Forests are migrating up mountain peaks

Satellite imagery reveals an unexpected shift in tree lines—but it may not have anything to do with climate change

It’s a hallmark prediction of climate change: As the world warms, trees will migrate not just toward the poles, but also up the slopes of mountains, eating away at fragile alpine ecosystems. Although advancing tree lines have been tracked at individual mountains, a new large-scale study has found something surprising: Over a span of 4 decades, the largest upward movement of these forests has come not near the poles, as one might expect, but instead in the tropics, where monitoring has been far more limited.

The survey, published late last month in Biogeosciences, looked at 115 peaks stretching from western Canada to Panama. Using imagery collected from 1984 to 2017 by the Landsat program, a long-running series of U.S. satellites, researchers found that tree lines at peaks in the northern United States and Canada had barely budged, even though global warming is happening faster at high latitudes than in the tropics. Meanwhile, “in Mexico and Central America, they are running up the mountainside,” says Townsend Peterson, an ecologist at the University of Kansas (KU) and co-author of the study. “It’s kind of scary how fast it is. It’s at a level of multiple meters per year. It’s too fast.”

Whether warming is causing this surge is not clear, Peterson and his co-authors concede. They excluded any mountains with obvious signs of human interference in the forest edge, such as grazing, logging, or farming, but such signs can’t always be picked up from space. And many of the advancing tree lines in Central America are gaining ground in regions where, theoretically at least, temperatures were already warm enough to allow them to grow, says Christian Körner, a botanist at the University of Basel. “They cannot make any claims regarding climate change.”

The study of tree lines is surprisingly contentious. Partly this comes down to definitions. For many ecologists and biologists like Körner, a mountain’s tree line is an isotherm: a fixed elevation representing the boundary where growing-season temperatures dip below 6°C and trees fail to grow. Even if there are no trees at the tree line—because of, say, past fires—this boundary does not change. Other scientists, meanwhile, continue to use tree line in its older sense, representing the place where trees stop on a mountain, even if the temperature barrier has not been breached.

Tree line studies have also tended to focus on the peaks of North America and Europe, reflecting the bias of researchers’ home turf. Some of these have found signs of upward advance—especially in the Alps, where temperatures have risen 3°C above preindustrial levels—whereas in other regions evidence has been weaker. But several years ago, Daniel Jiménez-García, a conservation biologist at the Meritorious Autonomous University of Puebla, led a study with Peterson and others that found a dramatic change in the forests of 15 Mexican volcanoes. Tree lines rose by an average of nearly 500 meters in 3 decades. And that got the scientists wondering whether they had missed an important trend.

Measuring these changes globally was challenging. The researchers turned to the Landsat record now available on Google Earth Engine, which allowed them to analyze shifts in greenery at more than 100 peaks across nearly 40 years of imagery in the cloud. When automated methods failed, Peterson went in and spent the better part of 3 months mapping out the forest edges by hand.

Cover photo :  Photographic comparisons, such as these shots of Red Castle Mountain in Utah, taken in 1870 and 2024, help verify satellites’ views of advancing tree lines. Drag the slider to compare the two images.(First image) William Henry Jackson; (second image) Joanna Corimanya, Anahí Quezada, and Town Peterson