The Western Arctic Caribou Herd, once the biggest in Alaska, is faltering, having fallen from a high of 490,000 animals in 2003 to only 152,000 as of 2023. But to the east, the Porcupine Caribou Herd appears to be thriving, with an all-time high of 218,00 animals recorded at the last census. That makes it, rather than the Western Arctic herd, the state’s largest.

Why are the herds following opposite trends? An answer, Alaska scientists say, is found in what is growing on the ground – and the way the warming climate has changed those plants.

Woody shrubs and even trees are spreading rapidly over Arctic regions of Northwest Alaska, the area where the Western Arctic herd ranges, said Roman Dial, a professor at Alaska Pacific University. But that plant transformation, which scientists refer to as “shrubification,” has been much slower on the eastern side of Arctic Alaska, the range for the Porcupine Caribou Herd, he said.

For caribou, growth of woody plants like alders and willows means problems. Caribou depend on tundra plants like lichen and mosses; the shrubs and trees taking over the terrain are reducing the availability of that food favored by the animals.

Dial has studied changes in Alaska plant growth for several years and has been traveling in the Brooks Range since he was a teenager in the 1970s.

Even though his recent years’ work in Arctic Alaska has been focused on plants, he said encountering willows and other woody plants covering what used to be open tundra west of the Dalton Highway made him think right away of animals.

“A lot of caribou trails were getting overgrown and disappeared, and you’d find really old antlers that were in skulls that were kind of buried in the tundra, so caribou hadn’t been there for a long time,” he said. “Right away it was, like: ‘Wow, caribou are changing their routes.’ And you could see it.”

The overgrown state of caribou trails that had been etched into tundra terrain over multiple years of migration was instructive, Dial said.

When he presented his studies during the December annual meeting of the Western Arctic Caribou Herd Working Group, the advisory panel representing villagers and others dependent on the herd, Dial described what might be a caribou’s view of the plant takeover, and he used a brief video from the field to illustrate his point.

He does not like walking through willows that can be eight feet tall, “and I don’t think caribou like going through willows either,” Dial told the working group. “If my antlers were all tender and velvet, I wouldn’t want to go through a bunch of tall willows. And also, when you go through willows, there’s bears in there.”

Changes in caribou habitat are linked to reduced Arctic sea ice, which itself is a direct result of accelerated climate warming in the Northern Hemisphere, Dial said.

Open water leads to more snowfall, he said, as there is more moisture sent into the atmosphere to fall as precipitation, More snowfall insulates the ground, keeping soil temperatures warmer through the winter, he explained. Warmer soil temperatures encourage plant growth and the spread of woody shrubs and trees. More woody plants on the ground make life harder for caribou, both by displacing their usual tundra food sources and by creating new obstacles to movement.

Open water does not affect Alaska’s western and eastern Arctic tundra regions equally, and the results are seen on the ground, Dial said.

Sea ice retreat usually forms later and melts earlier in the Chukchi Sea, which lies off the northwestern coast, than in the Beaufort Sea, which lies off Alaska’s northeastern coast. Utqiagvik – the nation’s northernmost community – is the point where the two Arctic seas meet. Relatively warm Pacific Ocean water flows into the Chukchi through the Bering Strait, making ice there more seasonal, meaning it forms and melts earlier each year. In contrast, an ocean circulation system called the Beaufort Gyre sends old multiyear ice from north of Canada into the Beaufort, making the freeze there a little more resilient. While ice retreat has been significant over the past decades in both seas, the characteristics of the Chukchi make it particularly vulnerable, and it has lost both the thickness and extent of ice at a faster rate than almost any marginal sea in the Arctic, according to climate scientists.

Both the Western Arctic herd range and the Porcupine herd range have become warmer in summer and snowier in winter, according to records kept by the Alaska Center for Climate Assessment and Policy at the University of Alaska Fairbanks. But the summer change has been more intense in the western range, particularly in coastal areas off Nome and Kotzebue, according to the data.

The work by Dial and his colleagues to track the changes involved an old-fashioned method: walking the ground.

The idea to do that was inspired by studies of shrub growth that is spreading up to higher elevations around Anchorage – and made necessary by the COVID-19 pandemic, which shut down National Science Foundation-funded air travel in 2020, Dial said. Some of the Alaska Pacific University students who were working with him on studying vegetation were enlisted to go north to be part of the Brooks Range expeditions.

In 2020 and 2021, they walked hundreds of miles of the terrain, smartphones in hand, looking down and recording changes in the plants growing on either side of the Dalton Highway. “It kind of added a new dimension to hiking,” he said. A study published in June details the findings from their treks in 2020.

A separate but related study, published about a year ago, examined tree rings to show a correlation between growth and proximity to open Arctic water. The study, which Dial did with Patrick Sullivan of the University of Alaska Anchorage and other scientists, focuses on white spruce trees from 19 different sites along the Brooks Range. Those trees were small, ranging from ankle to chest height, indicating that they were recent arrivals, Dial said.

The on-the-groundwork by Dial, Sullivan and their colleagues adds to past research that tracked the northward spread of woody plants by more distant methods. A 2018 study by scientists from the U.S. Fish and Wildlife Service, UAF and other organizations, for example, used 50 years’ worth of aerial photographs to identify shrub and tree expansion into the Arctic National Wildlife Refuge in northeastern Alaska, where the Porcupine herd ranges. The study found that shrubs had spread into the refuge’s tundra regions over the half-century period, but they had done so at a slower pace than in other Arctic Alaska tundra regions.

The growth of woody plants in Arctic tundra regions affects more than caribou.

In northwestern Alaska, where the growth has been most dramatic, it has attracted a proliferation of beavers, for example. And as beavers colonize the landscape, they are transforming it with thousands of new dams that pool water that, in turn, speed thaw of permafrost and feed into the cycle of shrub expansion.

Broader climate change impacts

Climate change impacts on the Western Arctic Caribou Herd go beyond the spread of shrubs displacing tundra plants.

Warm winter conditions in 2005 produced two days of rain in the herd’s winter range, creating a thick layer of ice that encased the tundra plants that the animals eat. A large die-off followed, according to the Alaska Department of Fish and Game.

Wildland fires that are becoming more common as conditions warm can also affect caribou by destroying the slow-growing lichen and other tundra plants the animals eat. That has long been known to be an issue for caribou in more southern and boreal regions, such as Interior Alaska’s Nelchina herd. Now wildfire has emerged as a threat to the Western Arctic Caribou Herd’s habitat.

The Western Arctic herd’s declines are part of a circumpolar trend.

Tundra caribou populations across the Arctic have declined by 65% over the last two to three decades, according to the 2024 Arctic Report Card released in December by the National Oceanic and Atmospheric Administration. Alaska’s Western Arctic Caribou Herd was one of those identified as having the most dramatic declines.

“Warmer summer and fall temperatures, changes in winter snowfall, and an increasing human footprint collectively stress Arctic caribou, altering their distribution, movements, survival and productivity,” the report card said.

The Porcupine herd, in contrast, was cited in the Arctic Report Card as one of the major herds with a stable or increasing population, thus going against the dominant trend. The 218,000 total last counted was an increase from 197,000 in 2013. Because the last full census of the Porcupine herd was completed several years ago, in 2017, that population is classified as stable rather than increasing.

For the Western Arctic herd, changes go beyond its sliding population numbers. The herd has also A key metric measured by federal and state biologists who study the herd is the date when southward-moving caribou cross the Kobuk River, a waterway that flows west from the Brooks Range into Kotzebue Sound. Over the decades, most collared caribou spend summers north of the river and winters south of it, though in five years since 2016, fewer than half of the collared animals went that far south in their fall migration, according to the data.

At the December meeting of the Western Arctic Caribou Herd, National Park Service biologist Kyle Joly said the average river-crossing date in 2023 was notably late: Nov. 8. “It was the latest-ever average time that they crossed. I actually had to extend my graph here because the number didn’t fit,” Joly told working group members.

Caribou that do cross have also shifted the location of where they do so, and where they spend the winter. There has been a notable lack of caribou on the Seward Peninsula in the western part of the traditional range, according to the data from collared animals. None were tracked into Bering Land Bridge National Preserve, Joly said.

On-the-ground observations match the data, he said. “Twenty-five years ago, Unalakeet was a great place to see caribou,” he said. Residents haven’t seen caribou there for several years, he said.

While the Western Arctic and Porcupine herds are following opposite trends, both face challenges from industrial development or potential development.

The biggest development project envisioned for the Western Arctic herd’s range is the Ambler Access Project, which would construct a road about 200 miles into the Brooks Range foothills to an isolated mining district. The Biden administration rejected a plan for road construction, but the project could be pushed forward by the incoming Trump administration.

Also in the area is the Red Dog mine, one of the world’s largest zinc producers. The 52-mile road that connects the mine site to the Chukchi Sea port used to ship out processed ore has already been shown to hinder caribou movement for at least part of the herd. The mine operator, Teck Resources Ltd., just won federal approval for exploratory work at what would be an expansion into a different zinc deposit, which would include an extension of the mine’s road.

There is also expanding oil development in the National Petroleum Reserve, on the eastern edge of the Western Arctic herd’s range, a planned graphite mine north of Nome on the Seward Peninsula and assorted smaller projects that are underway.

The encroaching development worries members of the Western Arctic Caribou Herd Working Group. The group has repeatedly expressed official objections to the proposed Ambler road, as well as concerns about the cumulative effects of multiple projects.

Those concerns were repeated at the December meeting.

“It seems like development is taking over. We’re living in a different time,” said Michael Stickman, a member from Nulato, an Koyukon village on the Yukon River. “We don’t want to lose our way of life.”

The Porcupine herd’s territory, in contrast, has been largely protected from development. But there are looming plans that would bring oil drilling rigs to the coastal plain of the Arctic National Wildlife Refuge, the place where the herd usually masses in summer to give birth to and nurture young calves.

Two congressionally mandated lease sales, one in 2021 and one held this month, failed to generate industry interest. Most of the bidding in the first sale, which resulted in no on-the-ground development, was from an Alaska state agency, the Alaska Industrial Development and Export Authority. This month’s sale attracted no bids.

However, President-elect Donald Trump has touted the refuge’s potential for producing oil, falsely claiming that it has the potential to hold more oil than Saudi Arabia. More lease sales, with more industry-favorable terms, could be held in future years in the new Trump administration.