Resistance of stream vertebrate populations and assemblages post-fire in Western OR

Wildfire frequency and intensity are expected to increase in many forested regions. Aquatic biota face multiple wildfire-induced changes in physical, chemical, and biological conditions, and these can vary with burn severity, post-fire weather, geomorphic events, stream features, and fluctuate over time. Aquatic vertebrate responses can range from extirpation to increased densities or no observed change. Because of the unpredictable nature of wildfires, many fire studies are limited in replication, further contributing to highly context-dependent biotic responses. We evaluated aquatic vertebrate populations and communities in the first three years post-fire across 30 fourth order streams within the Riverside, Beachie Creek and Holiday Farm fire boundaries to assess fish, amphibian, and crayfish trends over a range of post-fire aquatic habitat conditions. Pronounced changes in physical characteristics were observed with burned canopy cover ranging from 15 to 95%, with concomitant changes in stream light and temperature. In addition to multiple species of trout and salmon, these streams collectively contain fish (dace, sculpin, and lamprey), amphibian (salamanders and frog), and crayfish species often ignored in many salmonid focused post-fire studies allowing us to also evaluate aquatic vertebrate community structure. Preliminary findings suggest that within the first three years, total vertebrate biomass was greater and age-0 trout size was larger in streams draining more severely burned watersheds. However, relative to landscape variables (position, elevation, or pre-fire stand age) vertebrate communities were minimally influenced by fire severity or extent. The lack of high intensity precipitation following these fires allows this study to uniquely evaluate wildfire effects unconfounded by major geomorphic changes and further contributes to the growing body of empirical data on the persistence of aquatic vertebrates in post-fire conditions. The views expressed in this abstract are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency.

Impact/Purpose

Wildfires are a growing concern in the western United States, where wildfire severity and size has increased in recent decades. Wildfires introduce many physical and chemical changes to stream ecosystems, with potential for adverse impacts to fish and other aquatic life.  Many studies to date have focused on single fish species, and have tended to rely on observations from a small handful of sites.  This study assesses post-wildfire aquatic vertebrates across 30 different streams in western Oregon, USA, and examines multiple fish, amphibian, and invertebrate species. Despite some sites having relatively high wildfire severity and a large portion of watershed burned, there was relatively little short-term effect of wildfire on stream vertebrate assemblages after accounting for other factors that influence assemblage composition.  Future work will continue to monitor these aquatic assemblages to assess longer-term effects of wildfire.

Citation

Swartz, A., A. Coble, B. Penaluna, R. Flitcroft, J. Ebersole, AND M. Krawchuk. Resistance of stream vertebrate populations and assemblages post-fire in Western OR. Oregon Chapter American Fisheries Society, Bend, OR, February 26 - March 01, 2024.