump. We assumed this to mimic variability associated with repeat monitoring of a given clump(s) over time. We set high-visibility markers at known distances apart to provide scale references in the photographs. We removed camera and markers and replaced them between successive photographs. Scanned images of the photographs were spatially rectified using digital image–processing software. We determined the profile area of willow clumps by digitizing clump boundaries within rectified images. We examined variability associated with image analysis by asking 6 individuals (analysts) to determine profile area for a series of 5 images. We then compared the results across analysts. We calculated sampling error for each photograph by dividing root mean square error by the mean value. Results indicate field data collection produced minimal variability; sampling error averaged 1.82% (±1.05). Between-analyst sampling error averaged 1.63% (±0.73) across clumps and was <3% for all clumps. Trained analysts took <10 minutes per image to obtain profile-area estimates. These results indicate our technique produces quick and repeatable estimates of willow abundance, would be useful in evaluating change in abundance over time, and minimizes person-to-person variability.
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