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Forest can replace tundra at rapid rate

treeline

Photo courtesy Ryan Danby.

Forests of spruce trees and shrubs are taking over tundra in parts of northern Canada, forcing out other species. New research examining the last three centuries of arboreal history shows that this shift can happen at a much faster speed than scientists originally thought. The study from the University of Alberta appears in the March issue of the Journal of Ecology.

“The conventional thinking on tree-line dynamics has been that advances are very slow because conditions are so harsh at these high latitudes and elevations,” says UA biologist Ryan Danby. “But what our data indicates is that there was an upslope surge of trees in response to warmer temperatures. It’s like [the trees] waited until conditions were just right and then decided to get up and run, not just walk.”

Danby and UA’s David Hik reconstructed changes in the density and elevation of tree-line forests in the southwestern Yukon for the past 300 years. Using tree rings, they were able to date the years of establishment and death of spruce trees and reconstruct changes in treeline vegetation.

Danby and Hik found that a rapid change in response to climate warming during the early to middle part of the 20th century was observed at all locations. The tree line advanced considerably—as much as 85 meters (280 feet) in elevation—on south-facing slopes, where sunshine warmed the soil. Tree density increased significantly—as much as 65%—on cooler, north-facing slopes.

“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,” says Danby.

University of Alberta

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