Taking the Pulse of Western Washington's Old-Growth Forests

A National Park Service research brief, January 2023, on a study by Steven Acker, John Boetsch, Beth Fallon and Marie Denn of the North Coast and Cascades Inventory and Monitoring Network.

NPS photo.

Why these forests matter

Pacific Northwest forests cycle huge amounts of carbon and nutrients, filter pollutants from streams and help buffer climate change: an average acre locks away 147 tons of carbon, among the most of any forests on Earth. The oldest forests are also climate refuges. Heat-sensitive birds such as the hermit warbler fare better there, probably thanks to cooler microclimates and varied food, and older, taller forests proved more resilient in the Cascades' 2020 megafires.

But recent decades have brought higher temperatures and less snowpack. As it warms, more water evaporates from the ground, and even the region's rainiest forests have run a "climatic water deficit" — they get thirsty. Trees competing for scarce water become stressed and vulnerable to insects, disease and other threats. Forests worldwide are dying back with warming and drought. Is it happening here?

The study

Ecologists asked four questions about 2008–2018:

1. Did trees die faster than the background rate of 1 percent a year?
2. Did deaths rise in the second half of the decade?
3. Do trees of different sizes die at different rates?
4. Is drought behind individual tree deaths?

Starting in 2008, they set up 45 permanent plots in mature and old-growth forest at low, middle and high elevations, across Olympic, North Cascades and Mount Rainier National Parks and Lewis and Clark National Historical Park. Crews visited each plot every year from 2008 to 2013 and again in 2018. On the first visit they recorded each tree's species, size and condition and fixed a small numbered metal tag; on each return, they noted new deaths.

Study plots in four national parks in western Washington: low (yellow diamonds), middle (blue squares) and high elevation (orange circles). Figure from Acker and others, 2023.

Surprising results

• No rise in deaths. Contrary to expectations, no size group at any elevation died faster than the background rate, and deaths did not increase over the ten years.
• Small trees die more often, as expected, than their bigger neighbours.
• No overall link to drought. Just one climate pattern appeared: more trees died standing after winters with more moisture in the air. Could heavy snowpacks be shortening the growing season? Or could damp winters mean more snow falling as rain — and a long, dry summer ahead?

"Our forests as of 2018 are defying global odds in not demonstrating increased mortality. But we can't assume they are safe."
— Beth Fallon, plant ecologist, Mount Rainier National Park

Crews reach many plots off-trail through dense growth. "Your clothes get smeared with forest goo," says researcher John Boetsch. "It takes a lot of effort and dedication to collect these data in the first place." NPS photo/Acker.

What's next

Up to 2018, protected mature and old-growth forests here seemed to be standing strong against climate change. But the link between one climate variable and tree deaths hints at changes to come, so monitoring must continue — especially after extremes like the 2021 heat dome. A forest's heartbeat is slow: some trees in the study took root in the 1500s, and ten years is one short chapter of a long story.

"People conserved these forests. This is our legacy, this is our heritage, and we want to understand what's happening to it in the face of climate change and hopefully protect this biodiversity."
— Marie Denn, aquatic ecologist, NPS Water Resources Division

The big ideas

• Up to 2018, western Washington's mature and old-growth forests appear resilient to climate change.
• A hinted link between tree death and atmospheric moisture may foreshadow future losses.
• There is a chance now to conserve these forests, their biodiversity and their globally significant carbon.
• Continued monitoring is needed to spot change and adapt management.

Sources

• National Park Service, North Coast and Cascades Inventory and Monitoring Network, "Taking the Pulse of a Forest," January 2023. https://www.nps.gov/articles/000/pulse_forest.htm
• Acker, S.A., Boetsch, J.R., Fallon, B., and Denn, M., 2023, Stable background tree mortality in mature and old-growth forests in western Washington (NW USA): Forest Ecology and Management 532, 120817. https://doi.org/10.1016/j.foreco.2023.120817
• Print version of the brief: https://irma.nps.gov/DataStore/Reference/Profile/2296900
• Rewritten in hubnx's own words.