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 鈥� ravaged by wildfire 鈥� reached 121 degrees Fahrenheit (49.6 Celsius).
What become known as the 鈥渉eat dome鈥� 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鈥檚 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鈥檚 trees to orange, red and brown.
But, as recent research suggests, tree foliage didn鈥檛 simply dry out in the heat. Instead, it underwent 鈥渨idespread scorching.鈥�
鈥淎 lot of this reddening and browning of leaves was just that the leaves cooked. It really wasn鈥檛 a drought story,鈥� said Chris Still, professor at Oregon State University鈥檚 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鈥檚 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 鈥渉ot droughts.鈥�
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 鈥� a process called 鈥渉ydraulic failure.鈥�
In a paper earlier this year in the journal , Still made the case that damage to the region鈥檚 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.
鈥淚鈥檓 not trying to say that drought is not a huge and important factor,鈥� said Still. 鈥淏ut I think with events like the 2021 heat wave becoming more common and intense, it鈥檚 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.鈥�
Still鈥檚 argument includes the observation that 鈥渇oliage scorch鈥� was primarily found on the southern and western sides of trees and forests 鈥� a pattern that follows the track of the sun across the summer sky.
鈥淏asically, it was like a sunburn across the entire forest. It was quite disturbing,鈥� 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.
鈥淚t almost appeared as if the forest damage disappeared,鈥� 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. 鈥淥verall I agree 鈥� 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,鈥� wrote that study鈥檚 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鈥檚 current understanding of extreme heat鈥檚 effect on plants a worrying 鈥渂lind spot.鈥�
鈥淥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,鈥� he said.
What scientists call 鈥渢hermal tolerances鈥� have been established for just 1,028, or less than 1%, of the world鈥檚 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.
鈥淲ith those temperatures you might think 鈥榳ow, the air doesn鈥檛 get that hot,鈥� but that鈥檚 the temperature of the plant, not the temperature of the air. And those things can be quite different,鈥� 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.
鈥淧lants can control their temperature to some degree, but if the heat is extreme enough, some plants won鈥檛 be able to get through it even if they have a ton of water,鈥� he said.
Hammond has reached the same conclusion based on work in his lab. 鈥淚f temperature gets high enough, heat stress can kill living plant tissues even if they have water,鈥� said Hammond.
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