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 newly published 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 1.1 meters (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 meters (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 vast 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.
By the 1970s, aerial surveys recorded a population of 1,550 to 1,700 moose on the North Slope, about half of them in the middle Colville River drainage; by the 1980s, moose had colonized Northwest Alaska.
Four decades ago, there were Alaska Native elders who remembered the time in the early 20th century when no moose were in the region, Tape said. Today, it is difficult to find someone with a memory that goes that far back, though indigenous observations of moose absence are available in written records, he said.
Evidence shows that moose roamed the North Slope during a period much further back in time, the study points out. Moose bones have been found in Colville permafrost sites that date back to about 3,000 years ago, Tape noted. Past work by the USGS describes moose and humans arriving on the North Slope at about the same time as the demise of large ice age mammals like the woolly mammoth, between 14,000 and 10,000 years ago. A similar shrub expansion seems to have occurred at the end of the Pleistocene era.
The modern shrub expansion and the incursion of moose into the more northern region appear set to continue over the long term, the study says.
"Increases in shrubs and earlier snowmelt have occurred across most of the Arctic, notably along the northern edge of moose distribution, so increases in abundance and extended northward distribution of moose are anticipated elsewhere," it concludes.
But in the shorter term, the North Slope moose population has been volatile, prompting some hunting restrictions. The reasons for sudden drops in recent years are not yet known, Tape said. Habitat expansion is not the only factor affecting wildlife, and the moose could have been affected in recent years by wolf predation, parasites and disease or other forces, he said.