PORTLAND, Ore. (AP) - From June 25 to July 2, 2021, the Pacific Northwest experienced a record-breaking heat wave that sent the normally temperate region into Death Valley-like extremes, taking a heavy toll on trees and people alike.
Seattle and Portland, Oregon, recorded their highest temperatures ever, at 108 degrees Fahrenheit (42.2 degrees Celsius) and 116 Fahrenheit (46.6 degrees Celsius), respectively. In British Columbia, the small town of Lytton reached a temperature of 121 degrees Fahrenheit (49.6 degrees Celsius).
What has become known as the "heat dome" is estimated to have killed hundreds of people in Oregon, Washington and British Columbia.
As this human tragedy unfolded, a lesser-known ecological tragedy was unfolding, one that scientists warn will have grim consequences for the world's plants and the many species that depend on them.
Within a few days, the 2021 heat dome turned many of the green leaves and needles on the region's trees to orange, red and brown.
But as recent research shows, tree foliage didn't just dry out in the heat. Instead, it underwent "widespread scorching."
"A lot of this redness and browning of the leaves was because the leaves were cooking. It really wasn't a drought story," said Chris Still, a professor at Oregon State University's College of Forestry and a leading researcher on the effects of heat on trees.
Still is part of a growing number of scientists investigating what they say is a new, woefully underestimated threat to the world's plants: climate change-induced extreme heat.
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EDITOR'S NOTE: This story is part of a collaboration between The Associated Press and Columbia Insight examining the impact of climate on trees in the Pacific Northwest.
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In recent years, scientists in the Pacific Northwest have linked the decline of ten native tree species to drought.
In many cases, the conditions that led to this decline are known as "hot droughts."
Due to above-normal temperatures, hot droughts can be much more damaging to trees than droughts that are simply the result of a lack of moisture. Hot droughts don't just dry out the soil; they also dry out the air. This puts stress on trees and can cause water-bearing tissues in the trees to collapse - a process called 'hydraulic failure'.
In an article earlier this year in the journal Tree Physiology, Still argued that damage to the region's trees during the heat dome was caused mainly by direct damage from heat and solar radiation, and not indirectly by drought caused by the extreme heat.
"I'm not trying to say that drought isn't a big and important factor," Still said. "But I think as events like the 2021 heatwave become more frequent and intense, it is important to look at the response of trees and other plants to these events and not just drought, which has been the dominant paradigm. "
Still's argument includes the observation that 'leaf burn' was found mainly on the south and west sides of trees and forests - a pattern that follows the sun's trail across the summer sky.
"It actually looked like a sunburn throughout the forest. It was quite disturbing," said co-author Daniel DePinte, program manager of the U.S. Forest Service's aerial survey program, who observed the phenomenon from an airplane.
Multiple tree species were scorched, DePinte said, noting that the sun's role became apparent when the same trees were viewed from an orientation not exposed to direct sunlight.
"It almost seemed like the forest damage disappeared," he said.
The article was written in response to an earlier study published in the same journal, which argued a different position: that the heat dome led to widespread drought stress and hydraulic failures in trees in the Pacific Northwest. "In general, I agree... that heat damage played a major role in tree damage (during) the 2021 PNW heat wave. But in my opinion, hydraulic failure was just as important, if not more so," wrote the study's lead author Tamir Klein, professor of plant and environmental sciences at the Weizmann Institute of Science in Rehovot, Israel.
Exactly how hot is too hot for trees and other plants is the research focus of William Hammond, a plant ecophysiologist at the University of Florida.
Hammond called the scientific community's current understanding of the effect of extreme heat on plants a worrying "blind spot."
"One thing is certain: we know a lot more about how dry is too dry for plant survival than we know about how hot is too hot," he said.
What scientists call "thermal tolerances" have been established for only 1,028, or less than 1%, of the world's 330,200 recognized land plants, according to a frequently cited 2020 article in The Proceedings of the National Academy of Sciences.
No single thermal limit is suitable for all plant species, but generally extreme damage to plant tissues occurs around 122 degrees Fahrenheit (50 degrees Celsius), Hammond said.
"At those temperatures you might think 'wow, the air doesn't get that warm', but that is the temperature of the plant, not the temperature of the air. And those things can be very different," he said.
How different is something that Still has kept track of.
During the heat dome, he and his colleagues recorded air temperatures around a Douglas fir reaching 112 degrees Fahrenheit (about 44 degrees Celsius), the hottest ever recorded in the forest where the measurements were taken. However, the tree's needles reached 124 Fahrenheit (51.1 degrees Celsius) as a result of exposure to direct sunlight.
Still says observations like this and others like it in forests around the world challenge a common misconception, even among some scientists, that plants can withstand extreme temperatures and stay cooler than the air around them, especially if given access to water .
"Plants can control their temperatures to a certain extent, but if the heat is extreme enough, some plants won't be able to get through it even if they have a lot of water," he said.
Hammond has come to the same conclusion based on work in his laboratory. "If temperatures get high enough, heat stress can kill living plant tissue, even if they have water," says Hammond.
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Nathan Gilles is a science writer and journalist based in Vancouver, Washington.
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Columbia Insight is an Oregon-based nonprofit news website covering environmental issues affecting the Pacific Northwest.
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