by PORTLAND, Ore. (AP) — From June 25 to July 2, 2021, the Pacific Northwest experienced a record heat wave that sent the normally mild region into Death Valley-like extremes that took a heavy toll. both to trees and to people. .
Seattle and Portland, Oregon, recorded their highest temperatures ever, reaching 108 degrees Fahrenheit (42.2 Celsius) and 116 Fahrenheit (46.6 Celsius), respectively. In British Columbia, the small town of Lytton reached 121 degrees Fahrenheit (49.6 Celsius).
What became known as the “heat dome” is estimated to have killed hundreds of people in Oregon, Washington and British Columbia.
While this human tragedy was unfolding, a lesser-known ecological tragedy was occurring, one that scientists warn has grim repercussions for the world’s plants and the many animal species that depend on them.
In a matter of a few days, the 2021 heat dome turned many of the green leaves and needles on the region’s trees orange, red and brown.
But, as recent research suggests, the trees’ foliage didn’t simply dry out in the heat. Instead, he suffered a “general burnout.”
“Much of this reddening and darkening of the leaves was simply due to them being cooked. 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 and woefully underappreciated threat to the world’s plants: extreme heat driven by climate change.
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EDITOR’S NOTE: This story is part of a collaboration between The Associated Press and Columbia Insight, exploring 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 10 native tree species to drought.
In many cases, the conditions that have caused the decline are known as “hot droughts.”
Driven by above-normal temperatures, warm droughts can be much more damaging to trees than droughts that result simply from a lack of moisture. Hot droughts don’t just dry out the soil; They also dry the air. This stresses trees and can cause the water-carrying tissues inside them to collapse, a process called “hydraulic failure.”
In a paper published earlier this year in the journal Tree Physiology, Still argued that damage to the region’s trees during the heat dome was caused primarily by direct damage from heat and solar radiation rather than indirectly by the drought caused by extreme heat.
“I’m not trying to say that drought isn’t a huge, important factor,” Still said. “But I think that as events like the 2021 heat wave become more common and intense, it is important to look at the response of trees and other plants to these events and not just to drought, which has been the dominant paradigm.” .
Still’s argument includes the observation that “scorched foliage” was found primarily on the south and west sides of trees and forests, a pattern that follows the path of the sun across the summer sky.
“Basically, it was like a sunburn on the entire forest. It was pretty disturbing,” said co-author Daniel DePinte, aerial survey program manager for the U.S. Forest Service, who observed the phenomenon from an airplane.
Several species of trees were burned, DePinte said, noting that the role played by the sun became clear when the same trees were observed from an orientation not exposed to direct sunlight.
“It almost looked like the forest damage had disappeared,” he said.
The paper was written in response to an earlier study published in the same journal that held a different position: that the heat dome caused widespread drought stress and hydraulic failure in trees in the Pacific Northwest. “Overall, I agree…that heat damage played a big role in the damage done to trees (during) the PNW heat wave of 2021. But in my opinion, hydraulic failure was just as important , if not more,” wrote the study’s lead author, Tamir Klein, a professor of plant and environmental sciences at the Weizmann Institute of Sciences in Rehovot, Israel.
Exactly how much heat is too much for trees and other plants is the focus of research by 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 for sure: 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 an oft-cited 2020 paper in The Proceedings of the National Academy of Sciences.
There is no single thermal limit that fits all plant species, but in general 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 hot,’ but that’s the plant temperature, not the air temperature. And those things can be quite different,” he said.
How different is something that Still has been following.
During the heat dome, he and his colleagues recorded air temperatures around a Douglas fir that reached 112 degrees Fahrenheit (about 44 degrees Celsius), the highest ever recorded in the forest where the measurements were taken. The tree’s needles, however, reached 51.1 degrees Celsius (124 Fahrenheit) due to exposure to direct sunlight.
Still says observations like this and similar ones 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 when they have access to the air. water.
“Plants can control their temperature to a certain extent, but if the heat is extreme enough, some plants won’t be able to handle it even if they have a ton of water,” he said.
Hammond came to the same conclusion based on work in his laboratory. “If the temperature gets high enough, heat stress can kill living plant tissues even if they have water,” Hammond said.
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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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The Associated Press’ climate and environmental coverage is supported by several private foundations. See more about AP’s climate initiative here. The AP is solely responsible for all content.
