In northeastern North America, a 500-mile-long stretch of mountains runs from the Adirondacks in New York to the Gaspé Peninsula in eastern Quebec.
Nearly 70 million people live within a day's drive of one of the alpine zones, the area above tree line where trees don't grow due to the harsh conditions.
The ecological habitats, which include the alpine zones in the Green Mountains and the White Mountains, are home to many rare species of flowering plants, making the areas hotspots for biodiversity. However, with climate change and rising temperatures, the alpine zones are impacted by declining snow cover, atmospheric nutrient deposition, and high-impact outdoor recreation where invasive species may be tracked in by hikers, hunters, bikers, skiers, and others enjoying the mountain terrain.
While prior research has examined greening in regions such as the Alps, the Rocky Mountains, and the Arctic, in relation to climate warming and other processes, Jonathan Chipman '90, director of the Citrin Family GIS/Applied Spatial Analysis Laboratory at Dartmouth, noticed that a regional-scale assessment of what's been happening above tree line in northeastern North America was lacking.
Given that Dartmouth is home to remote sensing and technical expertise in processing satellite imagery and time series data, and the Appalachian Mountain Club has a huge amount of alpine ecology expertise on the ground from fieldwork and citizen science projects, Chipman teamed up with co-author Jordon Tourville, a terrestrial ecologist at the Appalachian Mountain Club , to research the topic further.
"This study was a perfect collaboration between a university and a nongovernmental organization in the region, as we don't just want to be doing this research in the abstract, but we want to help land managers, other stakeholders, and the public understand what is happening in the region," says Chipman.
In a new study led by Chipman using hundreds of satellite images over 40 years, the team provides the first regional analysis of greening trends in the northern Appalachian region.
Published in Ecosphere , the results show that 69% of the alpine zones studied have experienced significant increases in vegetation, representing 88% of the alpine area in the region.
"In other regions such as in the Alps, as an area near the top of the mountain warms, the tree line advances, growing higher up the mountain, but the situation here is quite different," Chipman says. "Our results show that significant greening and shrubification, in which alpine plants are being replaced by shrubs, is occurring inside the alpine zone, not just where the tree line is advancing."
This was especially evident for the Presidential Range in New Hampshire and Katahdin in Maine, where greening was prominent within the alpine zone at higher elevations. Increases in vegetation were observed, including in: cliff plants that grow out of rocky outcrops; cushion-tussock, a dense dome-shaped plant; and sedge meadows, a hardy grass-like plant.
However, according to the co-authors, this acceleration of growth may not necessarily be positive if rare alpine plants start getting crowded out by more aggressive flora.
"While these ecological changes are affected by a number of factors—the overall warming trend, the landscape such as the steepness of the slope and sun exposure, snow cover, soil chemistry, and nitrogen deposition from power plant emissions—what's notable in this region is the effect of wind," says Chipman.
All the mountains in the region have very high winds, especially in the winter. Mount Washington, in New Hampshire's White Mountains, has the highest recorded wind speed by a staffed weather station of 231 miles per hour.
"The wind causes the alpine zones to start lower than they would be elsewhere at this latitude," says Chipman. "What we're seeing is that this greening trend on mountains in the Northeast is kind of being held back on the windy side of the mountains, but there's this kind of growth that is being unleashed on the more sheltered sides, where it's occurring faster."
The results show that the region is warming due to climate change, but the observed greening trends don't necessarily match the expected pattern. The researchers suspect that other factors are adding to the impact of warming. "In the late 20th century these mountains were being doused with nitrogen from Midwestern power plant emissions, which acted as a fertilizer for some plant species more than others," says Chipman. "After the 1990 Clean Air Act amendments, these emissions were reduced, but it takes time for the ecosystem to adjust." So climate warming plus other factors makes for a pattern of greening that varies over space and time.
Using Landsat satellite imagery from 1984 to 2024, the researchers defined the alpine zones on every site south of the St. Lawrence River that were large enough to monitor from satellites, which totaled 50 square miles. They identified 35 major alpine zones and modeled the greenness of the landscape over time. They looked at where the greening is occurring regionally and within individual mountains.
One of the challenges of working with satellite imagery from the region is the cloud cover, resulting in fewer clear-sky days for imagery as compared to other regions. Furthermore, given the sparsity of data from the past, this creates the risk for "observation frequency bias," where it may appear as if there's a trend due to the limited amount of older data from the 1980s to 1990s when that may not be the case. To deal with this problem, the team came up with a method that takes every sparse observation in the past and turns it into the best possible predictor of what the data was at that time.
The pilot version of this work was conducted by co-author Irene Ko '27, a Women in Science Project intern in 2024 and a Citrin Lab research assistant from 2024 to 2025, who examined 22 alpine zones, demonstrating that the topic was something to study further.
The co-authors hope that their regional analysis of alpine greening in northeastern North America can be used to help inform future land and ecological management and stewardship practices.
Funding for this research was provided by Dartmouth's Women In Science Program, the William H. Neukom Institute for Computational Science, and the Jeffrey and Rona Citrin family.
Chipman is available for comment at: Jonathan.W.Chipman@dartmouth.edu .
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Ecosphere
Long-term trends in greenness in the alpine zone of northeastern North America from remote sensing
15-Sep-2026