This migration can be seen in these images of Canada’s Waterton Lakes National Park…

…and of Jackson Glacier in Montana’s Glacier National Park, for example.

But new research, published in the International Journal of Applied Earth Observation and Geoinformation, paints a more complicated picture: Between 2000 and 2020, 42% of tree lines shifted up, true. But 25% of them actually moved downhill.

Sabine Rumpf, an ecologist at the University of Basel in Switzerland, said many studies of tree line shifts tend to be concentrated in limited geographic areas. A preponderance are based primarily on data from North America, Europe, and the Himalayas, where researchers are more likely to have funding to head to the field to take measurements themselves.

“But that also means that a large proportion of the surface of our planet is so understudied,” Rumpf said. “And [to remedy] that, remote sensing data [are] really amazing because you can get a truly global picture, even though there’s nobody, or too few people, observing things in the field.”

Tree Lines Aren’t Living up to Their Potential

So the team set out to take a more global look. They used a world mountain map, developed in 2018, with a 250-meter resolution. They did exclude some regions from their analysis: cells with less than 10% high-mountain coverage (which have so few trees that they don’t have much of a tree line) and cells more than 95% covered with trees (which have so many trees that they don’t have much of a tree line). For their purposes, the team defined the “observed tree line” as the upper limit of trees that stand 3 meters or taller.

Then, said Rumpf, they used a model to calculate the potential tree lines for each area, because, thanks to human effects on the environment, “where these trees could be surviving is almost always higher than where the trees are currently.” The model looked at the growing season length and mean growing season temperature for each cell in the map’s grid. The researchers determined that if a cell had a growing season length of 94 days or longer, and an average growing season temperature of 6.4°C or higher, it could potentially host trees. Cells that didn’t meet both criteria were considered unable to be covered in forest, and thus above the potential tree line.

Jordon Tourville, a terrestrial ecologist with the Appalachian Mountain Club, said the overall findings are not surprising, because other studies have shown seemingly “paradoxical downslope shifts in some cases.” But he noted that whereas this study estimated potential tree lines based on temperature constraints, some scientists have suggested that factors such as nutrient availability and wind exposure are also important in determining tree line position.

Unsurprising, on Second Thought

Armed with this information about observed versus potential tree lines, the researchers hypothesized that areas with the smallest deviation between the two were mostly responding to climatic factors. In contrast, they speculated, areas with a greater difference between observed and potential tree lines were likely experiencing more anthropogenic disturbance, such as logging, agriculture, and infrastructure development.

Their hypothesis held up. In areas with less human disturbance, tree lines were moving upward more quickly (the researchers noted, though, that the upward migration of tree lines lagged behind the rate of climate change). In areas with more human disturbance, the upward spread of trees is suppressed, or even reversed.

Rumpf and several of her colleagues are located in the Alps, where glaciers are retreating, tree lines are climbing, and towns are generally more threatened by mudslides than by wildfires.

Some of the study’s findings, like a quarter of tree lines shifting down, or such a clear signal from wildfires in some areas, were at first unexpected. But after some reflection, Rumpf realized the diversity of data was a perfect example of why global-scale research is important.

“A lot of scientific funding is based in North America and Europe,” Rumpf said, which means many studies return similar results. “Then we do something global and we are surprised that things are different somewhere else on the globe?… I mean, well, duh.”

—Emily Gardner (@emfurd.bsky.social), Associate Editor

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Citation: Gardner, E. (2026), Tree lines are migrating. Some up, some down., Eos, 107, https://doi.org/10.1029/2026EO260146. Published on 12 May 2026.

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Planting more trees will decelerate climate change only if those trees are placed in optimal locations—primarily the tropics and subtropics—suggests new research published in Communications Earth and Environment. However, planting trees in locations like Alaska, Siberia, and large parts of the United States could actually lead to warming, said lead author and doctoral student at ETH Zurich Nora Fahrenbach.

Much of the current thinking in nature-based solutions, Fahrenbach said, is based on the idea that “more is better.”

As in, “we’ll plant a trillion trees, or we’ll plant more than a trillion trees, and we are going to get more cooling, right?” Fahrenbach said. “That’s something we show is just not the case.”

Fahrenbach researches reforestation potentials, or global maps that identify areas where trees could be planted to mitigate climate change. In this work, she and her colleagues compared three prominent reforestation potentials to determine the effect of tree placement on local and global temperatures.

One scenario involved reforesting about 926 million hectares focused mostly on the tropics and resulted in about 0.25°C of cooling by 2100. Another called for reforesting 894 million hectares, including large areas in northern temperate and polar latitudes, and resulted in 0.13°C of cooling by 2100.

The third scenario involved planting forests strategically over only 440 million hectares of mostly tropical and subtropical land (less than half of the area covered in the other scenarios) but also resulted in 0.13°C of cooling. Geography, the findings suggest, may matter more than quantity when it comes to the cooling benefits of reforestation efforts.

Let’s Get (Biogeo)physical

The researchers modeled all three scenarios using the same parameters: Trees were planted from 2015 to 2070 and then remained steady in their population until 2100.

All three models identified reforestation opportunities in regions such as the eastern United States, Amazonia, the Congo rainforest, and eastern China, as well as regions for which reforestation would not be as impactful, such as polar regions in the Northern Hemisphere. The researchers also found significant temperature changes across the Atlantic and Indian oceans as a result of atmospheric changes induced by reforestation, demonstrating an interconnected reality: Planting trees in one area doesn’t just change the local temperature but has effects across the world.

These local and nonlocal effects can be explained by a combination of biogeochemical and biogeophysical effects.

A biogeochemical effect relates to the movement of chemicals or chemical elements, such as trees absorbing carbon from the atmosphere.

A biogeophysical effect relates to the physical results of changing the land’s surface: Placing a tree in a snowy region, for instance, decreases the land’s albedo, meaning it causes the land surface to become darker and absorb more light, leading to more local heat. This rise in surface temperature also raises air temperature, creating cascading effects on wind patterns and oceanic currents.

Considering both processes together is essential for understanding whether a net cooling or net heating effect exists, but most policies focus only on biogeochemical effects, seeing trees solely for their ability to absorb carbon from the atmosphere, Fahrenbach said. They include prominent international policies such as the Paris Agreement and the United Nations’ Framework for REDD+.

“Really, we would also need to consider the biogeophysical effects,” Fahrenbach said. “That’s harder to do, right, considering those nonlocal effects, because just imagine, some country is going to plant a lot of trees, and that’s going to lead to warming somewhere else.”

A Call to Policymakers

Emilio Vilanova, a forest ecologist at the climate action nonprofit Verra, wrote by email, “The most important message for me is that this study emphasizes something that is often not well addressed in reforestation projects: Reforestation is not just about planting trees—it’s about designing where new forests go to maximize benefits and avoid unintended consequences.”

Vilanova also said the study puts the potential for reforestation efforts to address climate change in perspective. “Even very large reforestation efforts would only reduce global temperatures by about 0.13–0.25°C by the end of the century,” he said. “While meaningful, this finding also reinforces that reforestation is a helpful tool, not a stand-alone solution to climate change.”

Though the limited potential for change is sobering, the authors and Vilanova pointed out that this change does matter and that it matters how we think of our approach. They advocate for policies that adopt reforestation strategies based on location and that acknowledge both the local and nonlocal effects of reforestation.

“We really need to make sure that where we plant first, it has benefits locally, it has benefits globally,” Fahrenbach said.

—Andrew Meissen (@AndrewMeissen), Science Writer

22 April 2026: This article was updated to correct Nora Fahrenbach’s position at ETH Zurich.

This news article is included in our ENGAGE resource for educators seeking science news for their classroom lessons. Browse all ENGAGE articles, and share with your fellow educators how you integrated the article into an activity in the comments section below.

Citation: Meissen, A. (2026), Location, location, location: The “where” of reforestation may matter more than the extent, Eos, 107, https://doi.org/10.1029/2026EO260125. Published on 22 April 2026.

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Central Mongolia’s Hangay Mountains have long posed a conundrum. Rising 4 kilometers above sea level, the dome-shaped range plays a key role in shaping the region’s climate. But it couldn’t have formed in the same way as most equally tall mountain ranges.

“These mountains in central Mongolia are very far from any plate boundary, about 3,000 kilometers away from the Pacific margin,” said Pengfei Li, a geologist at the Chinese Academy of Sciences’ Guangzhou Institute of Geochemistry. “It’s very hard to understand why we have such a mountain range so far from the plate boundary.”

Li recently led research finding that geochemical evidence supports a compelling explanation of how these oddball mountains formed. The researchers proposed that at the site of the future mountains, a U-shaped bend in a tectonic plate led to an extra-thick lithosphere. A chunk of that heavy lithosphere eventually broke off and sunk into the mantle. Free of the extra weight, the crust then rebounded upward as the Hangay Mountains.

Bend and Snap

Tectonic plates are far from rigid. As they move above, below, and against each other, sections of the plates far from the boundary can develop curves and folds like a scrunched up tablecloth. Curved sections, called oroclines, are common around the world. At about 6,000 kilometers long, the Mongolian orocline is one of the longest, and the Hangay Mountains sit right at the curviest part of the orocline’s U shape.

Li and his colleagues suspected that the Hangays’ location along the orocline is no coincidence. During multiple field expeditions from 2018 through 2026, the researchers collected rock samples from several sites in the Hangay Mountains that showed signs of ancient volcanic activity. Uranium-lead dating of zircons within those samples showed that the area experienced volcanic activity in the early Cretaceous period 124–114 million years ago.

“When I saw the age, I was surprised,” Li said. “120 million years—no one had ever reported volcanoes [in the Hangay Mountains] during this period.…It’s the first discovery of volcanism for this period.”

The team also analyzed the samples for major and trace elements to determine the depth at which the rocks formed. Their geochemical analysis revealed that the rocks formed in the lithosphere 80 kilometers below the surface. They published these results in Geology in April.

It’s pretty odd that the rocks originated so deep, Li said, because the modern-day lithosphere is only 70 kilometers thick.

The team proposed that when the continental plate folded and created the Mongolian orocline 200 million years ago, the lithosphere bunched up and became thicker in the curve of the U shape. That thicker section of lithosphere, a root at least 80 kilometers thick, would have been unstable in the long term, Li explained.

The lithospheric root would have been too heavy to remain attached to the crust above for long, and a chunk of it would have eventually snapped off. When it sunk, or foundered, into the deep mantle, it would have melted and generated the volcanic activity recorded in the rocks the team studied. Free from the weight of that lithospheric root, the crust above would have rebounded into the dome-shaped mountain range visible today.

Complicated Yet Compelling

“The story that [the researchers] have put together to explain the massive Hangay topographic ‘dome’ of central west Mongolia is a compelling one that spans more than the past 200 million years of Earth history,” said Stephen Johnston, a tectonics researcher at the University of Alberta in Canada who was not involved with this research. Past research into the Iberian orocline suggested that oroclines might lead to lithospheric thickening, and this explanation of the Hangay Mountains fits that narrative.

“Their story, though complicated, makes a great deal of sense and in a way provides affirmation of a prediction made some time ago regarding oroclines,” Johnston added.

Johnston said that the new explanation of how the Hangay Mountains formed makes him wonder why it took so long—80 million years—between when the orocline formed and when the lithospheric root sank.

“This seems a long time for a gravitationally unstable mantle root to have remained attached to the overlying crust,” he said. He hopes that future work can help determine whether this process has taken place at other oroclines around the world and has simply been overlooked or whether there is something special about the Mongolian orocline.

Li and his team have turned their attention to how the formation of the Hangay Mountains shaped the region’s ancient climate. Today, the towering mountain range prevents moist air from northern Mongolia from reaching the parched Gobi Desert in the south. They hope to connect how a process deep underground, like lithospheric foundering, affected the paleoclimate and, consequently, the region’s habitability.

“It’s very new to try to understand the Earth’s habitability from a deeper sense,” Li said.

—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer

Correction 18 May 2026: The distance between the Hangay Mountains and the Pacific plate margin has been corrected. The location of newly discovered volcanic activity has been corrected.

This news article is included in our ENGAGE resource for educators seeking science news for their classroom lessons. Browse all ENGAGE articles, and share with your fellow educators how you integrated the article into an activity in the comments section below.

Citation: Cartier, K. M. S. (2026), Mongolian mountains rose when the crust bounced back, Eos, 107, https://doi.org/10.1029/2026EO260153. Published on 15 May 2026.

Text © 2026. AGU. CC BY-NC-ND 3.0
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