Forest acquisition could preserve water for summer streamflows 

The Nooksack Indian Tribe and collaborators are taking steps to acquire 6,000 acres on Stewart Mountain in the South Fork Nooksack River watershed to preserve mature forest and change harvest practices, leading to increased summer streamflows.

The effort is part of the tribe’s Climate Change Adaptation Plan, which includes 140 high-priority actions that, if implemented, are expected to increase the climate resilience of the species and habitats addressed in the plan.

The proposed Stewart Mountain Community Forest Initiative was influenced by about 20 years of watershed studies in Oregon forests that show a possible connection between young regenerating forests in clear cuts and lower streamflows. The reason: transpiration, the tree equivalent of human respiration. 

Transpired moisture, like the water vapor that humans exhale, is carried by air out of the watershed, reducing the water available to streams. Immature trees growing in a harvested area take in more water that would otherwise make its way to streams as compared to mature and old-growth trees. The rate of transpiration can be managed by humans by protecting mature and old-growth stands and extending the time harvest occurs, according to Oliver Grah, water resources program manager for the Nooksack Indian Tribe.  

Exactly how much water is taken in and transpired by an immature tree is not specifically known in the Nooksack River watershed. But the Oregon studies — completed in 2005, 2016, 2019, and 2020 — showed that harvested and regenerating areas had 50 percent lower streamflows than adjacent watersheds with mature trees, according to Grah.

“Trees are living things. They take in water and they transpire,” Grah said. “Rapidly generating trees need more water than mature trees, and they transpire up to three times more.”

Grah hopes others who depend on the Nooksack River for water will see the benefit in protecting old-growth and mature forests, and promoting late summer streamflows by changing harvest rotation from about 40 years to more than 80 years.

“If we can increase summer streamflow, it’s great for fish, and it also benefits agricultural and other water users too,” Grah said. 

Everyone who depends on the Nooksack River has a stake in its future and a responsibility to advocate for the river, said Trevor Delgado, the Nooksack Tribe’s historic preservation officer.

“It’s our home. We all share in that,” he said. “The Nooksack River is the backbone of this place and this place has a story.”

The tribe is advocating for the Stewart Mountain Community Forest Initiative because it aims to facilitate more late summer streamflows through community stewardship to change how and when trees are harvested.

“It’s not the forest industry’s fault, because the Forest Practices Act allows them to do what they’re doing,” Grah said. “The Forest Practices Act doesn’t consider the effects of tree harvesting on late-summer streamflows.” 

A change in forestry practices could help ensure the industry’s viability into the future while offsetting the effects of climate change, Grah said. One idea he supports: promoting the removal of narrower swaths of trees in higher elevations, which would promote more snow accumulation and later summer snow melt compared to the typical expansive clear cuts that are common today.

“All of these changes to how we manage our forests will take time, but if we are to offset the impacts of climate change on streamflows and water supply, we better get started now before those impacts get progressively worse into the future,” Grah said.

Above: Low summer streamflows on the South Fork of the Nooksack River threaten salmon recovery. Photo and story: Richard Walker