Satellites Reveal Four Decades of Greening on Northeast Alpine Summits

High above the tree line in northeastern North America, where stunted forests give way to windswept heaths, sedge meadows, and rocky cushion plants, the landscape is quietly changing. A new study led by researchers at Dartmouth College, published in the journal Ecosphere, provides the first region-wide analysis of vegetation trends across the alpine zones of the northern Appalachians, and its central finding is striking: the mountaintops are greening. Drawing on hundreds of satellite images collected over four decades, the team found that 69 percent of the alpine zones they examined have experienced significant increases in vegetation, a change that encompasses 88 percent of the total alpine area in the region. The result transforms what had been scattered local observations into a coherent, data-driven picture of ecological change across an entire mountain system.

The mountains in question form a roughly 500-mile-long arc stretching from the Adirondacks of upstate New York through the Green Mountains of Vermont and the White Mountains of New Hampshire to Mount Katahdin in Maine and the Gaspé Peninsula in eastern Quebec. Although these alpine zones cover only about 50 square miles in total, they punch far above their weight ecologically. They harbor numerous rare species of flowering plants found nowhere else in the region, making them recognized hotspots of biodiversity. They are also remarkably accessible: nearly 70 million people live within a day’s drive of at least one of these mountaintop habitats, a proximity that brings both public appreciation and mounting pressures from recreation, including the risk that hikers, hunters, bikers, and skiers inadvertently track in invasive plant species.

Jonathan Chipman, director of the Citrin Family GIS/Applied Spatial Analysis Laboratory at Dartmouth, initiated the study after noticing a conspicuous gap in the scientific literature. Greening trends, in which vegetation cover and productivity increase over time, had been documented in relation to climate warming in the Alps, the Rocky Mountains, and the Arctic, but no one had carried out a comparable regional-scale assessment of what was happening above tree line in northeastern North America. Chipman partnered with co-author Jordon Tourville, a terrestrial ecologist at the Appalachian Mountain Club, whose organization contributes extensive alpine ecology expertise from fieldwork and citizen science programs. The collaboration paired Dartmouth’s strength in remote sensing and satellite time-series analysis with the Club’s on-the-ground knowledge of the region’s fragile summit ecosystems.

The technical foundation of the study was the Landsat satellite record, the longest continuous archive of moderate-resolution Earth observation imagery available. The researchers used imagery spanning 1984 to 2024, a forty-year window long enough to separate genuine directional trends from year-to-year weather noise. They defined the alpine zones on every site south of the St. Lawrence River that was large enough to monitor reliably from orbit, identified 35 major alpine zones, and modeled the greenness of each landscape through time. Crucially, the analysis worked at two scales simultaneously: the team examined where greening was occurring across the region as a whole and within individual mountains, allowing them to detect patterns that a coarser regional average would have obscured.

Working in this particular region posed a distinctive data challenge. The Northeast is notoriously cloudy, which means far fewer clear-sky satellite observations are available than in arid or high-latitude regions where remote sensing studies are more commonly conducted. The sparsity is worst in the early part of the record, with relatively few usable images from the 1980s and 1990s. That imbalance creates a statistical hazard the researchers call observation frequency bias: an apparent trend can emerge simply because the limited older data happen to capture unusual years, rather than because vegetation is genuinely changing. To address this, the team developed a method that takes every sparse observation in the historical record and converts it into the best possible estimate of what conditions were at that time, effectively reconstructing a more complete and trustworthy time series from fragmentary inputs.

The pattern that emerged from the analysis was not the one the researchers might have predicted from simple climate logic. In regions such as the Alps, warming typically manifests as an upward advance of the tree line, with forest creeping into what was formerly alpine terrain. In the Northeast, Chipman explains, the situation is different. The study found significant greening and shrubification, a process in which low-growing alpine plants are replaced by shrubs, occurring inside the alpine zone itself, not merely at the advancing margin of the forest. This was especially evident in the Presidential Range of New Hampshire and on Katahdin in Maine, where greening was prominent at higher elevations well within alpine territory. Vegetation increases were observed across several distinct plant communities, including cliff plants that grow from rocky outcrops, cushion-tussock vegetation forming dense dome-shaped mats, and sedge meadows of hardy grass-like plants.

One of the most important physical factors shaping this pattern, the researchers argue, is wind. All of the region’s mountains experience very high winds, particularly in winter, and Mount Washington in New Hampshire’s White Mountains holds the record for the highest wind speed ever recorded by a staffed weather station: 231 miles per hour. These brutal winds suppress woody growth and push the alpine zone to lower elevations than it would occupy at this latitude elsewhere in the world. The study’s results suggest that the prevailing greening trend is being held back on the windward, exposed sides of the mountains, while on more sheltered sides the growth of shrubs and denser vegetation is being unleashed and proceeding faster. Wind exposure, in other words, acts as a spatial filter on how climate warming translates into visible ecological change across the rugged terrain.

Wind is not the only complicating factor. The researchers emphasize that the observed greening does not necessarily match the pattern expected from warming alone, and they suspect that additional influences are layered on top of the regional warming trend. Among these are declining snow cover, atmospheric nutrient deposition, soil chemistry, slope steepness and sun exposure, and a historical legacy of nitrogen pollution. In the late twentieth century, the region’s mountains were regularly doused with nitrogen emitted by Midwestern power plants and transported by prevailing winds, which acted as a fertilizer that benefited some plant species more than others. After the 1990 Clean Air Act amendments sharply reduced those emissions, the fertilizer effect diminished, but ecosystems do not adjust instantaneously. The result, the authors suggest, is a greening pattern that varies over both space and time as warming, nutrient history, topography, and wind exposure interact in different combinations on different peaks.

Whether this greening is good news is an open question, and the co-authors urge caution. Accelerated plant growth might sound benign, even positive, but in alpine zones it may not be. If aggressive, faster-growing flora begin to crowd out the rare, slow-growing flowering plants that make these summits biodiversity hotspots, the net effect could be a loss of the very species that define the habitat. Shrubification, in particular, represents a structural transformation of the ecosystem rather than a simple increase in life. The researchers hope that their regional analysis will serve as a practical tool for land managers and stewardship organizations, helping them target monitoring and conservation efforts at the zones and slopes where change is happening fastest. Chipman framed the project as a model of collaboration between a university and a regional nongovernmental organization, aimed not at research in the abstract but at helping managers, stakeholders, and the public understand what is happening in the mountains around them.

The study also had an educational dimension. A pilot version of the work was conducted by co-author Irene Ko, a Women in Science Project intern in 2024 and a Citrin Lab research assistant from 2024 to 2025, who examined 22 alpine zones and demonstrated that the topic merited full-scale investigation. Funding came from Dartmouth’s Women In Science Program, the William H. Neukom Institute for Computational Science, and the Jeffrey and Rona Citrin family. As satellite archives continue to grow and analytical methods for handling sparse, cloud-obscured records improve, the Northeast’s alpine summits, long studied plot by plot on foot, now have a four-decade regional baseline against which future change can be measured, and against which the fate of some of eastern North America’s rarest plants may ultimately be decided.

Subject of Research: Long-term satellite analysis of vegetation greening and shrubification in the alpine zones of the northeastern United States and Canada

Article Title: Study finds alpine peaks are greening in Northeast U.S., Canada

Article References: Study finds alpine peaks are greening in Northeast U.S., Canada. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: alpine zone, greening, shrubs, Landsat, remote sensing, northern Appalachians, climate change, tree line, biodiversity, nitrogen deposition, wind exposure, Ecosphere