Decoding stream life after wildfire

Photo Credit

Forest Service

Ecologist Laura Hauck filters stream water to collect environmental DNA (eDNA), which scientists analyze to identify aquatic and terrestrial species in the watershed.

Driven by exceptionally strong, dry east winds after a hot summer, the 2020 Labor Day fires spread very quickly. In two weeks, they burned nearly as much forest west of the Cascade Range crest as burned during the previous five decades.

Large, severe fires are a natural part of these wet forests, but they happen infrequently. The 2020 fires offered a rare opportunity to explore any number of research questions, including one that has intrigued ecologists for decades: when fires burn unevenly across a landscape, does the resulting patchwork of regenerating vegetation create habitat complexity that can lead to greater biodiversity?

Until now, researchers have explored that question almost entirely on land. But watersheds experience fire too. Laura Hauck, an eDNA ecologist, and Brooke Penaluna, a research fish biologist, both with the Pacific Northwest Research Station, and colleagues wanted to address this gap.

“What happens to the biodiversity beneath the waterline in streams embedded within landscapes that burn?” Hauck asks.

Signs of resilience one year after fire

Hauck and her research partners sampled aquatic life in 25 Oregon streams in summer 2021, about a year after the fires. The study’s stream sites represented a range of fire effects, from unburned to severely burned. Six unburned streams in Washington provided additional points of comparison.

“It was very interesting to look at shifts in total biodiversity across the different groups, from high severity burned locations to unburned reference sites,” says Hauck.

The clearest finding, however, was what the researchers did not see. Across the study sites, they detected no decline in the variety of aquatic life as fire severity increased.

Notably, several at-risk species were detected in burned watersheds. These included chinook and coho salmon, as well as Pacific lamprey and western river lamprey. All were detected in watersheds where 78 to 100 percent of the area had burned, suggesting that, even where there was extensive burning, some suitable habitat remained during the first year after the fires.

“The big takeaway from our results is that Pacific Northwest organisms might be more resilient than we knew,” says Hauck.

Reading a stream three ways

To get as complete a picture as possible of stream communities, Hauck and her team combined three methods. Electrofishing is a standard technique that sends an electrical current into the water to temporarily stun aquatic species so they can be captured, identified, and released. Another traditionally used field survey technique is the Surber sampler, a specialized tool that collects insects and other small creatures that live on the bottom of shallow streams.

The third method was eDNA metabarcoding, which analyzes genetic traces captured in water samples from the streams. "eDNA is one of the most powerful emerging tools for understanding how wildfires affect aquatic ecosystems,” said Penaluna.

“While electrofishing can tell you what animals are actively swimming in the water, eDNA metabarcoding can inform us as to what animals are living in the water and in the surrounding watershed, offering a more thorough picture of biodiversity,” says Hauck.

None of the survey methods captured the whole community. For example, the eDNA method missed caddisflies entirely, while the traditional streambed surveys found 38 different kinds. Electrofishing found a Cascades frog that eDNA missed. In other cases, eDNA added precision: it distinguished several sculpin species that could not be told apart in the field and provided information about terrestrial organisms that live within the watershed.

Comparing the three methods showed how difficult it is to assess an entire biological community. Each method captured a different part of the picture. Used together, they can give researchers, and the land managers who use their findings, a fuller understanding of how streams respond to disturbance.

Understanding wildfire recovery takes years

This study was the first to use eDNA metabarcoding to assess watershed-scale biodiversity after wildfire, but it captured only the first year of recovery. Conditions could change as streamside trees die, stream temperatures shift, and storms move sediment and woody debris.

For Hauck, this means the first-year findings are a beginning, not an endpoint. “As megafires increase in intensity and frequency, it is imperative that we continue to monitor recovery in affected areas,” she says. Tracking life in these watersheds over longer periods will show whether those early signs of resilience persist.