2969. Doctors Recommend Walking in the Woods

By Amitha Kalaichandran, The New York Times, July 12, 2018

On a damp Saturday morning last August, I joined 10 others in the woods outside Ottawa, Canada, as part of a “forest bathing” session offered by a local wilderness resort.  

First we sat in a circle on the leafy ground, each sharing a moment in nature from our childhood that filled us with joy. Next our guide, Kiki, a newly trained forest therapist who insisted we call her by her first name, led us on a mindful — and very slow — walk through the forest.

“What do you hear, smell, see?” Kiki asked, encouraging us to use all five senses to become deeply “immersed” in the experience.

An older woman in the group told us that she was undergoing a difficult and stressful period in her life, and that being among the trees felt “healing.” Others mentioned that the activity reminded them of walks they took as part of Boy Scouts or commented on the sounds: insects, birds, the rustling of leaves. I noticed the bright green acorns that dotted the forest floor, which reminded me of my childhood collection of acorns and chestnuts. Admittedly, I was also worried that the early morning rain was fertile ground for vicious mosquitoes (West Nile!) and ticks (Lyme!).

We ended the two-hour forest walk with a tea ceremony, sipping a concoction of white pine needles steeped in hot water.  I left feeling relaxed and more at peace, though with at least two dozen bites from mosquitoes that seemed immune to DEET.  

Kiki had been trained according to standards set by the Association of Nature and Forest Therapy, a professional group that has certified more than 300 people across North America to be forest therapy guides, among them psychotherapists, nurses and six M.D.s. The sessions are modeled after the Japanese tradition of shinrin-yoku,  or forest bathing.    

Over the years, I’ve had physician mentors recommend Richard Louv’s books, “The Nature Principle” and “Last Child in the Woods,” which describe the benefits of time spent in the wilderness, from stimulating creativity to reducing stress. Florence Williams’s best-selling book, “The Nature Fix,” has a chapter dedicated to the benefits of forest therapy.  And now, it appears that more North American doctors are starting to incorporate spending time in forests into their practice.

Some small studies, many conducted in Japan and Korea, suggest that spending time in nature, specifically in lush forests, might decrease stress and blood pressure (especially in middle-aged men), improve heart-rate variability and lower cortisol levels while boosting one’s mood. An analysis of studies from 2010 that focused on exercising in nature found improvements in self-esteem, particularly among younger participants. Overall effects on mood were heightened when there was a stream or other body of water nearby.

But other studies have shown mixed results. A cross-sectional study from Korea found no change in blood pressure with forest bathing, and a systematic review from 2010 found that while time in the forest may boost mood and energy, any effects on attention, blood pressure and cortisol may not be statistically significant. Another recent review from Australia underscored the challenges of drawing causal links to disease prevention, with the authors calling for robust randomized controlled trials.

Several theories have been proposed as to why spending time in forests might provide health benefits.  Some have suggested that chemicals emitted from trees, so-called phytoncides, have a physiological effect on our stress levels. Others suggest that forest sounds — birds chirping, rustling leaves — have a physiologically calming effect. Yet evidence to support these theories is limited.

On a recent visit to Japan, I met with Dr. Hirotaka Ochiai, a surgeon based at Tokyo Medical Center, and her husband, Toshiya Ochiai, who is currently the chief executive of the International Society of Nature and Forest Medicine. Dr. Ochiai is trained in forest therapy and currently conducts most of her sessions with volunteers within a forest in Nagano, about three hours from Tokyo, with the help of a local guide, and plans to offer forest therapy soon at one of Tokyo’s largest hospitals.

“I usually encourage participants to sit or lie down on the forest ground and listen to the sounds,” she says. “The hypersonic natural world can be soothing, and things are always moving even while we are still. It can be very calming.”

Last June the Northside Hospital Cancer Center in Atlanta began to formally offer forest therapy as part of a pilot project in collaboration with the Chattahoochee Nature Center. Twelve patients with newly diagnosed cancers recently signed up for a session, according to Christy Andrews, the executive director of Cancer Support Community Atlanta.

“It was a four-hour session that seemed to have an impact on the patients,” she said. “I remember one participant telling me afterward that it was a way to ‘steer away from cancer,’ and the group became very cohesive. I think it helped reduce the isolation in a way that’s different from a regular support group.”

Dr. Suzanne Bartlett Hackenmiller, an obstetrician-gynecologist based in Cedar Falls, Iowa, began guiding patients in her practice through the Prairie Woods in Hiawatha Iowa, though she has also led groups in forests around Des Moines. She became a certified guide through the Association of Nature and Forest Therapy three years ago and tries to tailor her offerings based on the group she is leading.

“I generally get a sense of where people are at. For some, it’s best for me to stick to the science, but others may literally want to hug a tree. The traditional tea ceremony at the end might turn some people off, so I’m conscious of that and adjust accordingly,” she says.

In one exercise, she has participants close their eyes as she guides them through experiencing the different senses, imagining feeling their feet growing into the ground like roots of a tree, for instance, listening to nearby sounds and observing how far they may extend, or smelling the air. It’s similar in many ways to a guided meditation.

“I recently held a session where four out of the 20 participants were in wheelchairs, so I found a local park that had plenty of trees and a paved sidewalk so everyone could enjoy it,” she says.

At the University of California, San Francisco, Benioff Children’s Hospital in Oakland, Dr. Nooshin Razani, a pediatric infectious disease doctor and director of the Center for Nature and Health, has offered a similar program for the past four years. The “Shine” program, linked to the East Bay Regional Parks District, offers “park prescriptions,” a movement that is growing in popularity, and aims to improve accessibility to nature for low income children.

One Saturday a month, Dr. Razani leads a group of up to 50 people through a lush forest of redwood trees and lakes on the outskirts of Oakland. The groups consist of patients ranging in age from a few months to 18 years, accompanied by at least one adult family member. A few of her medical colleagues — an orthopedic surgeon and primary care doctor — have also attended, and the Oakland-based pediatrics residency program at the medical centers invites doctors in training to join the group.  Shine recently celebrated its 60th park outing.

“The accessibility part is huge for me. Many children don’t have access to green spaces in their community,” Dr. Razani says. “We also have evidence that supports the mental health aspects of spending time in forests, and for the resident doctors who participate, it’s a way to show them how children interact with nature based on the developmental stage. Sometimes the doctors’ need is just as much as the patients’.” In February, Dr. Razani published findings of a randomized trial that found that park visits — regardless of whether they were led by a guide or not — were associated with a decrease in stress three months after the visits.

A few hours after my own forest walk, the woman in our group who had mentioned her stress emailed me to say that she had checked her blood pressure afterward and noticed it was lower than usual. “It would be nice to see if there was a meaningful change from before, if they collected that information,” she wrote.

She had hit on one of the biggest issues around guided forest walks and forest therapy. Is it an evidence-based activity with proven clinical benefits?

The science is still lacking to prove it.  But there is some evidence — as well as good old common sense — to suggest that spending time in nature is good for both the mind and body, whether done as a group or alone. It may be something we all need more of.

Amitha Kalaichandran, M.H.S., M.D., (@DrAmithaK) is a resident physician in pediatrics based in Ottawa, Canada.

2953. 2017 Was the Second Worst Year on Record for Tropical Forests

By Eliza Barcley, Vox, June 28, 2018

The principal driver of deforestation in tropical countries like Ecuador, pictured here, is agriculture. Photo: Ministry of Agriculture.

The current rate that we’re clearing the Amazon and other tropical forests in Southeast Asia and Central and West Africais putting us on a course for rapid, irreversible, and catastrophic climate change, with an intensifying cycle of extreme droughts, more heat, and more forest fires. All told, deforestation accounts for an estimated 15 percent of total greenhouse gas emissions.OSLO, Norway — Tropical forests are essential for keeping carbon in the ground and maintaining the climate. When Amazonian forests are converted into savannah, there’s less rainfall and more drought.

Given forests’ importance, new data from the University of Maryland released on Global Forest Watch, a forest monitoring site, is alarming: 2017 was the second-worst year on record for tropical forest loss. Some 39 million acres of trees, an area the size of Bangladesh, were destroyed. That’s about 40 football fields of trees lost every single minute.

2017 turned out to be only slightly better than 2016, which was the worst deforestation year to date due mainly to an El Niño-related drought and the major spike in fires it caused in Brazil.

“This is a crisis of existential proportions,” Ola Elvestuen, Norway’s minister of climate and environment, said Wednesday at the Oslo Tropical Forest Forum where the data was released. “We either deal with it or we leave future generations in ecological collapse.

What caused deforestation in 2017?

As the chart shows, deforestation in the tropics has been accelerating. Which means we are not keeping up with — much less reversing — the growing pressures on forests from people clearing and burning them for agriculture, livestock, and timber.

Governments, corporations, and local communities are taking the issue more seriously. But many factors contributed to the extraordinary destruction of forests last year.

Deforestation increased significantly in a few countries in 2017, most notably Colombia and the Democratic Republic of Congo. In Colombia, the peace process and the demobilization of the Revolutionary Armed Forces of Colombia (FARC) left a power vacuum in vast, remote forest regions. With the rebel group no longer controlling the forest areas (and the lack of any other government entity to regulate them), opportunists moved in to log and clear land to plant coca and other crops.

Overall in 2017, Colombia lost 1 million acres — 46 percent more than the previous year. In the DRC, some 3.4 million acres of forest were lost to agriculture, artisanal logging, and charcoal production.

Brazil had success slowing deforestation between 2004 and 2012 through better monitoring and crackdowns on illegal activity, incentives for farmers to not clear forests, and programs to cut off illegal soy and cattle producers from markets.

But in a dramatic turnaround, tree cover loss doubled there from 2015 to 2017. As the World Resource Institute’s Frances Seymour writes, this is “in part due to unprecedented forest fires in the Amazon ... [and] to a relaxation of law enforcement efforts in the midst of the country’s ongoing political turmoil and fiscal crisis.”

According to Carlos Nobre, a Brazilian scientist and expert on climate change, we’ve already deforested about 18 percent of the Amazon. Reaching 20 to 25 percent deforestation would cause the “system to flip to non-forest ecosystems in eastern, southern and central Amazonia,” he wrote with Thomas Lovejoy in a recent paper in Science Advances.

“We are very close to 20 percent,” he said Wednesday at the Oslo Tropical Forest Forum. “We need to stop completely Amazonian deforestation. We do not want the Amazon to become a global cattle ranch.”

In most tropical regions, demand for soy, beef, palm oil, and other commodities — as well as fires — is driving the bulk of deforestation. In Brazil, which lost 11 million acres of forest cover in 2017, the main use for cleared land is cattle pasture.

In the Caribbean, the total forest lost during the 2017 Atlantic hurricane season was small compared to Brazil and DRC. But the Category 4 hurricanes that pummeled the region still did a number on forests there. The island of Dominica lost 32 percent of its tree cover in 2017, while Puerto Rico lost 10 percent.

Why stopping deforestation is so difficult

So what to do about it?

In the past 10 years, we’ve seen more and more ambitious international commitments on forests and climate change, including the Paris climate agreement, REDD+, and the Sustainable Development Goals. Better satellite images, remote sensing, and other technologies are helping law enforcement respond to illegal deforestation far quicker. More and more companies are committing to only buying from producers who can certify they are improving the environment.

“It comes down to regulation, enforcement, and incentives,” said Norway Environment Minister Elvestuen.

Some countries actually saw a drop in tree cover loss in 2017. Indonesia achieved a decrease because of a moratorium on converting peatland, better education of farmers, and enforcement around burning forests, according to the new report.

But it’s not enough. According to Seymour, who spoke Wednesday in Oslo, “We know what it takes. And we’re not doing enough of it, and we don’t have enough help to do it.”

The list of things we need to do is very long and very challenging, especially given the growing appetite for meat and other commodities like palm oil, timber, and cocoa. We need to shift global diets and lower meat consumption. We need to persuade farmers not to burn forests.

In the case of Brazil, cattle pasture is the main use of cleared land. And farmers “have grown weary of being vilified as criminals, of unmet promises of positive incentives for shifting to sustainable production systems...” write Daniel Nepstad and João Shimada of the Earth Innovation Institute for Mongabay. “To gain the support of conservation-minded, responsible farmers for the deforestation agenda, a new narrative and set of actions is needed that recognizes, applauds and rewards them for their efforts as it effectively includes them in dialogues.”

Increasingly, forest advocates are talking about connecting people who live in or near remaining tropical forests with opportunities to restore them. The Nature Conservancy, for instance, has been heavily involved in a new initiative with 7 other institutions called Nature4Climate, that will try to engage farmers and others around reforestation and conservation agriculture.

“We can’t have self-righteous NGOs just telling people ‘you can’t cut down your forests,’ anymore,” Justin Adams, TNC’s managing director of global lands, told Vox. “We want to help countries manage land and trade carbon as an asset. There has to be an economic opportunity for tropical forest countries in restoring ecosystems.”

On the bright side, there seems to be more dialogue across the board, around the world, than ever.

2232. Corvids Hold the Key to Saving the World's Forests

By Conservation, University of Washington, February 24, 2016

Imagine you’re a tree. Like every other organism on this planet, you’ve got two life goals. The first is to survive long enough to reproduce, and the second is to actually reproduce. Your aim is to cast out your seeds as far as possible so that your descendants will have room to grow. After all, if you just drop your seeds straight to the ground, your children’s nascent root systems will compete with your own mature one and will have trouble surviving. The solution? Strike a bargain with an animal. You provide the food, and the animal provides the baby carriage to transport your precious cargo a safe distance away. According to a new study, some of the best animal couriers around are corvids, the family of birds that includes crows, ravens, rooks, jays, and magpies.

But the immobile tree makes a gamble: by making their seeds tasty and nutritious, birds are highly motivated to gather up as many as possible. Instead of scarfing them all up at once, they cache them and return later to feast. But even the cleverest western scrub jay or Clark’s nutcracker won’t quite remember where he or she hid every single seed. Some of those seeds will go un-eaten and, with luck, they will become new plants. Those plants will grow to become trees which will provide food for the corvids’ own descendants. The two sets of organisms have come to rely on each other. That’s why corvids are so integral to maintaining forested ecosystems.

The better we can understand the relationship between these brainy birds and the trees they rely on for food, argues University of Nebraska-Lincoln biologist Mario B. Pesendorfer in the journal The Condor, the better we can rely on that relationship when it comes to restoring and preserving those ecosystems.

Take the jays of North America. Florida scrub jays, western scrub jays, and island scrub jays can cache between 5,000 and 6,000 acorns during a good year. Oak forests depend on those years for their continued survival. When researchers compared acorns that had merely fallen to the ground and rolled away and those that had been cached by jays, it was the cached acorns that were more likely to become saplings. Caching is even more important following a forest fire; hidden acorns remain likely to sprout through the burned soil, while those resting above ground would burn right up.

And a study of blue jays found that a single population cached more than 130,000 acorns from just 11 oak trees one year. More than nine of every ten of those acorns were cached in habitats suitable for new oak trees. Some blue jays will even transport those acorns up to four kilometers away, allowing the oak ecosystem to expand.

Or take the nutcrackers of Europe. They spend most of the fall months collecting and hoarding seeds from pine trees. They can transport up to 150 pine seeds at a time in a special pouch in their throats and have been known to fly up to 32 kilometers away before hiding them in small caches of between one and 15 seeds. And because those seeds come from a variety of individual trees, the forests that emerge after they grow are genetically diverse. North American nutcrackers are also responsible for maintaining healthy, genetically diverse populations of trees such as the ponderosa pines that tower over the Rocky Mountains. As climate change alters the birds’ habitats, their caching efforts allow the trees they rely on to migrate. In other words, the nutcrackers facilitate the trees’ climate-change adaptations.

Healthy, diverse oak and pine stands, in turn, provide important resources for a host of other plant and animal species. They provide food and shelter for vertebrates of just about every taxon, as well as millions of invertebrates. They alter the chemistry of soil to facilitate the growth of other plants. They help watersheds their water.

Meanwhile, humans continue to degrade oak and pine ecosystems. The reasons for that damage are diverse: unnatural fire management protocols, for example, or overbrowsing by ungulates, or the introduction of invasive species. “Long-distance seed dispersal and subsequent ecosystem engineering processes could be crucial to the ability of oak and pine ecosystems to respond accordingly,” write the researchers. By ensuring the health of corvid populations, conservationists can in turn ensure the health of whole ecosystems.

In Germany, foresters keep jays on the payroll. Old oaks that provide an abundance of acorns are preserved, while young oaks from elsewhere in the forest are harvested. If seed availability is low in a given year, foresters provide the jays with baskets of acorns and allow nature to take its course. This program allowed jays to contribute to the growth of between 2,000 and 4,000 trees per hectare. Other research has shown that by protecting the jays, forested lands become more profitable.

And in Sweden, the work of Eurasian jays in urban habitats means that humans do not have to be paid to plant acorns. At $2 per acorn, the birds saved $9,400 per hectare each year. Those funds could then be reallocated towards other conservation efforts. Several Mediterranean islands have employed a similar scheme to economically reforest abandoned agricultural fields.

Possibilities for taking advantage of corvid scatter-hoarders clearly abound. Pesendorfer and his colleagues argue that if biologists can better understand the circumstances in which the birds can most efficiently and effectively transport and cache their bounty, conservationists can in turn leverage that information to enhance forest conservation efforts. This would “provide a promising way forward in modern, cost-effective conservation,” they say. – Jason G. Goldman | 24 February 2016

Source: Pesendorfer, M. B., Sillett, T. S., Koenig, W. D., & Morrison, S. A. (2016). Scatter-hoarding corvids as seed dispersers for oaks and pines: A review of a widely distributed mutualism and its utility to habitat restoration. The Condor, 118(2), 215-237. DOI: 10.1650/CONDOR-15-125.1.

2066. Forests Can Help Alleviate World Hunger

By Emily J. Gertz, Takepart, May 6, 2015

Growing more food for the globe’s rising population is the major driver of deforestation. But a new report, endorsed by dozens of forest scientists, has found that leaving those trees standing would do much more to curb hunger than would converting them to cropland or pastures.

More than 1 billion people worldwide already depend on forests for food and crucial nutrients, according to the report, which was published Wednesday by the International Union of Forest Research Organizations.

For communities in Machakos County in eastern Kenya, the report stated, forest tree fruits such as pawpaws, mangoes, and loquats are major sources of vitamin A, and guava is a source of vitamin C.

The wild animals, fish, and insects in forests are also important sources of protein, iron, and fats. In the Rio Negro area of Brazil’s Amazon, the report noted, communities obtained 70 percent of their protein from fish caught in flooded forests and rivers.

Healthy forests also help communities withstand changing climate conditions, civil unrest, and shifting food prices, according to the researchers.

Although tree crops such as rubber, palm oil, and coffee are worth tens of billions of dollars a year in international trade, the report warned that razing natural forests and replacing them with tree crop plantations does not help reduce hunger. Instead, tree crop plantations increase food insecurity by wiping out local sources of food and eradicating agricultural as well as natural biodiversity.

There is little evidence for the “land sparing” argument that such plantations save forests by producing their crops more efficiently, the report noted. But there is a great deal of evidence that “land sharing,” or managing the land for multiple uses and biodiversity, can increase the supply of forest foods and support animal species that are important to agriculture, such as the birds, bats, and bees that pollinate crops

But unless governments integrate forests into their national food policies and international agreements, these benefits are all but invisible.

The report made several recommendations for ways that political leaders could “reimagine forests and food security.” Governments need to restore degraded forests to healthy biodiversity to increase their forest food supplies. They can also “target” particular forest foods for “improved harvest and/or cultivation”—in essence, putting more effort into managing which foods grow in forests. Better education on nutrition, particularly for women and children, can also help communities improve how they gather and manage forest-based foods.

Economic policies and programs must also help communities develop income from forest-related foods and other products, the researchers said.

“We know that forests already play a key role in mitigating the effects of climate change,” Christoph Wildburger, the coordinator of IUFRO’s Global Forest Expert Panels initiative, said in a statement. “This report makes it very clear that they also play a key role in alleviating hunger and improving nutrition.”

1806. Logging Undermines Dominance of Invertebrates Such as Ants and Worms

By Science Daily, April 13, 2015

Invertebrates perform essential functions for the smooth running of the ecosystems in tropical forests. For example, creatures such as termites and millipedes help dead leaves decompose and release their nutrients back into the soil, and carnivorous ants and spiders act as predators of herbivorous invertebrates that would otherwise munch through all the foliage.

Nearly a half of all tropical rainforests worldwide have been logged, and this often causes heavy changes to the number and type of invertebrates, with many species being lost from the ecosystem.

New research led by biologists from Imperial College London, published in Nature Communications, has discovered that when invertebrate diversity declines, their vital functions can be carried out by other members of the ecosystem.

The result shows that rainforest ecosystems have a remarkable resilience to change. "Invertebrates are often thought of as the controllers of tropical forests, so it's surprising that people can upset their dominance to this level," said lead author Dr Robert Ewers from the Department of Life Sciences at Imperial.

Although the ecosystem can continue to function with vertebrates taking more of a leading role, Dr Ewers says the situation leaves rainforests vulnerable: "The forest will keep maintaining itself, but it will be much more susceptible to further change. Relying on vertebrates is a bad tactic -- they are less diverse and vulnerable to new challenges such as land use change.”

For example, the switch to agricultural plots such as palm plantations would cause biodiversity to drop faster. "Knocking out one or two invertebrates might not be too bad, as there are many others to take their place, but knocking out one or two vertebrates could now be disastrous," said Dr Ewers.

By excluding certain organisms from patches of both natural and logged tropical rainforest in Borneo, the team were able to determine their contribution to the ecosystem. They found that in logged forests, the essential activities were still carried out at approximately the same speed, but that invertebrates contributed much less.

For example, the overall rate of invertebrate predation in natural and logged rainforests was the same. However, invertebrates were responsible for only around 60 percent of the activity in logged forests, compared to nearly all of it in natural forests. Instead, birds and bats were responsible for preying on many more invertebrates in logged forests.

The same trend was seen for seed disturbance, a vital function that helps maintain tree diversity, which was taken up by small mammals such as mice and tree shrews.

The decomposition of leaf litter also seemed to be unaffected by the lack of invertebrates, although this was not a function picked up by vertebrates. Instead, the researchers think the litter may have continued to decompose thanks to a changing microclimate or the activities of soil bacteria.

Story Source:

The above story is based on materials provided by Imperial College London. The original article was written by Hayley Dunning. Note: Materials may be edited for content and length.

end story_source

Journal Reference:

1 Robert M. Ewers, Michael J. W. Boyle, Rosalind A. Gleave, Nichola S. Plowman, Suzan Benedick, Henry Bernard, Tom R. Bishop, Effendi Y. Bakhtiar, Vun Khen Chey, Arthur Y. C. Chung, Richard G. Davies, David P. Edwards, Paul Eggleton, Tom M. Fayle, Stephen R. Hardwick, Rahman Homathevi, Roger L. Kitching, Min Sheng Khoo, Sarah H. Luke, Joshua J. March, Reuben Nilus, Marion Pfeifer, Sri V. Rao, Adam C. Sharp, Jake L. Snaddon, Nigel E. Stork, Matthew J. Struebig, Oliver R. Wearn, Kalsum M. Yusah, Edgar C. Turner. Logging cuts the functional importance of invertebrates in tropical rainforest. Nature Communications, 2015; 6: 6836 DOI: 10.1038/ncomms7836

1793. Climate Change Threatens to Kill Off More Aspen Forests by 2050, Scientists Warn

By Justin Gillis, The New York Times, March 30, 2015

Aspen forest in autumn

The beloved aspen forests that shimmer across mountainsides of the American West could be doomed if emissions of greenhouse gases continue at a high level, scientists warned on Monday. That finding adds to a growing body of work suggesting forests worldwide may be imperiled by climate change.

The new paper analyzed the drought and heat that killed millions of aspens in Colorado and nearby states a decade ago. Such conditions could become routine across much of the West by the 2050s unless global emissions are brought under control, the study found.

“I think of aspens as a good canary-in-the-coal-mine tree,” said William R. L. Anderegg, the Princeton University researcher who led the new study, released online Monday by the journal Nature Geoscience. “They’re a wet-loving tree in a dry landscape. They may be showing us how these forests are going to change pretty massively as that landscape gets drier still.”

The study found that large aspen die-offs were a near-certainty only if greenhouse emissions were to continue at the runaway pace that has characterized the last decade. If global emissions are brought under control, the chances will improve that large stands of aspens could be preserved, the paper found.

In the fall, stands of trembling aspens are among the most breathtaking sights in the West, turning hillsides an iridescent golden hue.

Dr. Anderegg grew up camping and hiking in the aspen forests of southwestern Colorado and was dismayed when the trees started dying a decade ago. He has devoted part of his early scientific career to understanding the dieback — and the implications of it for forests elsewhere.

A central focus of the research has been to get a better handle on exactly how trees die in droughts, crucial for predicting how they will fare as global warming proceeds. Dr. Anderegg’s research on aspens suggests that when the ground gets too dry, air bubbles appear in the tiny tubes that carry water through the tree.

“These air bubbles block the pipes and interrupt water transport, giving the tree a kind of heart attack, basically,” Dr. Anderegg said.

He and his collaborators have devised a computer model that, when programmed with climate parameters, can predict aspen mortality with about 75 percent accuracy, and they are working to improve it. Applying their model to the rainfall and temperature conditions expected in coming decades as the climate warms under business-as-usual emissions yielded the prediction of a major aspen die-off.

Depending on exactly how dry the soil gets in the hotter climate, the mortality could extend beyond the West, with aspens — and perhaps many other types of trees — dying across the country, Dr. Anderegg said.

At a global scale, forests have been responding to the rising concentration of carbon dioxide in the atmosphere with accelerated growth, allowing them to pull large amounts of the gas out of the air and thus helping to limit the effects of human emissions. How robust this forest “carbon sink” will remain through time is among the most important topics in climate science.

Dr. Anderegg’s paper fits with other recent findings suggesting that forests may not be as resilient to global warming as once hoped. For instance, a paper published two weeks ago found that the ability of the vast Amazon forest to pull carbon dioxide out of the air was weakening through time, with trees growing faster and dying earlier.

Craig D. Allen, a forest expert with the United States Geological Survey who was not involved in the new research, said Dr. Anderegg’s work was a step toward understanding what might happen across broad landscapes.

But, he warned, a huge amount of work is still needed on other tree types, in other locales, before the picture becomes clear. “There’s just a lot of variability between species,” Dr. Allen said. He noted that aspens have relatively shallow roots, limiting their ability to tap deep water in a drought, whereas other trees could be more resilient.

Forest experts, including Dr. Allen, are particularly worried about future “hot droughts,” similar to the one that struck Colorado and nearby states in the early 2000s. Huge stands of aspens died, and heat-loving beetles killed millions of acres of pine trees.

These droughts are characterized not just by a lack of rainfall but by high temperatures that suck residual moisture out of the soil. They are predicted to increase in a warming climate.

In addition to killing forests, these types of droughts may make food production more difficult, as is becoming evident in California, which is suffering through the fourth year of an especially warm drought.

The frequency and intensity of such lethal droughts later this century will most likely be reduced if efforts to control carbon dioxide emissions are successful over the next few decades, scientists believe.

“The more we lower emissions, the less the risks become,” Dr. Anderegg said. “The choice is in our hands.”