Satellite data reveals dramatic upslope forest migration in Central America, raising new questions about tree lines, warming, and the future of alpine ecosystems
Forests on the Move—But Not Where Expected
A core prediction of climate science is now playing out—but with a twist. Tree lines are migrating upslope, not where warming is most extreme, like the Arctic, but in the tropical mountains of Mexico and Central America. A new study, published in Biogeosciences, shows forest edges advancing by multiple meters per year—a pace that’s “too fast,” according to ecologist Townsend Peterson, co-author of the study.
- The research spanned 115 peaks from western Canada to Panama.
- Using Landsat satellite imagery from 1984 to 2017, researchers mapped shifts in forest boundaries.
- Surprisingly, tree lines in northern regions barely moved, while tropical peaks saw significant upward expansion.
Climate Change: The Expected Culprit—Or Not?
Although the findings might seem to confirm climate change impacts, scientists urge caution in drawing direct links. The team deliberately excluded mountains with obvious human activity, but subtle land use impacts can go undetected in satellite imagery.
- Many tropical tree lines are expanding within zones already warm enough to support trees, says botanist Christian Körner.
- This suggests trees may be reclaiming areas cleared for grazing or logging decades ago.
“They cannot make any claims regarding climate change,” Körner cautions.
A Debate Rooted in Definitions
The field of tree line ecology is marked by disagreement—even about what “tree line” means.
- Some define it as a climate threshold (an isotherm around 6°C mean growing-season temperature), independent of actual tree presence.
- Others define it more practically as the visible upper edge of forest cover.
This disagreement complicates interpretation of shifting tree lines and what they mean in a warming world.
A High-Tech Global Survey—With Manual Labor
To measure tree line change on a large scale, researchers leveraged Google Earth Engine to access four decades of satellite data. When automation proved insufficient, Peterson manually traced forest boundaries, spending three months mapping tree lines by hand.
“It’s a coarse method,” admits ecologist Matteo Garbarino, “but works well at this large scale.”
Tropical Surprises: Why These Forests Are Racing Uphill
Why is the change most dramatic in the tropics? Researchers suggest a few key reasons:
- Moisture is more abundant at high elevations in tropical zones, making forests more responsive to even slight temperature increases.
- Some areas may be experiencing ecological recovery after historical clear-cutting for agriculture or pasture.
- The still-rare focus on tropical mountains may mean these trends have simply gone unnoticed until now.
What’s Next: AI and Historical Photos
Lead author Joanna Corimanya hopes to train AI models to expand this global tree line survey. The team is also analyzing historical photographs from as far back as the 1870s to validate the trends observed in satellite imagery.
“Alpine ecosystems are shrinking,” Corimanya says. “We need to document it—even if the causes are still being debated.”
A Shrinking Alpine World
Whether caused by climate change, land use recovery, or both, the result is the same: alpine habitats are disappearing. These high-altitude ecosystems, home to specialized and often endangered species, are being squeezed between rising forests and warming skies.
- More research is needed to distinguish climate-driven change from land use legacy.
- But documenting the trend itself is critical for biodiversity conservation and ecosystem forecasting.
A global satellite study finds tropical forests are moving upslope faster than anywhere else, with tree lines advancing by meters per year. While climate change may play a role, researchers say historical land use and moisture levels could also explain the trend. The loss of alpine ecosystems is happening regardless—and it’s accelerating.
