From June 25 to July 2, 2021, the Pacific Northwest experienced a wave that sent the normally temperate region into Death Valley-like extremes that took a heavy toll on trees as well as people.
Seattle and Portland, Ore., recorded their hottest-ever temperatures, reaching 108 degrees Fahrenheit (42.2 Celsius) and 116 Fahrenheit (46.6 Celsius), respectively. In British Columbia, the small town of Lytton 害羞草研究所� ravaged by wildfire 害羞草研究所� reached 121 degrees Fahrenheit (49.6 Celsius).
What become known as the 害羞草研究所渉eat dome害羞草研究所� 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 happening, one that scientists warn has grim repercussions for the world害羞草研究所檚 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害羞草研究所檚 trees to orange, red and brown.
But, as recent research suggests, tree foliage didn害羞草研究所檛 simply dry out in the heat. Instead, it underwent 害羞草研究所渨idespread scorching.害羞草研究所�
害羞草研究所淎 lot of this reddening and browning of leaves was just that the leaves cooked. It really wasn害羞草研究所檛 a drought story,害羞草研究所� said Chris Still, professor at Oregon State University害羞草研究所檚 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害羞草研究所檚 plants: climate change-driven extreme heat.
In recent years, scientists in the Pacific Northwest have linked the decline of 10 native tree species to drought.
In many cases, conditions that have brought about the decline are known as 害羞草研究所渉ot droughts.害羞草研究所�
Driven by above-normal temperatures, hot droughts can be far more damaging to trees than droughts that result simply from a lack of moisture. Hot droughts not only dry out soil; they also dry out the air. This stresses trees, and can cause water-carrying tissues inside them to collapse 害羞草研究所� a process called 害羞草研究所渉ydraulic failure.害羞草研究所�
In a paper earlier this year in the journal , Still made the case that damage to the region害羞草研究所檚 trees during the heat dome was triggered primarily by direct damage from heat and solar radiation rather than indirectly by drought caused by the extreme heat.
害羞草研究所淚害羞草研究所檓 not trying to say that drought is not a huge and important factor,害羞草研究所� said Still. 害羞草研究所淏ut I think with events like the 2021 heat wave becoming more common and intense, it害羞草研究所檚 important to look at the response of trees and other plants to these events and not just at drought, which has been the dominant paradigm.害羞草研究所�
Still害羞草研究所檚 argument includes the observation that 害羞草研究所渇oliage scorch害羞草研究所� was primarily found on the southern and western sides of trees and forests 害羞草研究所� a pattern that follows the track of the sun across the summer sky.
害羞草研究所淏asically, it was like a sunburn across the entire forest. It was quite disturbing,害羞草研究所� said co-author Daniel DePinte, U.S. Forest Service aerial survey program manager, who observed the phenomenon from an airplane.
Multiple tree species were scorched, DePinte said, noting that the role played by the sun became clear when the same trees were viewed from an orientation not exposed to direct sunlight.
害羞草研究所淚t almost appeared as if the forest damage disappeared,害羞草研究所� he said.
The paper was written in response to an published in the same journal that argued a different position: that the heat dome led to widespread drought stress and hydraulic failure in Pacific Northwest trees. 害羞草研究所淥verall I agree 害羞草研究所� that heat damage played a big role in the damage caused to trees (during) the 2021 PNW heat wave. But in my view, hydraulic failure was as important, if not more,害羞草研究所� wrote that study害羞草研究所檚 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害羞草研究所檚 current understanding of extreme heat害羞草研究所檚 effect on plants a worrying 害羞草研究所渂lind spot.害羞草研究所�
害羞草研究所淥ne 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,害羞草研究所� he said.
What scientists call 害羞草研究所渢hermal tolerances害羞草研究所� have been established for just 1,028, or less than 1%, of the world害羞草研究所檚 330,200 recognized land-based plants, according to a frequently cited in The Proceedings of the National Academy of Sciences.
No single thermal limit fits all plant species, but in general extreme damage to plant tissues occurs around 122 degrees Fahrenheit (50 Celsius), Hammond said.
害羞草研究所淲ith those temperatures you might think 害羞草研究所榳ow, the air doesn害羞草研究所檛 get that hot,害羞草研究所� but that害羞草研究所檚 the temperature of the plant, not the temperature of the air. And those things can be quite different,害羞草研究所� he said.
Just how different is something Still has been tracking.
During the heat dome, he and colleagues recorded air temperatures around a Douglas fir tree reaching 112 degrees Fahrenheit (about 44 Celsius), the hottest ever recorded in the forest where the measurements were taken. The needles of the tree, however, reached 124 Fahrenheit (51.1 Celsius) due to exposure to direct sunlight.
Still says observations like this and similar ones in dispute a common misconception even among some scientists that plants can withstand extreme temperatures and stay cooler than air around them, especially when given access to water.
害羞草研究所淧lants can control their temperature to some degree, but if the heat is extreme enough, some plants won害羞草研究所檛 be able to get through it even if they have a ton of water,害羞草研究所� he said.
Hammond has reached the same conclusion based on work in his lab. 害羞草研究所淚f temperature gets high enough, heat stress can kill living plant tissues even if they have water,害羞草研究所� said Hammond.
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