Due to disappearing sea ice, the tree line is advancing in northern regions

Due to disappearing sea ice, the tree line is advancing in northern regions

Where sea ice disappears, trees grow. In the Arctic, hard, abrupt boundaries can be seen between boreal forests of mainly pine and spruce and the treeless tundra. Ecologists have long known that those tree lines shift to higher latitudes as average temperatures rise.

But it now appears that the disappearance of sea ice in the Arctic Ocean also allows nearby trees to expand their habitat northward. That’s because sea ice influences precipitation in the region, researchers from Alaska wrote this Thursday Science. They base their conclusions on calculations using satellite images and annual rings of trees.

However, it remains complicated to determine exactly what causes tree lines to shift or not. The location of tree lines also depends on, for example, topography, animal grazing and human influences.

Dark leaves and needles

The advance of boreal forests northwards has implications for global warming. On the one hand, a larger forest area provides cooling through the absorption of carbon dioxide (CO2). On the other, the dark leaves and needles of trees absorb sunlight that would otherwise have been reflected by snow.

As early as the 18th century, explorer Alexander von Humboldt knew that with higher temperatures the tree line in the Arctic shifts northward. When the average temperature drops below six degrees Celsius, trees no longer grow. Due to global warming, areas further north will rise above this temperature limit, and forests can therefore expand northwards. This also happened after the last ice age.

But even in places in the Arctic where increased temperatures allow trees to grow, the Alaskan researchers saw treeless tundra. So there had to be more going on besides temperature.

Sea ice influences snowfall in the region, the researchers knew. Ice covers the sea like a kind of lid, preventing the water underneath from evaporating. But when sea ice disappears, water vapor spreads through the air and falls as snow elsewhere. And snow keeps the soil relatively warm, retains nutrients and protects saplings from the wind. So where sea ice retreats, trees could advance, was the idea.

To test that idea, the researchers used annual rings of 1,636 trees, satellite images and data on precipitation and temperature in the area. Annual rings reflect the growth rate of a tree in a given year.

Fewer young trees

“We indeed saw that the growth rate of the trees varies not only with changes in temperature, but also with variations in sea ice,” the first author of the study, Roman Dial, emails from Alaska. “Areas close to sea ice also have fewer young trees and those trees grow more slowly than areas further from sea ice. In addition, many tree lines that have moved over the past 50 to 100 years are found near places where ice was disappearing 40 years ago or where there was already little ice.”

“Brave study,” says Michiel van den Broeke, professor of polar meteorology at Utrecht University and not involved in the study. “To look for connections between sea ice and tree lines, the authors had to combine many different data. It is interesting to investigate the effect of retreating sea ice on the environment, especially because more and more sea ice is disappearing due to global warming.”

Michiel van der Molen, meteorologist at Wageningen University & Research who was also not involved in the study, is also enthusiastic about the study. “They have well substantiated their conclusion. But it remains complicated to find out what exactly determines the locations of tree lines. I have often been to Siberia for field work. The sharp tree lines were sometimes along a river. The pine cones of conifers may have difficulty crossing the river. And reindeer that eat small trees also influences where the border is.”

Also read
Where is the tree line? That is difficult to determine. ‘The border we see is often not the natural border at all’