ANCHORAGE, Alaska — Shrubs expanding northward into a warming Arctic — and growing taller as they did — paved the way for moose to expand their range northward too.
That’s the finding of a recent study by scientists with the University of Alaska Fairbanks and the U.S. Geological Survey.
The long-legged ungulates were absent from Alaska’s northernmost tundra regions in the second half of the 19th century and the early 20th century, but in recent decades, populations have spread along the rivers and streams that flow into the Arctic Ocean, said the study, which was published in the journal PLOS ONE.
The proliferation of woody plants along rivers and streams made that moose expansion possible.
Those plants grew to only about 1.2 yards tall in the period around 1860 — meaning snow covered most of them in winter, leaving little opportunity for animals to graze, according to the scientists’ calculations. But by 2009, the riparian plants in were growing to nearly 2.19 yards in height, allowing much more moose browse to poke out of the snow.
Photos show the spread of shrubs since the mid-20th century, but figuring out plant height back to the 19th century required calculations based on temperature records, said Ken Tape, an ecologist with UAF’s Institute of Northern Engineering and the study’s lead author. A strong correlation exists between temperature and plant height, Tape said.
It turns out that summer temperatures have increased 23 percent since 1860, he said.
“That’s actually huge, like an eight-week summer to an 11-week summer,” he said. “A 23 percent increase in temperature means a doubling of shrub height.”
Previous work by Tape and others, detailed in a 2011 study, shows how shrubs have spread into various parts of the Arctic — mostly alders in northern Alaska but also willows, evergreen bushes and birch in various parts of northern Canada and Russia.
The North Slope moose are the northernmost in North America and are right on the edge of the worldwide distribution, Tape said. For the most part, moose habitat in the circumpolar North ends where the trees do, he said.
“But we’re seeing that break down as moose move into the tundra,” he said.
The shrub growth — and movement of moose — is limited to the stream and floodplain areas that thread through the tundra, he said. For the most part, the rest of the tundra’s expanse still lacks tall plants and the animals that eat them, he said.
“Moose are confined to what’s sticking out above the snow,” he said. “You’re not going to see a moose standing out on the tundra.”
The northward and westward shift of Alaska’s moose population mirrors a similar shift in the population of snowshoe hares, the subject of an earlier study led by Tape.
To reconstruct the changes in moose distribution, Tape and his colleagues used a combination of historic records and modern observations. They did field work in 2010 to measure the height of shrubs growing along the Colville River and Chandler River, areas where North Slope moose congregate. They used a trove of photographs showing how riparian — that is, stream-side — shrub cover spread over northern Alaska from 1950 to 2000, changes detailed in a 2001 study that Tape co-authored. They used data from sediment cores that preserve records of past Colville-area vegetation and show how it has changed over time.
The absence of moose in these areas during the 19th and early 20th centuries is backed by archaeological information, reports from Alaska Natives and documents left by early explorers, the study says. Moose populations have since spread — not just to the far north of Alaska but also to similar areas in Canada and Russia, the study says.
