High above the tree line on Katahdin, where biting winds and sub-zero winters hold the rocky landscape in a frozen grip for much of the year, the mountaintop is turning noticeably greener.
Four decades worth of satellite observations reveal that alpine tundra across the Northeast are undergoing widespread vegetation expansion, not just at the advancing tree line but well above it, too. The new growth is transforming rare, high-elevation ecosystems once thought buffered from rapid change.
And Katahdin, Maine’s highest peak at 5,268 feet, is greening faster than any other New England summit. Vegetation has increased there by 7.9% from 1984 to 2024, according to a recent study in the journal Ecosphere by scientists at Dartmouth College and the Appalachian Mountain Club.
“Greening isn’t necessarily good or bad, but it’s happening,” said Dartmouth College geographer Jonathan Chipman. “The reason may be a concern. It might mean rare plants are being crowded out by aggressive interlopers that can now live in areas when they couldn’t survive in the past.”
The study used thousands of NASA images to track greening on 35 alpine summits from New York to Quebec. Co-author Jordon Tourville provided field perspective as an alpine ecologist with the Appalachian Mountain Club to explore the potential reasons for the greening.
As a geographer, Chipman said he is often asked to work with other researchers to analyze or map data in their fields of study. As an avid climber, he has long wanted to use his skills to research what is happening at the tops of New England’s summits.
“I grew up in New England,” Chipman said. “I climbed Katahdin when I was a kid. I climbed Mount Washington too many times to count. A lot of people, including me, are very concerned about the future of these precious little sky islands.”
For Mainers who hike Baxter State Park or look up at the Longfellow Mountains, the findings suggest that environmental pressures are actively reshaping the summits of New England that are so isolated that naturalists call them “sky islands.”
These summits support fragile plant communities, such as cushion plants and arctic sedges, that survived the last ice age and thrive in conditions too harsh for trees. Some animals are found only in these areas, such as the Katahdin Arctic butterfly, or breed only here, like the American Pipit.
There are only about 50 square miles of alpine ecosystem in the Northeast, Tourville said.
“That makes it the rarest ecosystem type in the region,” Tourville said. “We suspect that because they’re so small, and because the plants there are already existing really at the edge of what plants can withstand, that any amount of change will destroy this system.”
Across Maine’s six studied alpine summits — Katahdin and North Brother in Baxter State Park and Sugarloaf, Bigelow, Saddleback and Mount Abraham in the Longfellow Range — greenness expanded over the four-decade study period, but not at the same pace.
While Katahdin clocked a 7.9% greening trend, the average greening rate of all the peaks studied was 5.2%. Records show a 5.8% increase at North Brother at Baxter, a 5% increase at Sugarloaf, a 4.8% increase at Saddleback and a 4.7% increase at mounts Bigelow and Abraham.
Ground mapping on Katahdin shows that greening is hitting alpine sedge meadows particularly hard. The windswept lawns of Carex bigelowii appear to be undergoing invasion by the woody dwarf heath shrubs that thrive in warmer temperatures and higher nutrient levels.
Baxter State Park officials acknowledged that the shift is a serious concern. They have been collaborating with other researchers to map Katahdin’s alpine plant boundaries, monitor trail corridors and track shrub encroachment into sedge meadows, according to park manager Kevin Adam and natural resources director Nava Tabak.
Strict “stay-on-trail” policies, supported by visitor quotas, summit stewards, signage and string fencing are the park’s most vital tools to help the fragile root systems withstand atmospheric stresses, Adam and Tabak said.
But researchers suspect warming temperatures are hardly the only environmental force at play.
The greening trend accelerated through the 1980s and 1990s before flattening around 2005 to 2009, a trajectory that aligns with ecological recovery following federal Clean Air Act regulations that curbed acid rain and sulfur pollution across New England peaks.
“A lot of people focus on temperature, but I think we missed the nitrogen signal,” Tourville said. “The soils in the alpine zone are extremely nitrogen poor. For decades, we essentially dumped nitrogen fertilizer up there. That probably had a pretty profound impact.”
Topography and severe weather also shape the landscape. Sheltered east- and north-facing slopes across the region’s peaks showed rapid greening, but exposed ridges seem to have resisted plant expansion.
For park stewards, the 40-year satellite record provides a crucial baseline to guide future conservation. But it also highlights a fundamental disagreement over how to manage wilderness in an era of global climate change.
Tourville argued that traditional, hands-off wilderness management may no longer be enough.
“My message to them was, it might be time in the near future to rethink that stance in terms of hands-off, no management up in the park, especially in the alpine zone, because climate change does not respect park boundaries,” Tourville said. “And climate change is not hands-off.”
Baxter State Park leadership firmly defended the park’s “Forever Wild” mandate, arguing direct human intervention is not the answer.
“Our response to stressors from climate extremes, as illustrated by this research, is to continue to minimize human impacts so that the natural communities and species have the best chance to adapt,” Adam and Tabak said in an emailed statement. “Though our approach to stewardship of the Park’s alpine areas is unique, these areas may over time serve as an experimental control that can be compared with other northeastern alpine areas that are more directly managed.”
This story was originally published by the Maine Trust for Local News. Penelope Overton can be reached at poverton@pressherald.com.