2021 4-Day OR/WA Heatwave: "No Examples Of Heatwave-Induced Foliar Death At This Scale In The Historical Record"

Using satellite imagery, researchers have discovered that nearly 5% of Oregon and Washington’s western forest canopy — about the size of Rhode Island — was scorched and damaged in the multi-day 2021 heat dome. The wave of extreme heat across the Northwest from June 25 to 29, 2021, brought the temperature in Portland to 116 degrees, killing 70 people in Multnomah County alone.

It also took a toll on the region’s forests, turning tree canopies from green to red, orange and brown within hours. That’s a sign that the leaves were scorched, healthy leaf tissue died and chlorophyll production — a pigment that gives plants their green color and helps them absorb sunlight — stopped, researchers from Oregon State University and the U.S. Forest Service found. Their study, published Oct. 30 in the journal Global Change Biology, described it as an “unprecedented amount of foliage death.” As leaves die, trees experience acute stress that leads to slow growth, declining immunity to pests and fungus and in some cases accelerated death.

EDIT

Still and researchers suggest that current models projecting how the region’s forests will continue to change under a warming climate are overly optimistic because they lack much analysis from extreme events like the heat dome. “The heat dome was an uncontrolled test of the thermal tolerance of trees in their native environments,” Still said in a statement. “To our knowledge, there are no examples of heat wave-induced foliar death at this scale in the historical record.”

The forest canopy damage from the 2021 heat dome showed up on satellite images from southern British Columbia in Canada to as far south as Roseburg. The temperature during the dome reached 120 degrees in parts of British Columbia, clocking in as the highest temperature on record in Canada. In western Oregon and Washington, Still and his team identified 725,368 acres — more than 1,000 square miles — of damaged tree canopy. Those forest acres were more vulnerable to heat damage because of a number of factors the researchers identified. Among them, some were more obvious — if the trees were on a sun-facing slope or if their surroundings didn’t offer much cover from the sun. Others were less obvious. There was higher leaf scorch among trees in urban areas, agricultural valleys and forests that had burn scars from the 2020 Labor Day fires.

EDIT

https://oregoncapitalchronicle.com/2025/11/13/2021-heat-dome-left-rhode-island-sized-damage-in-oregons-washingtons-western-forests/