White spruce trees are expanding into the Arctic tundra with stunning speed, with potentially serious consequences both for the region and the world.

In the summer of 2019, Roman Dial and his friend Brad Meiklejohn hired a single-engine bush plane out of Kotzebue, on the northwest coast of Alaska. Even those wings could only get them within a five-day hike of where they wanted to be: deep in the tundra, where Dial had noticed peculiar shadows showing up in satellite images. 

On the fourth day of that hike, the pair was walking along a caribou trail when Meiklejohn yelled, “Stop!” Dial thought his friend had seen a bear. But it was something more troubling: a stand of white spruce trees. The plants were well formed and chest-high, like small Christmas trees. And from a planetary perspective, they were bad news, because they were not at all where they were supposed to be. In this Alaskan tundra, fierce winds and biting cold favor shrubs, grasses, and grass-like sedges. The growing season is supposed to be just too short for trees to get a foothold, even if their seeds manage to fly north.

The journey confirmed what Dial suspected, that the shadows in the satellite images were in fact out-of-place trees that are part of a phenomenon known as Arctic greening. As the Arctic warms more than four times faster than the rest of the planet, that’s bringing down the ecological barriers for plants in the far north, and more vegetation is marching toward the pole. “The next day we found more and more as we headed east, until we discovered an Arctic savanna of white spruce trees,” recalls Dial, an ecologist at Alaska Pacific University. “Sounds funny to say, it was maybe the most exciting hike I’ve ever been on.”

Arctic greening is a blaring warning light on the climate damage dashboard, both for the region and the world at large. The proliferation of shrubs is one thing—they’re small and grow relatively quickly—but long-lived white spruce are another thing entirely. “When you see trees growing, you know that the climate has really shifted,” says Dial. “It’s not like five years of weather, or 10 years of weather. It’s 30 years of climate that’s established new trees in new places.”

Writing this month in the journal Nature, Dial and his colleagues put hard numbers on what they discovered in the Alaskan tundra: White spruce, both as individuals and as a population, are growing exponentially there. The population is now moving north at a rate of 2.5 miles per decade, faster than any other conifer treeline that scientists have measured, in what should be one of the most inhospitable places on the planet for a tree.

This one’s probably five years old.Courtesy of Roman Dial

Usually, spruce seeds don’t travel more than a few hundred feet from a tree. But Dial is finding young white spruce growing from seeds that must have traveled 5 to 7 miles—and over mountains, no less. The population isn’t so much moving north as it is leaping. “These new colonists, you’d think that they’re beyond the treeline, they should just be struggling. But they’re actually growing really rapidly,” says Dial. “They’re happy as pigs in poop—they’re just going gangbusters out there in the Arctic tundra and alpine tundra. They’re way ahead. They’re even doing better than the shrubs.”

Exactly why they’re doing so well demands more research, but Dial speculates that the colonists have access to untapped nutrients in the soil. By contrast, back at the treeline, existing generations of white spruce have already extracted the goodies from the soil, perhaps slowing their march. “If you want to study how forests are going to move, it’s probably not appropriate to go to a treeline, because a treeline is where they’re kind of stalled out,” says Dial. “If you want to figure out, ‘how can a business do better?’ you probably don’t go study a struggling business. You go look for startups that are doing well.” 

And, boy, business is booming for white spruce right now. Soil microbes in this part of the Arctic are providing a sort of stimulus package for them. The microbes multiply as the dirt warms, processing nutrients for the trees to use. “Obviously, warming is the driving factor—warmer conditions, longer growing seasons,” says macroscale ecologist Scott Goetz, who studies Arctic greening at Northern Arizona University but wasn’t involved in this new research. “So it’s all become much more suitable, and I think nutrients are just part of the story.”

Courtesy of Roman Dial

The white spruce colonists are likely warming the Arctic landscape too. Normally, snow cover makes these northern lands reflect the sun’s energy back into space—in scientific parlance, the land’s “albedo” is high. But trees are darker, so they have a lower albedo and absorb heat, which warms the area. “The albedo effect is the big thing,” says Goetz. “They absorb a lot more energy.” 

(This is also why the Arctic, in general, is warming so much faster than the rest of the planet: As sea ice disappears, it exposes darker waters underneath, which absorb more of the sun’s energy.) 

Counterintuitively, by acting as a sort of snare to trap snow against the ground, a forest further heats the soil. A thick layer of snow prevents the chill of winter from penetrating into the ground, and the extra trees block cold winds. (That trapped snow also provides lots of moisture for those trees in the future, creating a feedback loop.) 

In this animation, each dot represents a new tree.Courtesy of Roman Dial

Thawing permafrost is the aspect of Arctic greening that concerns scientists the most. These frozen soils are loaded with dead organic matter that hasn’t fully decomposed, but will decay rapidly once it thaws. Microbes then begin munching on the material, spewing both carbon dioxide and methane, an extremely potent greenhouse gas. Permafrost is now thawing so fast that Arctic land is collapsing, gouging great big holes in the landscape. “The implications are significant,” says Goetz. “It always comes around to the permafrost in the end, because that’s the big carbon reservoir that’s being mobilized.”

Yet the Arctic isn’t a monolith. In the part of northwest Alaska Dial studied, white spruce are on a northward sprint—but to the east, they don’t seem to have had the same success. “It’s not happening everywhere. The next question to answer is, why?” asks Dial. “What are the climatic controls on what determines where trees move fast?”

Figuring that out will require still more satellite imagery and multi-day hikes into the tundra—and if all goes according to plan, no bears.