Quantifying the Variability of “Fixed-Width” Buffers on Harvested Lands in Western Oregon and Washington

Abstract

In contemporary forest management, buffers of unharvested trees are left along streams to protect riparian and aquatic ecosystems. Buffer regulations often focus on specific minimum width requirements, which aid in straightforward regulation and application, but minimum widths also suggest buffered edges are uniform and contain little variability. Conceptual papers suggest that alternative buffer configurations may offer greater flexibility in landscape-level protection, increase forest complexity, and enhance aquatic and riparian biodiversity. However, before considering alternatives to fixed-width buffers, it is necessary to quantify the inherent variability in current buffer practices present on the landscape. In this study, we used aerial imagery to quantify variability of buffer widths in two hundred randomly selected recently harvested units on managed land in Oregon and Washington with both fish-bearing and non-fish-bearing sections of stream. Wider buffers on larger streams had a greater magnitude of variability, but when normalized by stream size, variation was greater in smaller streams, and overall, variation ranged from 25% to 50% of the mean width. Despite local variability, buffer widths rarely fell below 9.14 m (30 ft). The variation quantified here provides an initial measure of variability to inform future management, given emerging interest in variable retention buffers. Study Implications: In contemporary forest management, riparian and aquatic habitat protection usually relies on minimum buffer width regulations. Minimum requirements create the perception that buffered edges are uniform distances from streams containing little variability, so conceptual papers have suggested alternative buffer configurations with variable retention edges to enhance ecological benefits. However, there are several reasons why, in practice, buffer widths may vary substantially from minimum sizes within and across harvest units under current forest management, but this potential variation has not been quantified. Before considering alternative riparian management options, it is necessary to quantify and understand the inherent variability in current practices. By utilizing high-resolution aerial imagery and digital elevation models, we quantified buffer width variation. Using a set of two hundred recently harvested units, we demonstrate notable buffer width variation across managed Pacific Northwest forests. We attribute the variation in buffer widths to the presence of road crossings, tributary junctions, underlying valley slope and slope variation, and flexibility in regulations that may be overlooked in broad evaluations of strict minimum widths. Understanding fundamental information about buffer width variability provides information about current practices and provides a standard against which proposed increases in variability may be compared.