Secret Forest Walk
Why don't conifers spread farther north, where it's getting warmer? Researchers have an answer and say where to expect new forests.
Alaska's conifer forests: Trees spread differently than researchers expected.
Photo: Getty Images
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At first glance, the boundary line appears to have been drawn arbitrarily. East of the Brooks Range in northern Alaska, the white spruce forest breaks off suddenly, leaving an expanse northward of arctic tundra, a steppe area dominated by lichens, mosses, and small shrubs. There is an invisible factor that determines where the tree line is: temperature. Therefore, one would expect boreal forests, which form a ring around the Northern Hemisphere, to advance towards the poles as a result of climate change; Especially since the region is warming twice as fast as the global average.
But so far they have not acted according to the textbook: yes, they have pushed north in places. But in other countries, they have made little or no progress despite the rising temperature. A mystery that American ecologists now believe they have solved.
Sea ice sets the pace
The team led by Romain Dial of Alaska Pacific University in Anchorage looked at 19 sites along the Brooks Range, a mountain range that extends more than 1,000 kilometers from the Bering Sea in the west to the Beaufort Sea across Alaska and extending into northwestern Canada. Using satellite images and tree ring data, they analyzed treeline changes and compared them with the decline of Arctic sea ice over the same period.
They discovered a relationship: Where sea ice retreated further, conifer forests spread northward; However, as a layer of ice masses has remained off the coast all year to this day, it has remained rooted in place. A subsequent meta-analysis confirmed the suspicions.
But why should distant sea ice determine the pace of the advance of boreal conifer forest into Arctic tundra? Scientists now explain this in “Science” magazine: If the Arctic Ocean is exposed in the fall, the water heats up even more because the darker sea surface can absorb more sunlight.
If cold air now flows over warm water surfaces, evaporation occurs and moisture-saturated air parcels migrate to land. The result: more snowfall in the winter. “This snowfall protects the seedlings from harsh winter conditions,” says Dayal. “Thanks to the warmer temperatures, the soil is also enriched with nutrients and the trees can grow better. So they benefit from the marine conditions.”
Changes coming in Siberia
This is exactly what can already be observed in the Chukchi Sea, says Dial: In the marginal sea of the Arctic Ocean between Siberia and Alaska, the extent of sea ice decreased by more than 12,000 square kilometers in October, that is, in October. Near the time it reached its lowest point, since 1979, it is roughly the same area as Northern Ireland. The forests there were able to expand.
However, in the Beaufort Sea and in eastern Siberia, sea ice still exists, thus preventing forest expansion. According to the authors, no major changes are expected there in the next few years, while they expect major unrest to come in Western Siberia.
This is exactly a big step forward, says Ronnie Rothbarth of Wageningen University in the Netherlands, who was not involved in the study: The new findings will now allow “at least an approximate prediction” of where the forest is likely to expand in the future. Years become. “Until now this has only been possible to a limited extent,” says the ecologist.
Why this is important can be explained by the consequences of colonizing forests: not only can it attack carbon stores in the soil, but it also causes regional temperatures to rise because darker forest surfaces absorb more sunlight, according to the Science study.
Last but not least, developed forests will also put pressure on tundra ecosystems, and thus on the food supplies of indigenous groups that collect berries and hunt reindeer and caribou. “Most of the animals and plants they depend on will disappear as the landscape changes,” Dial says.
Climate performance is decreasing
While the Arctic tundra in North America, Sweden and Siberia is retreating, it is expanding northward: a team led by geographer Michael Grimes of the University of Leeds recently determined through analysis of satellite images that Greenland has been an icy surface for the past three decades. Of the size of Albania lost. Then tundra plants spread over the bare rocks. The area of wetlands has doubled in the same period, and the area of wetlands has quadrupled, according to the results of a study conducted in Scientific journal “Scientific Reports”.
Such a transformation can also be observed in the boreal coniferous forest. However, the world's largest terrestrial ecosystem is expanding more slowly in the north than it is retreating in the south, scientists led by Rothbarth discovered last year in the journal «Nature Communications » Steadily.
Progress took a long time, while drier and warmer conditions in the south caused relatively rapid changes. “The decline of boreal forests on their southern border is progressing rapidly, exacerbated by deforestation, fires or insect infestations,” says Rothbarth. “This has serious consequences for biodiversity and carbon storage and thus the global regulation of global climate through these forests.”
However, the empty space left by boreal conifer forests does not remain vacant for long: deciduous forests from temperate latitudes take their place.
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