Long-term monitoring reveals extensive variability in stream and riparian response to wildfires across the inland Pacific Northwest

ABSTRACT Objective Wildland fires in the western United States have grown larger and more intense over the past half century. These fires directly and indirectly affect riparian zones and streams, altering a variety of vegetative and stream conditions. However, current understanding of fire impacts to stream conditions depends on—and may be biased by—which streams were burned and studied in the past. For example, past research has been disproportionately focused on catchments that experienced high-severity wildfire resulting in dramatic postfire instream responses. In this study, we aim to provide a broader context for how wildland fires ranging in size (from 405 to ≥200,000 ha) and severity (from low to high) alter the near-stream and instream environment by analyzing stream and riparian data collected in 227 catchments both before and after fire occurrence. Methods We used paired t-tests to identify significant differences in stream and riparian habitats pre- and postfire. We then constructed linear models to assess whether fire severity and the proportion of the upstream catchment burned, obtained from the Monitoring Trends in Burn Severity program, help to explain variation in postfire responses. Landscape covariates, including precipitation, elevation, and stream gradient, were also incorporated into linear models to assess their ability to explain postfire stream response. Results On average, stream conditions changed in a manner consistent with previous research (e.g., increased water temperature, sediment loading, and wood accumulation). However, we observed extensive variation in the direction and magnitude of postfire responses in stream and riparian habitat conditions. The extent and severity of wildfire were major predictors for vegetative cover and stream temperature but were less important for other stream habitat variables, such as large wood and fine sediment. Conclusions Stream responses to fire in the inland Pacific Northwest are highly variable and difficult to predict based on wildfire characteristics alone. The observed variability in stream response led to changes in stream and riparian habitats that were both favorable (e.g., increases in woody debris) and potentially detrimental (e.g., increased sediment loading) for fish native to the inland Pacific Northwest. A better understanding of how stream channels and riparian zones respond to fire will help managers in determining when intervention and resources may be needed to protect important aquatic habitats.