Science Corner
Cheryl Lowe, NOSC Board Member

Last month I hiked through a burned forest area on the east side of the Olympics. Tunnel Creek is a lovely trail thru mature forest parallel to a babbling brook (at least until the trail crosses the creek and heads up to a ridge). Last year a wildfire swept through the lower portion of that watershed. Seeing the charred trees and thinking about landscape changes over time made me wonder what scientists think might be the impacts of a growing number of larger and more intense forest fires throughout the PNW on salmon.

Pacific salmon have evolved and adapted over time to a constantly changing landscape, whether it be natural disturbances like windstorms, floods, shifting river channels, fluctuating temperatures or wildfires. Over time, distinct species and even populations of the same species in diverse locations evolved specific behaviors or strategies to adapt and thrive in a particular place. In the last 200 years, dams, timber harvest, and the removal of beaver as major ecological engineers have also contributed to the decline of highly complex salmon and trout habitats in our region.

Increasing frequency and size of wildfires throughout the west as a result of wildfire suppression and changing climate can only mean bad things for salmon, right? Maybe, but not necessarily all bad. As our understanding of fire-impacted landscape increases, scientists are beginning to understand how the complexity of natural systems in time and landscape influence the answer to that question.

In one effort to predict what might happen, scientists at the US Forest Service’s Pacific Northwest Research Station used computer models and field data to predict the potential effects of wildfire on spring Chinook salmon and bull trout habitat in the Wenatchee River sub-basin. By looking into the specifics of how various spring Chinook life stages utilize stream habitats, they found that in the years following a wildfire, more in-stream wood from the accumulation of dead trees falling into the stream improves habitat quality for adults and overwintering juvenile spring Chinook. On the other hand, in the short term, fine sediment washing into streams after forest fires can degrade egg and fry habitat downstream. For bull trout that depend on cold water temperatures found in headwater streams, debris flows or landslides following wildfires may temporarily isolate these populations even more, adding to the challenges of a warming climate. That said, the scientists also suggest that “Forest management activities, such as enhancing river network connectivity through fish passage barrier removal and reducing predicted fire intensity and sizes, may increase the resilience of bull trout in the face of disturbances such as climate change and wildfire" (USFS Pacific Northwest Research Station).

At the Wild Salmon Center in British Columbia, researchers are also looking at long-term benefits as well as challenges of increasing frequency and size of wildfires.

“The fish we love and are trying to conserve are highly adapted to these landscapes,” says Dr. Gordon Reeves, a long-time Wild Salmon Center research collaborator. “In places from Yellowstone to the John Day and Montana, native fish populations have been shown to recharge, grow stronger after fires.”

He explains: “Fire is a natural event and it’s important from an ecological perspective. Following fires, landslides and debris flows introduce sediment to rivers, and sediment is one of the main building blocks of new habitat for salmon.” The Wild Salmon Center notes that over time, water pressure flushes gravel downstream, meaning rivers need regular influxes of new sediment for salmon to thrive. According to Dr. Reeves, “that recharge typically comes from the kind of large sediment pulses that follow wildfires—a bonanza of materials ranging from fine silt to huge boulders and wood.”

“It’s worth remembering”, says Dr. Reeves, “that ecological processes are complex—never simply all good or all bad—and that complexity should be part of how we talk about our interventions. Acknowledging that wildfires hold a silver lining for salmon means we can be more strategic in how we approach wildfire management.”

For fisheries, tactics for increasing resilience of salmon to altered hydrology and higher stream temperature include restoring stream and floodplain complexity, reducing road density near streams, increasing forest cover to retain snow and decrease snow melt, and identifying and protecting cold-water refugia (Wild Salmon Center).

It can be overwhelming to think about how many challenges our regional salmon populations face. Now, understanding a bit more about how wildfires also have the potential to provide some benefits as they reshape habitat, makes me look at that recently burned forest I walked through in a different way. Somehow a little more hopeful and connected to the longer time span of response, adaptation and habitat complexity.

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