How Hot is Too Hot?

Faced with widespread tree die-offs, scientists are racing to determine the upper thermal limit of the world’s trees.

ON MONDAY, JUNE 28, 2021, forecasts called for yet another day of record-breaking temperatures in the Pacific Northwest. Phil Hunter and his wife decided to beat the heat by leaving their home and Christmas tree farm near Tacoma, Washington, for the higher, cooler elevations offered by nearby Mount Rainier.

When they left their fir trees in the early morning, they were green. By the afternoon, Seattle and Portland had reached the unprecedented temperatures of 108°F and 116°F, respectively. That evening, as Hunter drove down the mountain on his way back home, he saw the first signs of what this extreme heat was doing to trees. “We started seeing that the trees on the side of the road were burnt. The trees were really, really red on one side,” Hunter recalls.

“They slowly died after that. There was no way to rehydrate them.”

When he arrived home, the tree farmer saw his firs — green and verdant that morning — were now orange, red, and brown, scorched in the heat. This surprised Hunter, because it wasn’t as if his trees didn’t have access to water. The previous day, he had irrigated them, something most Christmas tree farmers in Washington and Oregon don’t typically need to do. “I don’t know what trees I saved by continuing to irrigate. I only know what I lost,” he says. Hunter estimates that within a matter of hours many of his trees had been irrevocably harmed. “They slowly died after that. There was no way to rehydrate them.” In the end, he lost 1,500 trees to the heatwave, amounting to a 20 percent loss of revenue for the year.

What became known as the 2021 “heat dome” lasted from June 25 to July 2, 2021. During those eight days, the Pacific Northwest was enveloped in a high-pressure bubble of extreme temperatures reminiscent of Phoenix or Death Valley. The heat dome killed hundreds of people in Oregon, Washington, and British Columbia. And while the Pacific Northwest’s human residents suffered in the extreme heat, so did its trees. In a matter of days, the green needles and leaves on many of the region’s trees turned shades of red and brown. This widespread “foliage scorch,” as scientists would later name it, affected wild forests, commercial tree plantations, and Christmas tree farms alike.

Scientists would later determine that the 2021 heat dome was so rare and extreme for the region that it was “virtually impossible without human-caused climate change.” In fact, they say the heat event was so unlikely that a similar event had the probability of occurring just once in the past 1,000 or even 10,000 years. Climate change has upended all that. In recent decades, heatwaves have increased in their duration, frequency, and intensity, a trend that is expected to persist and worsen as climate change accelerates in the years ahead.

This emerging heatwave trend has scientists concerned, not only for human health and wildlife, but also for the health of plants, those key organisms that make up the base of most terrestrial food webs on Earth. For decades, people — even some scientists — thought trees and other plants had an innate ability to handle extreme temperatures. It was believed that plants could stay cooler than the air around them, especially when given access to water. But experiences like Hunter’s, and new research, now suggest that if temperatures get hot enough, having access to water doesn’t help.

While more intense heatwaves are expected to impact plants of all kinds, including domesticated food crops, some of the biggest impacts will be borne by the long-lived perennials we call trees, severely limiting their ability to provide ecosystem services and sequester carbon and hence slow climate change. And as Hunter witnessed, heat is already killing trees.

Along with creating more intense heatwaves, climate change is also causing average temperatures to rise, leading to an indirect effect of heat that scientists are now calling “hot drought” — dry conditions caused not so much by a lack of precipitation but by an increase in air temperature. From western redcedars in the Pacific Northwest to beeches in Germany, eucalyptus trees in Australia to pines in the Mediterranean, tree die-offs caused by these warmer, drier hot-drought conditions are occurring in forests around the world. It’s against this hot-drought backdrop that heatwaves enter the picture as a new and largely understudied threat to Earth’s forests.

According to a growing body of research, during heatwaves already stressed trees could be at or approaching temperatures at which photosynthesis starts breaking down.

Faced with widespread tree die-offs, scientists are now racing to figure out just “how hot is too hot” for the world’s trees and other plants. At the heart of this race is a looming question: What’s likely to hurt trees more, direct damage from extreme heatwaves or the indirect effects of prolonged hot drought?

ON A COLD, misty morning in the fall of 2023, I met up with Chris Still near Oregon State University’s campus in Corvallis, Oregon, before heading to an experimental forest in the nearby Cascade Mountains. A professor at the university’s College of Forestry, Still is a tall, soft-spoken man in his fifties with graying hair and a beard. His eyes strike me as sad, but behind them stirs the mind of an iconoclast. Still is one of a growing number of scientists arguing that extreme heat can directly harm trees and other plants, representing a new and largely underestimated threat.

I became interested in his groundbreaking work on the 2021 heat dome after I noticed the western redcedars in the woods next to my house in Vancouver, Washington, were dying. It’s been over two years since the heat dome, and much of the foliage scorch on the region’s surviving trees — including on living cedars near my home — has been replaced by new green growth. Nonetheless, Still suspects signs of the heatwave can be found in the tree rings of this forest.

The second-growth forest here is dense and dominated by Douglas-fir trees. At eye level, thin metal strips, which end in a spring mechanism and a counter, encircle the trees. These devices, called dendrometers, measure the trees’ growth. The trees, in turn, surround a weather station that’s been collecting local meteorological and climatological data for decades. At first glance, the forest looks healthy. But as I look closer, I see the skeletons of dead trees, Douglas-firs that died following the heat dome.

a man gazing at a tree trunk

Chris Still, a professor at Oregon State University’s College of Forestry, inspects a dendrometer at an experimental forest in the Cascade Mountains. Still’s research shows that the direct effects of heat and solar radiation can cause extensive damage to trees. Photo by Nathan Gilles.

old growth douglas fir in Pacific Northwest

In recent years, 10 Pacific Northwest species, including western redcedar and Douglas-fir (pictured), have seen diebacks and declines that scientists have tied to hot-drought conditions. Photo courtesy of US Forest Service - Pacific Northwest Region.

The combined climate and tree growth data at this site show a clear trend of hot drought. Although annual precipitation levels have remained at or near normal here, the forest’s air temperatures have risen steadily over the decades. Consequently, its soil and air have become drier. The trees have responded to these stressors by growing less, and they have less carbon to use for defense. Still suspects the heat dome added a further stress, which he thinks will show up in the tree ring data when it’s analyzed. For some of the Douglas-firs this has proved disastrous. Bark beetles have taken advantage of the weakened trees, leading to their death.

This forest’s experience is not unique. Although the lasting impact of the heat dome is largely unknown so far, in recent years, 10 Pacific Northwest species, including western redcedar and Douglas-fir, have seen diebacks and declines that scientists have tied to similar hot-drought conditions, which, recent research shows, have increased in frequency in western North America over the past century. But die-offs due to this indirect effect of heat are not limited to the Pacific Northwest; they appear to be a global phenomenon, affecting both wet and dry regions alike.

Hot droughts are often more damaging to trees and other plants than droughts that result from a lack of precipitation alone, what scientists call “meteorological droughts.” In fact, although hot droughts can exacerbate meteorological droughts, they can occur even when precipitation stays normal. That’s because hot droughts not only dry out soil but also dry out the air.

During these periods of “high atmospheric aridity,” the air essentially sucks the water from a plant’s leaves through a combination of evaporation and transpiration. Tied to photosynthesis, transpiration in plants is somewhat similar to sweating in animals. It helps plants shed heat by releasing water vapor into the atmosphere through their stomata — tiny holes found on leaves and other photosynthesizing parts that help plants take in carbon from the atmosphere.

However, transpiration only gets a plant so far. If the pull of the atmosphere is great enough, water-carrying tissues inside the plant can experience extreme tension, and air can sneak into the tissues, leading to embolisms. This can stop the movement of water from root to stomata and with it the cooling effects of transpiration. The result is something like the plant version of heat stroke. Scientists call this “hydraulic failure,” and some attribute the damage seen during the 2021 heat dome to this.

If hydraulic failure [in plants] is a heat stroke, then direct heat damage is a burn.

But this indirect effect of heat is not, according to Still, what caused the foliage scorch during the 2021 heat dome, which he says was caused by the direct effect of heat and solar radiation. “The causative factor of a lot of this reddening and browning [of foliage] was likely the combination of the intense heat plus the intense sun. Really, it was just that the leaves cooked, and it wasn’t just because they were droughted [experiencing drought conditions],” he says.

In other words, if hydraulic failure is heat stroke, then direct heat damage is a burn.

Scientists, including Still, are now actively trying to determine how much havoc each of these phenomena can cause. Although this might seem like an exercise in futility, as Still reminds me, knowing the difference matters, because the emerging heatwave trend represents “new territory” for the Earth’s climate.

Still lays out his case for direct heat and sun damage in a 2023 paper in the journal Tree Physiology where he notes that the pattern of foliage scorch that occurred during the heat dome followed the path of the afternoon and evening summer sun, suggesting, as one coauthor put it in a separate interview, that the region’s forests got a “sunburn.” In contrast, places that experienced the heat but didn’t have direct exposure to the afternoon and evening sun didn’t scorch. The scorching, Still says, also happened far too quickly for hot drought and hydraulic failure to be a significant factor.

Still’s case for direct heat damage is a departure from previous ideas. Over the past 30 years, hydraulic failure has become a “dominant narrative” among researchers hoping to understand the effects of heat on plants, a narrative that now includes the idea that climate change is driving hydraulic failure as hot droughts become more common globally. But it’s a narrative that Still says just doesn’t fully fit what happened during the 2021 heat dome’s extreme temperatures.

“There’s definitely a hydraulic, or a water stress, component that plays into this,” he says. “But there’s all sorts of reasons why just heat alone is actually quite a damaging thing.”

However, Still says, understanding the effects of heat on plants is a complex problem. Among other things, it’s difficult to “disentangle” the indirect effects of hot drought from the direct effects of heat and solar radiation. Previous research into the 2021 heat dome, for instance, suggested that many of the tree species that browned had reached their hydraulic limits. What’s more, it’s these hydraulic limits rather than heat alone that are behind current tree die-offs. Future tree die-offs, however, are likely to be another story altogether, one that involves exceeding temperature limits as well as hydraulic ones.

TO DATE, SCIENTISTS have determined the “thermal tolerances” of just 1,028 — a mere 0.31 percent — of the world’s recognized land-based plants, according to an extensive review of the scientific literature published in 2020 in The Proceedings of the National Academy of Sciences. This number, however, is somewhat misleading.

The term “thermal tolerance” includes both the extreme high temperatures and the extreme low temperatures at which plant tissue ceases to function. According to the study’s supplementary material, not all of the 1,028 studied plants have had their upper and lower thermal tolerances examined, with research typically focusing on only one or the other. It also notes that the scientific literature is potentially biased, with more estimates of heat tolerance for heat-tolerant plants and more estimates of cold tolerance for cold-tolerant ones.

To William Hammond, plant ecophysiologist at the University of Florida, the scientific community’s limited understanding of the upper thermal tolerances of plants represents a troubling “blind spot.”

a forest in warm light

Some research indicates that plants in warmer climates are more at risk of future warming than plants in cooler ones, as they are already closer to temperatures that can lead to potential breakdown in photosynthetic systems. Photo by Red Charlie.

“Extremely hot temperatures remain our highest confidence predictions for climate change. Pair that with the blind spot of how hot is too hot [for plants], and it keeps me up at night,” Hammond tells me.

At his lab, Hammond has studied the thermal tolerance of 58 plant species, including gymnosperms, angiosperms, nonvascular plants, and Coffea arabica (aka the coffee plant), the necessary ingredient, he jokingly (and energetically) tells me, that makes his work possible. Hammond says no one single thermal limit fits all plant species, but in general needles, leaves, and other photosynthetic tissues start to feel the effects of warm temperatures at around 104° F. However, this represents just the low end of the high thermal limits that plants can experience.

According to Hammond, the upper thermal limit for most land plants is around 122°F, with some tolerating slightly more and some tolerating slightly less. (Though there are some notable exceptions, including species in the cactus, amaranth, and fig families, which in the controlled environment of the lab have been measured withstanding temperatures from 140˚F to 160˚F.)

What happens at these upper thermal limits is the stuff of nightmares, suggesting that words like “burn,” “scorch,” and “cook,” are more like literal descriptions than colorful analogies. Extreme temperatures make plant cell membranes “leaky,” as the heat — ultimately a kind of movement — jiggles the membranes, allowing what’s inside the cell to seep out. The proteins in the cells’ chloroplasts — the machinery of photosynthesis — also start to denature the protein’s elaborate shapes, rendering them useless. (This stops photosynthesis and with it a plant’s ability to suck climate change-causing carbon from the air.)

Cell lipids, which are fatty acids, also don’t respond well to extreme heat.

“The membranes that hold [plant cell] organelles together are made of lipids,” explains Hammond. “And as with any lipid, if it gets too hot, it can become more liquid: Think of your butter melting.”

What happens at these upper thermal limits is the stuff of nightmares.

While it’s not yet known how, when, and if the death of a plant’s photosynthetic tissues will lead to the death of the whole plant, let alone whole forests — trees, for instance, can draw on stored carbon during rough times — Hammond says what is clear is that past a certain temperature, plants can’t cool themselves enough to avoid tissue damage even if they have access to water.

This is something Chris Still has also noted in his work.

A 2022 analysis that Still and colleagues conducted of temperature data collected from the upper canopies of multiple forest types in North and Central America — including tropical and temperate forests as well as dry and wet forests — revealed that leaf temperatures in the upper canopies were frequently warmer than the air around them. Still observed the same phenomenon at a forest in Oregon during the 2021 heat dome. He recorded a Douglas-fir tree reaching 124ºF, a full 12 degrees above the surrounding air, and beyond critical temperature limits of both mature and young Douglas-fir needles.

Still says his findings and similar ones made by other scientists dispute a common misconception, held even by some scientists, that plants can stay cooler than the air around them by transpiring, especially if they have access to water.

“When a plant transpires, just like when you sweat, it can cool that way,” he says. “But one of the things we pointed out [in the 2022 study] is, even if leaves are losing a lot of water from transpiration and evaporative cooling at the top [of the canopy], the amount of cooling they can do is still not enough to overcome the amount of radiation they’re receiving from the sun.”

WHILE SCIENTISTS working to understand the direct effects of extreme heatwaves, which they say is going to be an increasing factor in years to come, the researchers I spoke with all agreed that hot drought is already exposing forests to continuous stress.

Just how bad hot drought conditions have gotten for the world’s forests was revealed in a study led by William Hammond, published in 2022 in Nature Communications.

The study combines nearly 50 years of data from 154 peer-reviewed papers on tree die-offs from 675 different locations in forests around the world. From this data, Hammond and his coauthors conclude the forest die-offs were due to a “hotter-drought fingerprint” that was linked to climate change. Significantly, they found this same fingerprint across nearly every biome examined, and, the study notes, this pattern held regardless of whether the forest was found in a wet region or a dry one.

“The climate keeps on moving in the wrong direction [for trees],” says study coauthor Henrik Hartmann, Professor at the Max Planck Institute for Biogeochemistry, and a leading expert on global tree mortality.

Hartmann is the lead author of a 2022 paper in Annual Reviews that is one part science and one part personal testimonies written by the scientists who have observed tree die-offs. He writes one of these testimonies on his home region of Thuringia, Germany, where mortality rates in some species have increased 20-fold in recent years. One of the hardest hit has been the iconic European beech, a tree that was seemingly on the rebound until recently.

“What this global data on trees is telling us is that every degree matters.”

Hartmann and I talk over Zoom. Like many of the scientists I spoke with, a sadness seems to follow him. “Ten years ago, in Germany, we thought beech would be dominating all of our forests, and now it’s over,” he tells me. “Within five years, we’ve basically given up on the beech, which is very harsh.” Beeches are unlikely to disappear from the landscape entirely, “but the 45-meter-tall, majestic, individual trees that we have here still in some regions, I think that’s over, that’s gone.”

The loss of culturally important trees like the beech is likely to lead to a loss of stability both psychologically and ecologically that is already being felt, Hartmann says. “I think we should be really concerned, especially for the cascading effects in forest ecosystems. It is heartbreaking because I have children. And I really don’t know what their world is going to look like. I don’t know whether they will have an environment that actually can provide them [with] all the wealth that we had. It is a threatening situation.”

For Hartmann’s colleague Hammond, however, there is something of a silver-lining to the global hot-drought phenomenon. His 2022 study concludes that limiting global warming to 2ºC above preindustrial levels will result in less than half of the number of forest die-offs predicted under 4ºC of warming. “What this global data on trees is telling us is that every degree matters,” Hammond says. “I look at that and I don’t get depressed. This is why every action we can take to limit further warming of the Earth has got to be our top priority.”

And yet, Hammond says, we should not underestimate the effects of extreme heat during heatwaves. Extreme heat might still “surprise us,” he tells me. It’s an opinion that Chris Still shares.

“You can imagine situations where multiple heat events in addition to [hot] drought and other stressors like bark beetle can eventually lead to large-scale mortality of forests,” Still says. This, he says, could lead to the Pacific Northwest’s trees dying off in large numbers, changing the look and structure of the forests for generations. “The implications, I think, are pretty profound.”

LIKE HARTMANN, PHIL HUNTER, the Christmas tree farmer, too, knows what the loss of stability feels like. I met up with him in October of 2023 at the 20-acre farm he’s run since 1986. In that time, he’s seen ups and downs, but nothing like what’s happened in the last few years. Since the heat dome, Hunter’s wife has passed away. One year to the day of her death, in February of 2022, his seizures started. He was soon diagnosed with brain cancer. “I’m not afraid of dying,” he tells me. “We all got to do it. No one is getting out of here alive.”

a man handles a tree branch

Most of the trees on Phil Hunter’s farm look healthy and green; says the healthy trees are a drought-tolerant fir species imported from Turkey. Photo by Nathan Gilles.

a dessicated, dead fir tree

Some are clearly dead, the result of hot-drought conditions that have occurred since the heat dome. Photo by Nathan Gilles.

Hunter is a tall man in his 70s. He wears a baseball cap with the words “Pacific Northwest Christmas Tree Association” embroidered on it. He knows his farm intimately — the land, the trees, the soil and how it drains. He wears this intimate knowledge like a second skin. He also wears what looks like a hair net with a long power cord that trails from his head down his neck into a backpack he carries. It’s a medical device that sends mild electrical shocks to his brain to help slow the spread of his seizure-causing cancer. External signs of the cancer are also visible in bandages that peek out under the cap and net on his clean-shaven scalp, remnants of a recent surgery.

On my tour of his farm, I see row after row of Christmas trees. Most look healthy and green, but some are clearly dead, their needles an ocher color, the result of hot-drought conditions that have occurred since the heat dome. The trees that are thriving are drought-tolerant fir species from Turkey, he tells me, his speech slow and measured, a consequence of his brain condition. He continually apologizes for it. He agrees to do our interview despite his health issues, he tells me, because he feels a sense of responsibility to get the word out.

“I think we have to recognize that with the climate changing, we’ve got to adapt, because this isn’t the only time it’s going to reach 105 or 112,” he says, “and it only has to happen one day to have the heat kill.”

You Make Our Work Possible

You Make Our Work Possible

We don’t have a paywall because, as a nonprofit publication, our mission is to inform, educate and inspire action to protect our living world. Which is why we rely on readers like you for support. If you believe in the work we do, please consider making a tax-deductible year-end donation to our Green Journalism Fund.

Get the Journal in your inbox.
Sign up for our weekly newsletter.

Subscribe Now

Get four issues of the magazine at the discounted rate of $20.