Canadian tundra turning green

Posted: 8 March 2007

Northern Canada's tundra is disappearing at a rapid rate, with forests of spruce trees and shrubs taking over the once frozen landscape, new research finds. The study offers further evidence of climate change and the authors warn it shows that the shift in the Canadian tundra can happen at a much faster speed than scientists originally thought. This report is from ENS.

The research examines changes in the treeline between forest and tundra ecosystems, a prominent landscape feature in both Arctic and mountain environments.

Scientists have long believed that the treeline will advance as global temperatures continue to increase, but the new study shows that such a shift will not always occur gradually.

"The conventional thinking on treeline dynamics has been that advances are very slow because conditions are so harsh at these high latitudes and altitudes," explained study author Ryan Danby, a biologist with the University of Alberta. "But what our data indicates is that there was an upslope surge of trees in response to warmer temperatures. It's like it waited until conditions were just right and then it decided to get up and run, not just walk."

Coniferous trees are invading the tundra
Coniferous trees are invading the tundra
Coniferous trees are invading the tundra, a consequence of the changing climate. Photo © Adrian Jones courtesy IAN
The study, published in the "Journal of Ecology," analyses reconstructed changes in the density and altitude of treeline forests in southwestern Yukon over the past three centuries.

The research team used tree rings to date the year of establishment and death of spruce trees and reconstruct changes in treeline vegetation. They found that a rapid change in response to climate warming during the early mid 20th century was observed at all locations.

On warm, south-facing slopes the treeline advanced as much as 278 feet in elevation. Tree density increased as much as 65 per cent on cooler, north-facing slopes.

Positive feedback

"The mechanism of change appears to be associated with occasional years of extraordinarily high seed production - triggered by hot, dry summers - followed by successive years of warm temperatures favorable for seedling growth and survival," said Danby.

Danby noted that there is also the concern about a "positive feedback" effect, which has been associated with the decrease in the Arctic ice cap. As the treeline advances, he explained, the reflectance of the land surface declines because coniferous trees absorb more sunlight than the tundra

This light energy is then re-emitted to the atmosphere as heat, adding to warming and further fueling the advance of the treeline.

"These results are very relevant to the current debate surrounding climate change because they provide real evidence that vegetation change will be quite considerable in response to future warming, potentially transforming tundra landscapes into open spruce woodlands," said Danby, a participant in an International Polar Year project that will be examining treeline dynamics across the circumpolar north.

Danby added that the shift will have adverse impacts on tundra species such as caribou and wild sheep, which will also be forced upwards as tundra habitats fragment and disappear.

"The problem is that in mountainous areas you can only go so high so they get forced into smaller and smaller areas," said Danby.

Danby added that the changes are of particular importance in these northern regions where indigenous people still rely heavily on the land. Caribou and sheep populations have already declined across southwestern Yukon, he said.

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