Jordan Tourville – an Appalachian Mountain Club staff scientist and ecologist and lead researcher on a paper, Forests on the Move, that tracked the rate of tree line ascent on Katahdin and Mount Washington – has calculated that the tree line is moving up Katahdin about 10 feet a decade. Shawn Patrick Ouellette/Staff Photographer

Mythologized by artists, hiked by explorers and revered by the Wabanaki, Katahdin is a towering symbol of Maine’s untamed natural beauty, its storied logging and sporting traditions, and its Indigenous peoples and culture.

Now the state’s tallest peak is becoming a symbol of climate change. Global warming is fueling the uphill march of its mountain tree line, with warmer temperatures and longer growing seasons enabling firs and spruce to grow higher up the 5,267-foot mountain than ever before.

By comparing old and new aerial photos, Jordon Tourville, an Appalachian Mountain Club ecologist, has calculated that the tree line is moving up Katahdin about 10 feet a decade. A photogrammetric analysis found that the alpine habitat had shrunk by 26 acres from 1991 to 2018, a yearly loss of about an acre.

Tree lines serve as a climate change barometer, Tourville said. As temperatures rise, so will the tree lines. That worries Tourville because he loves the world above that line. It makes him feel like he is walking on the moon. It’s both peaceful and dangerous, virtually untouched by man.

“As someone who hikes for fun as well as for work, I just love the view and the feeling that you get when you emerge from the tree line on a good day,” Tourville said. “Usually, it is rainy, foggy and cold, but that has its own reward. Either way, you’re experiencing what few ever do. It’s one of the last wild places left.”

But Tourville is aware his research on tree line movement proves that Katahdin’s summit isn’t as pristine as it feels, that climate change is threatening the very wildness that has drawn people to hike, paint and study the mountain for centuries.

His findings prove that even this massive granite dome forged 400 million years ago can feel the heat.

A mountain’s tree line is where the forest gives way to alpine shrubs, mosses and rocks. In Maine, that occurs between 3,800 and 4,200 feet, depending on wind, exposure, soil depth, rain and temperature conditions. Regardless of location, it is too cold and snowy for even spruce to thrive above the tree line.

Their vertical structure makes trees more vulnerable to cold than ground-hugging shrubs, mosses and grasses. Trees need an average temperature of about 43 degrees Fahrenheit, and a growing season of at least 94 days with temperatures above 43 degrees, to thrive. Summer warmth is particularly important.

Tree growth is sparse at Katahdin’s tree line, where stunted spruce and fir no more than a foot and a half tall struggle to survive the harsh conditions. It is called “krummholz,” a German word for “crooked wood.” Beyond the tree line, the alpine zone extends up to the summit, which is another 1,000- to 1,400-foot climb.

In 1846, after climbing but not summiting Katahdin, the American philosopher Henry David Thoreau wrote: “Nature here was something savage and awful, though beautiful. Here was no man’s garden. … It was the fresh and natural surface of the planet Earth, as it was made forever and ever.”

Hikers traverse the Knife Edge near South Peak on Katahdin in Baxter State Park in 2016. Tree growth is sparse at Katahdin’s tree line, where stunted spruce and fir no more than a foot and a half tall struggle to survive the harsh conditions. Beyond the tree line, the alpine zone extends up to the summit, which is another 1,000- to 1,400-foot climb. Gregory Rec/Staff Photographer, file

The decrease in air temperature with increasing elevation creates the alpine climate. In the eastern U.S., temperatures usually drop about 1.4 degrees Fahrenheit for every 1,000 feet of elevation. This weekend, in what is usually Maine’s hottest month, summit temperatures are expected to dip as low as 34 degrees.

But climate change is expected to drive up Maine’s average annual temperature by as many as 10 degrees by 2100, according to scientists who advise the Maine Climate Council. They referenced Tourville’s work when noting a link between rising tree lines and rising temperatures in their June report to the council.

The council is tracking how rising temperatures are impacting trees in Maine, the most heavily forested U.S. state. They provide wildlife habitat, protect water quality, absorb 60% of our carbon emissions and support an industry that employs 33,000 people and pumps $8.5 billion into the state economy.

Tourville and his colleagues, AMC’s David Publicover and SUNY College professor Martin Dovciak, also measured Mount Washington’s tree line ascent and found it moving up about 9 1/2 feet a decade. Mount Washington is losing about 44 acres of alpine ecosystem per decade.

The team published its research in the Journal of Biogeography in August 2023. In December, scientists at the Southern University of Science and Technology in China published research using remote sensing to confirm that tree lines rose at about 70% of the world’s mountain ranges.

In their paper, published in Global Change Biology, the Chinese team calculated that a typical tree line moved up an undeveloped mountain about 4 feet each year. They found considerable variability, however. Some tree lines did not move at all, while tropical mountain tree lines are advancing quickly.

Will these findings inspire people to stop burning the fossil fuels that emit the heat-trapping gases that cause climate change and allow tree lines to creep uphill? Tourville would like to think so. He hopes the moving tree lines become visual proof of climate change, a smoking gun for those who remain skeptical.

But Nava Tabak, the director of natural resources at Baxter State Park, where Katahdin is located, said people won’t notice that on their own. Most alpine zone visitors wouldn’t notice climate-driven changes unless they kept careful records over many years. That’s what makes Tourville’s work so important.

“That is part of the challenge with many of the impacts of climate change,” Tabak said. “Humans tend to shift their perceptions with time, and it requires well-designed long-term studies to detect such changes, which is referred to as shifting baselines.”

Educating the public is critical because tree line ascension threatens Maine’s unique alpine ecosystems.

Some alpine specialists prefer this fragile ecosystem, like the rare Katahdin Arctic butterfly only found on Maine’s tallest peak or the American pipit, which only nests above the tree line. The alpine zone also serves as a climate haven for rare plants like the arctic willow, which grows nowhere else in Maine.

Tourville’s research is only the beginning of what old photos can reveal about Maine’s changing climate.

Peter Howe, a Ph.D. student in the University of Maine School of Forest Resources, explains how he will scan and digitize more than 750,000 aerial surveyor photos dating back to 1946 and use photogrammetry software to create three-dimensional photomosaics that will identify changes in New England forests through time. Photo courtesy of the University of Maine

University of Maine graduate student Peter Howe is digging into a collection of even older aerial survey photos donated to the university by the Sewall Co. in 2019 to create a spatially accurate photomosaic of Maine’s forests over time. The Acadia Science Fellow is digitizing the 700,000-piece archival collection.

These historic aerial photos dating back to 1949 cover all of Maine, not just Katahdin, and will enable Howe to build upon Tourville’s work at Maine’s two dozen other bald mountains and calculate how the rate of tree-line movement has changed over time. The oldest Katahdin photos date back to 1957.

It will take Howe time to digitally process the old photos and find shared reference points with the new images that make it possible to take precise measurements within stitched-together photomosaics and calculate the distance between old and new tree lines.

“By photographing a common object from different angles, you can get depth,” Howe said. “It is similar to the parallax effect with our eyes. Each eye sees the world from a slightly different angle, allowing our brain to create a three-dimensional image of the world around us.”

When Howe is done, the photomosaics and the backup data will be available to scientists and the public, and will help managers like Rebecca Cole-Will, who runs Acadia National Park’s resource management program, decide how best to restore ecosystems, including those on their mountain summits.

“We cannot have a thriving ecosystem in the future if we don’t understand how (it) evolved,” Cole-Will said.