To find out what will happen to the world’s forests in the face of climate change, researchers want to see how trees die.
While we’re suspended above the ground on a sunny October day, it would be simple to concentrate on the blue ridges of the hills and the small villages nestled between them. But Richard Peters, who is with me in a steel gondola attached to the manoeuvrable arm of a crane, shows me the treetops below, tinged with the golden and copper hues of autumn. ” This guy is definitely on the verge of death,” he says of a tree.
We fly over the bare, bare branches of a beech tree that has lost its crown due to drought, of a fir tree whose tip is bare of needles, and in the distance we see the bald skeletons of conifers ravaged by bark beetles.
Peters shouts instructions and the crane operator steers the 50-meter-long boom in a circle, allowing the gondola to glide across the forest roof as a breeze gently caresses the leaves. It’s a surreal way of looking at the forest canopy, and for Peters and the other scientists painting here, it’s much more than that. They stop at this forest in Switzerland’s Hölstein region in the Jura Mountains to take careful measurements of around 80 of the 480 trees, directly in the domain where they breathe.
About 14 species of European trees grow here, mainly beech and fir, according to a long-term study led by plant ecologist and physiologist Ansgar Kahmen of the University of Basel. When the project was announced in 2018, the goal was to simulate the effects of drought through the roofs of buildings just above the forest floor to intercept rainfall. But that summer and early autumn, it was the weather itself that sparked the experiment, with rainfall almost partly cut off and temperatures 3 degrees warmer than before amid the worst drought Central Europe has ever experienced. noted in 250 years.
Many trees were destroyed; Ten fir trees succumbed on the two-hectare site (about five acres). Countless trees were tested to their limits that year and in the years that followed.
Forest scientists around the world are alarmed to find that droughts, exacerbated by fires and bark beetle infestations, are causing trees to be cut down on scales never seen before: from large expanses of U. S. forests to the dry forests of Australia, where roots can pass as far as 50 meters (more than 160 feet), to temperate regions and rainforests where such occasions have long been considered unthinkable. “Even other very competent and experienced people in the box were surprised to see how temporarily those forests were disappearing,” says Henrik Hartmann, an ecophysiologist at the Julius Kühn Institute, the Swiss Federal Center for Crop Plant Research in Germany and lead author of a review of forest mortality in the 2022 Annual Journal of Plant Biology.
Droughts have affected many of those ecosystems before, but what’s different now are the “warmer droughts” caused by sweltering temperatures. And more dramatic forest disappearances are taking place, Hartmann and his colleagues warn. Determine how many of those sacrifices will be located and where it is critical; Forests are important pockets of life on Earth and act as planetary air conditioners, absorbing up to one-third of the fossil fuel emissions that contribute to global warming and that humanity produces each year. Some experts hope that if trees are lost rushing and releasing more carbon into the air, forests may become producers of carbon dioxide, accelerating climate change.
But predicting the long term is a primary challenge, to the point that major weather predictions from the Intergovernmental Panel on Climate Change (IPCC) likely particularly underestimate tree mortality due to drought. Scientists don’t even know how many trees are dying right now; Much of it only records deaths at well-studied sites, so many are likely to die unnoticed.
Most importantly, much of the clinical information about how trees respond to drought is outdated and based on incomplete attention to tree physiology, making it difficult to build accurate models. Predicting what the future holds means unraveling the silent processes taking place within the planet. tree bodies as they suffer from warmer, drier weather and, ultimately, how trees die.
It may seem that scientists still don’t know exactly how droughts kill trees. On the other hand, they rarely have a chance to abide by the death of a tree from start to finish, as it can take years or even decades for a tree to recover. They succumb, and several centimeters get closer and closer to the edge, without anyone noticing.
This is part of what makes tracking Hölstein so valuable. If a tree dies in this forest, scientists will hear it loud and clear.
After driving down a winding highway near Basel, he arrived at the closed site before dawn on a Thursday in mid-October, accompanied by Peters, a tree physiologist at the University of Basel, and David Steger, a PhD student. at the University of Basel. The same establishment on responses to underground drought. We put on helmets, in part to protect ourselves from falling beech branches broken by drought, and scientists spring into action. Steger looks for the evergreens to inspect today and shines a flashlight. their trunks so that ecophysiologist Günter Hoch, perch, perched on top of the gondola, can step out into the canopy to collect twig samples.
Squinting in the dark, I see tools attached to trees to monitor their important functions: the contraction and swelling of bark tissue as trees drink, the movement of sap from roots to leaves, and the overall circumference of the trunk. The machinery gives the impression of a gigantic open-air intensive care unit.
Hoch drops a bag full of twigs, and Peters and I head to a small cabin to take a key measurement: the water content of the leaves, an indicator of a tree’s stress. The leaves are dotted with tiny valves called stomata, through which trees bring in carbon dioxide and oxygen and allow water to escape; Like transpiration, this loss of water cools the tree. When water exits, it generates a negative tension that draws more water through the xylem (the diversion of water funnels the trunk and branches of a tree) into the leaves. We also measure this negative stress, known as stress.
The water potentials of the leaves are negative, but the less the better. When measured before dawn, they show whether the trees were able to rehydrate overnight, replenishing the water they had lost the day before. That’s why I’m here at five o’clock in the morning. , watching Peters assess the condition of the twigs of red spruce (Picea abies), common fir (Abies alba) and pine (Pinus sylvestris). Push each twig into an airtight chamber and slowly introduce pressurized air until bubbles of sap erupt. of the cut end of the twig. The amount of pressure needed to make this happen is equivalent to the strain of the water the twig was undergoing.
Peters is pleased to note that the water potential values of the twigs are high, in the diversity of -0. 6 to -0. 7 megapascals. Thanks to recent rains, the trees had recovered from the grueling summer of the year, when their leaf water prospects dropped to -2 megapascals because the trees were dehydrated. “They’re pretty satisfied trees,” Peters says.
During the summer and fall of 2018, the scientists observed that the potential daytime water from the branches of 10 fir trees they were tracking in the forest dropped below -2. 3 megapascals. Obviously, the drought, accompanied by a heat wave, had pushed the trees to their limits. Warm air can hold exponentially more water than cooler air, leading to a scenario where, degree after degree, more moisture is removed from the stomata and the tree as a whole. Trees can close their stomata to prevent this water from leaking, but still some of the water escapes.
According to the team, once the roots ran out of water, the spruce began to dehydrate, as they depleted their internal water reserves and lost water through their needles. The tension on the water columns of the trees became so wonderful that the liquid water vaporized. , creating an air pocket called an embolism that obstructs the xylem. If a tree has too many embolisms, the entire water-carrying formula won’t be able to deliver water to the canopy when moisture returns to the soil, Kahmen says, which is what happened to five fir trees whose water prospects had dropped below -7 megapascals. They died from hydraulic failure, mainly from thirst. ” The processes we observed in common spruce were quite remarkable and unprecedented,” says Kahmen. “It is rare to demonstrate that water failure is the only mechanism of mortality. “
The results, published in 2021, suggest that the common spruce is much more vulnerable to drought than in the previous idea, which is problematic since the tree is planted in much of Europe for its wood. The paintings have also fueled a heated debate about exactly how drought kills trees. Although hydraulic failure is thought to be the fatal blow (and indeed it was for Norway spruce), some scientists argue that drought may cause starvation in trees in the first place. Trees burn their energy reserves faster at warmer temperatures because their metabolism speeds up. And if they’ve closed their stomata to protect themselves from water loss, they can’t cool down or absorb as much carbon dioxide as they want for photosynthesis and to produce essential sugars like metabolism, water absorption, and embolism repair. This is a vicious cycle that, in turn, makes them more prone to hydraulic breakdowns.
To what extent does it boil down to starvation or dehydration?Or does it have the species? Getting an answer is a reminder of the ultimate cause of death for people with interrelated health conditions, says Alana Chin, a tree ecophysiologist at ETH Zürich. “That’s part of why we’re surprised by these tree mortality events, because we don’t quite know how it works. “
Death and agony are the end of the road. But an equally vital question is what makes trees vulnerable to drought. Many trees have tricks to avoid harmful degrees of dehydration and embolism. Scots pine, a slow-growing, disjointed-looking conifer, closes its stomata quickly, at least compared to spruce. . Its durable needles prevent water from escaping; its slightly thinner xylem channels can make it difficult for embolisms to appear; And the water stored in the tissues of its bark helps it persevere during periods of drought. “It’s essentially a breath-holding species,” Peters says.
The common spruce, by contrast, prioritizes photosynthesis and expansion at the expense of safety; It is lazier at the end of its stomata and has less water storage capacity in its trunk.
European beech, a relatively fast-growing species, is also susceptible to drought, but it can lose its leaves to avoid wasting water through its stomata, and in severe droughts, it can lose entire branches.
But stomata and xylem aren’t everything. During the 2018 drought, Kahmen and his colleagues found that some species were doing remarkably well even though they left their stomata wide open. This includes oaks and what Peters proudly calls the “supertree” of the Basel site: Sorbus torminalis, also known as the wilderness service tree. The team suspects that the secret of these trees possibly lies in long roots that bring water from deep soil layers that species such as beech and fir trees can’t reach, thus maintaining solid prospective water and photosynthesis. As long as the trees have a straw in the water, it’ll be fine, Hoch says.
Other factors, some of which are still unknown, are also the fate of a tree in times of drought. European beech trees, for example, suffer in Hölstein, but Steger believes in drier ground near Berlin, perhaps because they are more deeply rooted there. These hardy Scotch pines do well in the clay soils of Hölstein, but drought has killed them en masse in other parts of Switzerland, in sandier, faster-draining soils.
It’s shocking to see,” Chin said. To see the Scots pine die, among other things, due to the obvious tension of the water and on a very giant scale. . . It’s not something that’s never been noticed before. “
Scientists around the world have also been surprised by the effects of droughts on forests, which are supposed to be resilient.
In 2015, a delay and a poor rainy season led to the death of large numbers of trees in a forest in Guanacaste, northwestern Costa Rica, which periodically alternates between rainy and dry seasons. Again, the most severely affected species were the most vulnerable to hydraulic failures. , for example because they leave their stomata open for as long as imaginable or because they have xylem or shallow roots that are prone to embolisms. “Most of the species you find there can go five months without rain,” says Jennifer Powers, a forestry ecologist at the University of Minnesota. But when you give them seven months without rain, it doesn’t matter if it rains two meters during the rainy season. “
One of the most unexpected episodes occurred in 2011 when, after an era of severe drought and a series of heatwaves, scientists detected that many trees were wasting their leaves in the northern Jarrah Forest in southwestern Australia. Eucalyptus, basically the jarrah tree Eucalyptus marginata, grows enthusiastically after wildfires and suffers up to seven months of drought each year, sucking groundwater through roots that can grow up to 50 meters (more than 150 feet) deep. Traditional wisdom was that the forest was bombproof, says ecologist Joe Fontaine of Murdoch University in Perth. But that year, many trees lost their crowns completely and grew back from the trunk, only to abandon the trunk and regrow from the base before dying, “at most like aftershocks of an earthquake. “” recalls Fontaine.
While many experts were surprised, perhaps they shouldn’t have been, says Tim Brodribb, a plant physiologist at the University of Tasmania. These fast-growing trees close their stomata late, their xylem transports water temporarily but is prone to embolization, and drink water voraciously to the point of exhaustion, as has probably been the case in the Jarrah forest due to years of declining rainfall. “Everyone thinks eucalyptus trees are very resilient, but they’re really just sleazy pioneers,” Brodribb says.
The basic dilemma is that trees sometimes have to make trade-offs: they can waste their carbon on immediate expansion or building a strong hydraulic system, but they typically can’t do both. Eucalyptus trees have opted for the former, allowing them to massively dominate most of Australia’s forests, but causing them to die out en masse during droughts. Conversely, Callitris, a genus of cypress circle conifers discovered in other forests on this continent, opted to invest in its hydraulic system, sacrificing its ability to temporarily regrow and compete in a fire-prone landscape.
Callitris’ xylem is so physically powerful that when Brodribb asked a colleague to spin a piece in a centrifuge to determine when it would expand harmful levels of embolism, they had to do it at such high speeds that the centrifuge ruptured.
The challenge is that many tree species have opted for a risky strategy, which would possibly be too risky in today’s warming world. In a 2012 study, Brodribb and his collaborators amassed data on 226 tree species from 81 sites around the world. They gathered knowledge on the water potentials at which harmful embolisms occur and on the average water potentials at which species occur in nature. They found that 70% of the species were very close to this harmful threshold, because, for example, it took them a long time to close their stomata, they had a weak xylem, or they had to work harder to rehydrate due to a shallow root system.
Surprisingly, this is true for all types of forests examined. From dry forests to temperate forests to tropical forests, many tree species settle for being at the breaking point of hydraulic fault because it helps them outperform other trees.
But even if this strategy worked long before human-caused climate change, the maximum excessive droughts caused by today’s emerging temperatures are too much for trees to withstand. ” Drought is different in the Amazon than in Arizona,” says Craig Allen, But in the region, trees are adapting to local situations disrupted by climate change. They run up against the thresholds of what they can tolerate.
Warmer temperatures don’t just push trees to the hydraulic edge. The droughts they cause exacerbate other stressors, such as fires: A spring drought likely contributed to Canada’s record 2023 wildfire season, for example. Even at the edge of the Amazon rainforest, drought makes it easier for other people to burn plantation spaces and fires to spread farther, though the forest’s wet interior still looks resilient, says Adriane Esquivel-Muelbert, an ecologist at the University of Birmingham.
Around the world, some studies estimate that fires are now burning about twice as many forests as they did in 2001. In 2021, a bad year, fires fed 9. 3 million hectares, a domain the size of Portugal.
For most trees that die from droughts, diseases or insects like bark beetles are usually the fatal blow. This applies to European spruce trees, as well as exceptionally hardy species such as hedgehog pines and giant sequoias of the Sierra Nevada in the United States. Between 2014 and 2020, forest ecologist Nathan Stephenson saw 33 giant sequoias die; He suspects that a mixture of fires and drought disrupted the flow of water to the treetops, leaving them unable to expel defensive resins against the beetles. A species of beetle that has never killed redwoods invaded the crown downwards. What eventually wiped them out was a local crustal beetle that was too weak to kill them under general conditions, but could eventually kill them under excessive conditions,” says Stephenson, a senior scientist at the U. S. Geological Survey.
This constellation of crises is also widespread in the mountainous regions of northern New Mexico, where Allen documented the effects of a regional megadrought that began in 2000. Regular droughts had plagued the region before. But this time, a century of fire suppression and a large accumulation of dense plants in a past rainy era, as well as warmer temperatures that began with drought, unleashed an inferno, Allen says.
Bark beetle populations have skyrocketed in the heat, and more larvae survived the winter and crammed more generations into one season. Between 2002 and 2004, they each devoured more than one million hectares of stone pine (Pinus edulis) and ponderosa pine (Pinus ponderosa). ) in the southwest. The trees probably couldn’t collect enough carbon or water to produce the resin that would protect them. The fires delighted in the dense vegetation.
Time and again, the thick stands of conifers Allen had known for decades have turned into shrubs and grasslands, with so few trees that he can now see the nearest mountain ranges, dozens of miles away. “A lot of my favorite trees and forests from the ’80s and ’90s are no longer alive,” he says. “I know ecosystems are dynamic, I know that intellectually. But it’s one thing to know, it’s another to revel in the magnitude. of the transformations that have taken place in this landscape.
Until recently, many scientists thought that, in general, higher carbon emissions would be good news for forests, because of the undeniable explanation for why plants need carbon dioxide to grow. If there’s more in the air, they get more carbon dioxide for each and every molecule of water lost, allowing them to grow their tissues faster and use water more efficiently.
That is why the first computer models of plant expansion in the context of climate change showed a widespread greening of the planet and, in fact, recent satellite studies show that there was an expansion of global plants in the 1980s and 1990s.
But there is growing evidence that those benefits can be offset by the warming effects of carbon emissions. According to a 2019 study, global greening stopped more than 20 years ago and plants have been declining ever since, all due to the effects of warming magnifying drought. .
As the environment warms, thirst increases, and this dating is exponential, so that for every degree Celsius of warming, the environment can retain 7% more water. The carbon dioxide bonus is of little use to trees that close their stomata to themselves. by water loss or outright death due to overfed droughts.
Some studies recommend that trees can’t create wood under too much drought stress, even when they photosynthesize; instead, they can expel carbon through their roots. Instead of feasting on carbon dioxide and helping to combat climate change, forests can suffer greatly from peak emissions. And when they rot or burn, they release carbon into the air, amplifying global warming.
The noxious trio of drought, insects, and fires “could make the difference between moving the Earth’s surface from a carbon sink to a carbon source,” says Anna Trugman, a substitute ecologist at the University of California, Santa Barbara.
The scientists at Hölstein are well aware of these issues. Before leaving the site, Peters and I climbed a series of ladders inside the crane scaffold and watched Steger, in the gondola, use a device to measure the rate of photosynthesis of conifer leaves.
From this height (where Peters obviously feels comfortable, but where my knees are clearly shaking) we have a clever view of the rainproof plastic roofs, wedged like greenhouses on the forest floor. Peters says that if more excessive droughts occur, they will be hesitant to use rooftops, fearing that some trees will die immediately.
At present, scientists don’t know how many trees will succumb to the drought. In fact, despite dramatic stories of devastated forests, some researchers remain reluctant to say with certainty that drought mortality is a worsening trend, simply because there is not enough knowledge about global tree loss to be known.
Since 2010, Allen and others have compiled a database on reported forest loss due to heat and drought around the world. The database has 88 episodes and now lists more than 1,300. But this is not a complete picture of what is happening in the world. forests, says Hartmann; Some of the world’s largest forests (whether boreal or tropical) are understudied.
It’s also very difficult to predict the magnitude of imaginable deaths in the future, because the mathematical simulations used to wait for plant responses are based on superseded assumptions about how trees respond to drought, Hartmann says. When he and his colleagues recently used a “dynamic global plant style” to see if any of the staggering mortality events can be expected in places like Germany, Australia, and the southwestern United States, he couldn’t reliably expect the history of population mortality.
Most plant models do not incorporate the hydraulic processes that Kahmen and others are now learning are essential for a tree’s survival, as they are complex and still poorly understood. Instead, they largely focus on processes that occur in leaves, such as photosynthesis. When trees die in those models, they usually starve to death when their stomata are closed and they can’t get enough carbon, and many researchers, Hartmann adds, are unlikely to be the sole cause of death in drought.
Trugman says stylists are slowly beginning to understand and have begun to incorporate metrics that reflect the stress on a tree’s hydraulic system, such as how a tree temporarily loses its ability to bring water. She cites a style described in a 2018 study that predicted when individual trees on an experimental turf would fail hydraulics. However, it is difficult to achieve this on larger spatial scales. A big unknown is the vast and dynamic global nature of groundwater, how much it can contribute to forests, and what types of trees. they have roots that can succeed in them.
It will be years, Trugman adds, before water processes are sufficiently understood and modeled to be well represented in the mortality forecasts of the IPCC’s major climate projections, which aim to advise government policy.
Another mystery is the extent of pesky stressors, such as wildfires and insect infestations, in the future; This last point is also not reflected in any of the IPCC models. There are many species of bark beetles, and researchers know little about how each will respond to environmental changes. “We’re going to see new interactions that we’ve never seen before. “noticed before,” Stephenson says. In some cases, it will simply be being able to anticipate them in advance. “
This article appeared in Knowable Magazine, an independent journalistic effort of Annual Reviews. Sign up for the newsletter.
This article was originally published on October 1, 2023.