AD Header Dropdowns

AD Main Menu

More robust shrubs transform Arctic tundra, studies say

Doug O'Harra
NOAA photo

If you’re a patch of frozen Arctic tundra, it might be the slogan for a real-life horror movie. Call it "Invasion of the Willow Dwarfs: Attack of the Living Forbs." But this is no pulp fiction.

Rising summer temperatures and longer growing seasons have dramatically increased the spread of shrubs and other vegetation across Arctic tundra. This climate-driven ecological shift feeds on itself — accelerating the warming and boosting the advance of forests into open spaces, according to several recent studies that drew on computer models, field measurements, and snow-melt data. Not to mention herds of hungry reindeer.

An ambitious on-the-ground investigation of tundra on three continents, for instance, found shrubs growing taller and dumping more litter on the ground, accompanied by the disappearance of bare ground. Add it up, and Arctic climate will now warm even faster.

This advance of shrubs and northward crawl of boreal forests into the Far North’s vast tundra has become one of the most important indicators that climate warming is now shaping Arctic ecology on a scale not seen in thousands of years.

To get a vivid sense of what’s going on, check out before-and-after photos of tundra (here and here), along with the color shots posted in the article Shrub Takeover One Sign of Arctic Change published by NOAA’s Climate Watch magazine.

Most scientific studies of this shrub invasion rely on remote glimpses by satellites. But the new report appearing the journal Nature Climate Change took a different tack.

Using observations taken from plots maintained over decades, an international team of scientists analyzed how tundra vegetation had changed between 1980 and 2010 across 158 different plant communities at 46 locations, including eight areas on Alaska’s North Slope.

“Remote-sensing data indicate that contemporary climate warming has already resulted in increased productivity over much of the Arctic,” wrote plant ecologist and tundra expert Sarah Elmendorf and 46 co-authors in the paper. “We found trends of increased height of the plant canopy and maximum observed plant height for most vascular growth forms; increased abundance of litter; increased abundance of evergreen, low-growing and tall shrubs; and decreased abundance of bare ground.”

The findings were complicated and subtle. Much of the increased growth was measured in fractions of centimeter per year. And not all areas with increased warming automatically jump-started the greenery. Other factors included winter versus summer precipitation, and the amount of grazing on the sites.

“Vegetation changes were not always tightly linked with summer temperature trends,” the scientists explained. For example, changes in moss and lichen abundance were not associated with temperature trends, and even for shrubs, numerous warming sites in warm regions actually experienced shrub declines.”

Once the tundra brushes up, so to speak, it begins to directly alter the timing of seasons, according to a different study by the Finnish Meteorological Institute that used satellite observations to analyze how the new growth impacted Scandanavian snowmelt between 1995 and 2011.

The study also relied on a surprising twist to supply the variable that explains why Norwegian winters have been ending sooner than Finnish winters: reindeer with the munchies.

“The study revealed that vegetation was thicker in Norway. One reason is that reindeer grazing is more intensive in Finland,” explained research scientist Juval Cohen, in this story about the findings.

With fewer reindeer disturbing and scarfing down vegetation, Norwegian tundra grows denser and darker, and ultimately absorbs more solar energy than the relatively whiter, and less brushy, tundra in Finland. That factor — how much solar energy gets reflected back into space versus how much gets absorbed by the land and vegetation — is called “albedo.”

“With the exception of the differences in vegetation, the other prevailing conditions -- such as temperature, precipitation and solar radiation -- were almost the same in both countries,” the story explained. “The difference in the melting of snow between Finland and Norway affects the albedo on land. During the snowmelt period, the albedo was almost always higher in Finland."

Does that mean reindeer should be called to the rescue?

“Reducing vegetation or retaining bare tundra would postpone the melting of snow in spring,” Cohen said. “This, in turn, could possibly slow down global warming.”

A third study released in January used computer modeling to warn that it doesn't take a new forest to upset Arctic ecology. Incremental spread of taller shrubs into the Far North tundra increases the rate of warming that degrades permafrost bit by bit. A total conversion from tundra to forest doesn’t have to happen to trigger dramatic regional changes, the authors said. Height of the plants is key.

"Changes in abundance and size of shrubs are frequently observed in these regions," explained lead author Celine Bonfils of Lawrence Livermore National Laboratory, in this story. "Small shrubs are already present in most tundra areas, ready to grow under more favorable conditions. But until now, most climate model studies have only focused on the climate effects induced by a complete tundra-to-forest conversion."

An increase of shrub cover from 32 percent to 51 percent of the land north of 60 degrees north latitude will substantially warm the air during spring and summer, Bonfils and her co-authors explained. The darker shrubs and trees make the land less reflective of the sun’s rays — reducing albedo — and also increase humidity through greater transpiring of the plants.

The computer runs (at the National Energy Research Scientific Computing Center) “show for the first time that the strength and timing of these two mechanisms greatly depends on the height of the shrubs, and the time at which branches and leaves protrude above the snow," Bonfils said here.

“Taller, and aerodynamically rougher, shrubs lower the albedo earlier in the spring and transpire more efficiently than shorter shrubs, thereby increasing soil warming and making the permafrost less stable.”

Contact Doug O'Harra at doug(at)alaskadispatch.com