After devastating wildfires, watersheds surprisingly thick with fish and amphibians

CORVALLIS, Ore. – In the aftermath of historically severe wildfires in 2020, a study of Cascade Range watersheds found that stream vertebrates are doing surprising well, highlighted by flourishing fish populations.

“Our work looked at the three years following megafires in western Oregon and suggests that fishes are thriving and amphibians are persisting,” said Oregon State University postdoctoral researcher Allison Swartz, who led the study.

Swartz and collaborators at OSU, the National Council for Stream Improvement, Inc., the U.S. Forest Service’s Pacific Northwest Research Station and the U.S. Environmental Protection Agency examined 30 watersheds in moist conifer forests on the western slope of the Cascades.

Federal, state and private lands comprised the study area.

Unique aspects of the research were its focus on larger streams that host non-salmonid fishes like dace, sculpin and lamprey, as well as salamanders, frogs and crayfishes, omnivorous crustaceans important to understanding aquatic food webs; crayfish appear to be persisting too.

Previous ecological studies, the researchers note, have tended to mainly focus on economically and culturally important trout, salmon and steelhead.

The watersheds, among the nearly half-million acres burned by the Riverside, Beachie Creek and Holiday Farm fires around Labor Day in 2020, experienced varying levels of burn severity and post-fire salvage logging and replanting.

“Understanding the fire ecology of freshwater ecosystems is critical to our learning to co-exist with fire in ways that are socially and ecologically just,” said co-author Meg Krawchuk, an associate professor in the OSU College of Forestry.

The scientists found that total vertebrate, total fish and trout densities were greater in streams draining more severely burned watersheds than those draining less burned or unburned watersheds. Channel reorganization events, such as landslides after wildfire, are known to affect fish and amphibian densities but had not occurred in the study area.

“Despite experiencing high-severity megafires, vertebrate assemblages and populations seem to be buffered from fire-induced changes if adequate physical habitat and food availability are maintained post-fire,” Swartz said. “Sculpin, amphibian and crayfish densities did not appear to be influenced by burn severity. In areas with higher degrees of salvage harvesting and replanting, there were lower frog densities but greater densities of trout less than a year old.”

The researchers note that a combination of increased temperatures, shifts in precipitation, higher fuel aridity and prior forest management practices is resulting in longer fire seasons and greater frequency, size and intensity of fires in many parts of the world, including the Pacific Northwest.

Wildfire can change the structure and function of riparian and freshwater ecosystems in a number of ways that can affect stream vertebrate populations, the scientists say.

“More research exists on connections between burn severity and streamflow, sediment and stream temperature,” Swartz said. “Less work has been done looking at how burn severity and post-fire salvage harvest influence populations of all of stream vertebrates present and the overall assemblage.”

The National Council for Air and Stream Improvement, the Pacific Northwest Research Station, the Environmental Protection Agency and Weyerhaeuser provided funding for the study, which was published in Nature Communications Earth and Environment.

Brooke Penaluna and Becky Flitcroft of the Forest Service, Ashley Coble of the National Council for Air and Stream Improvement and Joe Ebersole of the EPA also contributed to the research.