by PORTLAND, Ore. (AP) – From June 25 to July 2, 2021, a record-breaking heat wave swept through the Pacific Northwest that sent the relatively temperate region to Death Valley-like extremes, causing severe damage to trees. as well as people. .
Seattle and Portland, Ore., recorded their hottest 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).
The so-called “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 has dire consequences for the world’s plants and the many animal species that depend on them.
In just 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 suggests, it’s not just the heat that dries up tree foliage. Instead, he underwent “extensive retirement.”
“A lot of this reddening and browning of leaves was caused by cooking the leaves. It wasn’t really 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, largely underestimated 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, investigating 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, this is called a “hot drought”.
Driven by above-normal temperatures, hot droughts can be far more damaging to trees than droughts caused simply by a lack of moisture. Hot droughts not only dry out the soil; they also dry the air. This stresses trees, and can cause water-bearing tissues within them to collapse — a process known as “hydraulic failure”.
In a paper earlier this year in the journal Tree Physiology , Still made the case that the region’s trees were damaged during the heat dome primarily by direct damage from heat and solar radiation rather than indirectly by drought caused by the extreme heat.
“I’m not trying to say that the drought isn’t a huge and important factor,” Still said. “But I think, with events like the 2021 heat wave becoming more common and more severe, it’s important to look at the response of trees and other plants to these events and not just drought, which is the dominant paradigm it.”
Still’s argument includes the view that “defoliation” was found mainly on the south and west sides of trees and forests – a pattern that follows the sun’s track across the summer sky.
“Basically, it was like a sunburn all over the forest. It was quite disturbing,” said co-author Daniel DePinte, the U.S. Forest Service’s aerial survey program manager, who witnessed the phenomenon from an airplane.
Many tree species were retired, DePinte said, noting that the sun’s role was clear when the same trees were viewed from an orientation that was not exposed to direct sunlight.
“It looked like the forest damage was almost gone,” he said.
The paper was written in response to an earlier study published in the same journal that argued a different position: that the heat dome was the result of widespread drought stress and hydraulic failure in Pacific Northwest trees. “Overall 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,” the study’s lead author wrote said Tamir Klein, professor of plant and environmental sciences at the Weizmann Institute of Science in Rehovot, Israel.
The research focus of William Hammond, a plant ecophysiologist at the University of Florida, is exactly how hot is too hot for trees and other plants.
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 and too dry for plant survival than we know about how hot is too hot,” he said.
Scientists call 1,028, or less than 1%, of the world’s 330,200 known land plants “thermally tolerant,” according to an oft-cited 2020 paper in The Proceedings of the National Academy of Sciences.
No single thermal limit fits all plant species, but in general, major plant tissue damage occurs around 122 degrees Fahrenheit (50 Celsius), Hammond said.
“With those temperatures you might think ‘wow, the air doesn’t get that hot,’ but that’s the temperature of the plant, not the temperature of the air. And those things can be very different,” he said.
Just how different something is Still tracking.
During the heat summit, he and his colleagues recorded air temperatures around Douglas fir trees that reached 112 degrees Fahrenheit (about 44 Celsius), the hottest temperature ever recorded in the forest where the measurements were taken. The tree’s needles, however, reached 124 degrees Fahrenheit (51.1 Celsius) due to exposure to sunlight.
Observations like this and similar ones in forests around the world still point to a common misconception even among some scientists that plants can withstand extreme temperatures and stay cooler than the air around them, especially when given access to water to them.
“Plants can regulate their temperature to a certain extent, but if the heat is extreme, some plants won’t be able to get through even if they have a ton of water,” he said.
Hammond reached the same conclusion based on work in his laboratory. “If the temperature gets high enough, heat stress can kill living plant tissue 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 non-profit news website that covers environmental issues affecting the Pacific Northwest.
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Associated Press climate and environmental coverage is supported by several private foundations. See more about the AP climate initiative here. The AP is solely responsible for all matters.
