Forest canopy damage caused by ‘heat dome’

Oregon State University

News Release 4-Nov-2025

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Douglas-fir on left and western hemlock (the thin tree) on right.  Note the new growth on the Doug fir seems most impacted. Photo by Dave Shaw, OSU College of Forestry)

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Credit: Photo by Dave Shaw, OSU College of Forestry

CORVALLIS, Ore. – A satellite imagery analysis shows that the 2021 “heat dome” scorched almost 5% of the forested area in western Oregon and western Washington, turning foliage in canopies from a healthy green to red or orange, sometimes within a matter of hours.

Damage to foliage leads to a range of problems for trees including reduced photosynthesis and increased vulnerability to pests and disease, scientists at Oregon State University say.

The study by researchers at OSU and the U.S. Forest Service identified 293,546 hectares of damaged forest, a total area of more than 1,000 square miles that’s nearly the size of Rhode Island. They took a deep dive into the affected areas to learn the factors that made some stands more vulnerable than others to the extreme heat event experienced by the Pacific Northwest in June 2021.

Over three days, the heat dome brought temperatures as high as 116 degrees Fahrenheit in Portland, 117 in Salem and 121 in Lytton, British Columbia, the highest temperature ever recorded in Canada. The coastal town of Quillayute, Washington, checked in at 110 – 45 degrees above its average high temperature for the day.

The forest analysis showed that sun exposure, microclimate and aspect – the direction a slope faces – were factors that made some areas more sensitive to the heat dome. Other factors were tree species, stand age, the timing and pattern of budburst – when dormant buds open and begin to grow – and the presence of foliar pathogens such as the fungus that causes Swiss needle cast in Douglas-fir trees.

“The extent of foliar mortality shown in this study, particularly in iconic, old-growth forests, suggests that longer-lasting or hotter heat waves in the future could lead to even more widespread impacts on invaluable Pacific Northwest forests,” said OSU College of Forestry doctoral graduate Adam Sibley.

The researchers found that culturally and economically significant species like western redcedar, western hemlock and Sitka spruce were disproportionately prone to heat damage, including in old-growth stands where they dominate the canopy. The scientists say the findings highlight the multifaceted challenges posed to forests by extreme heat waves, as well as the need to better understand their impact on forest ecosystems as the climate warms.

“The heat dome was an uncontrolled test of the thermal tolerance of trees in their native environments,” said Chris Still, a tree physiologist in the College of Forestry. “To our knowledge, there are no examples of heat wave-induced foliar death at this scale in the historical record. This study provides the first spatially comprehensive estimate of forest canopy damage from the heat dome and provides land managers with important information in advance of future extreme heat waves.”

The research revealed extensive foliar scorch on Washington’s Olympic Peninsula, home to Olympic National Park, recognized as both a World Heritage Site and an International Biosphere Reserve. Sixty-nine percent of the park’s forest is old growth dominated by western hemlock, western redcedar and Sitka spruce, among the species shown to be most sensitive.

“If heat waves increase in frequency and severity, we may see major changes in the composition of old-growth forests, as well as reduced productivity for plantation forests,” said Sibley, a former research associate at Oregon State and now a remote sensing scientist at Chloris Geospatial in Boston.

Also contributing to the study were the College of Forestry’s Matthew Gregory, David Shaw, Nina Ferrari, Alex Dye and Mark Schulze; David Rupp of the College of Earth, Ocean, and Atmospheric Sciences; and Chris Daly of the College of Engineering.

The National Science Foundation, the U.S. Geological Survey and the Forest Service’s Pacific Northwest Research Station funded this research, which was published in Global Change Biology.