Stream vertebrates after wildfire and salvage harvest: 3 megafires, 3 years, 30 mid-order streams in Western OR
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Wildfire frequency and intensity are expected to increase in many forested regions globally. Aquatic ecosystems and biota face multiple wildfire-induced changes in physical, chemical, and biological conditions, all of which can vary with burn severity, post-fire weather events and forest management, stream features, and yearly. Previously observed fish and amphibian responses range from extirpation to increased densities or no observed change, so aquatic vertebrate responses to fire remain equivocal. 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 ecosystem processes, vertebrate populations, and communities in the first three years post-fire across 30 fourth-order streams within the Riverside, Beachie Creek and Holiday Farm fires to assess stream ecosystem function and fish and amphibian population and community trends over a range of post-fire aquatic habitat conditions. Pronounced ranges 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 salmonids, these streams collectively contain fish (dace, sculpin, and lamprey), amphibian (salamanders and frogs), and crayfish species often ignored in many salmonid focused post-fire studies. 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, and vertebrate communities were minimally influenced by fire severity or extent. Vertebrate diversity was lower in more severely burned watersheds in the year-one post-fire but did not differ in years two or three indicating a short-lived response. 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.