The sudden surge of forest cover took place from 1925 to 1950, a period of warming probably not caused by human-generated greenhouse gases, the researchers noted.
Instead the change corresponds with the peak of a well-documented warming trend that followed the Little Ice Age between the 14th and 19th centuries.
The trigger for the treeline change was a summer of high seed production followed by several years of warm temperatures that allowed the seeds to take root, grow, and survive, Danby said.
Such large seeding events typically follow dry, hot summers and are cyclical in nature, he noted.
"You need the seed source, but you also need subsequent climate warming for those seeds to grow and mature into trees. So indeed, it is a threshold point for the system," he said.
According to Danby, scientists expected treeline to gradually inch forward as the harsh environment slowly adjusted to warmer temperatures.
"What our study shows is that that's not necessarily the case. It can actually advance forward quite rapidly," he said.
In addition, Danby said, current global warming is allowing the forest stands that took hold during the surge to grow even thicker.
"We had this initial advance from 1925 to 1950," he said, "and then with this more recent bout of warming since about 1980, we see an infilling of these stands with seedlings and young saplings that are now coming up and resulting in a thicker, denser forest."
Matthew Sturm is a geophysicist with the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory at Fort Wainwright, Alaska.
He said the team's finding is consistent with episodic shifts noted elsewhere in the scientific literature.
"Conditions aren't ripe, then all of a sudden they go ripe and the seeds are there, and these forests get a foothold and they move," he said.
The finding also fits with a larger picture that's emerging of how tundra responds to climate change, Sturm noted.
His own research concentrates on how tundra landscapes hundreds of kilometers from the nearest tree are responding to today's warming temperatures.
"The shrub component is increasing in abundance and vigor and height and size," he said.
"That's kind of an analog to the trees marching north as well, and marching up the hills."
Danby said the tundra changes in the northern latitudes of the Arctic are of particular concern to tundra species like caribou and Dall sheep.
"They rely on these tundra habitats, and as these habitats become shrubbier and tree density increases, they're forced to smaller and smaller habitat areas," he said.
Several recent studies, he added, indicate smaller and fragmented habitats can be detrimental to wildlife.
Caribou and Dall sheep are food sources for Canada's native First Nations peoples, but Danby said the recent decline in caribou and sheep numbers is unexplained by hunting alone.
"The question pops up: What is the other driver in this system that is causing these declines? And habitat change comes to mind," he said.
These changes could accelerate in the future, Danby cautioned.
Tundra, like the polar ice caps, reflects sunlight. Darker coniferous trees absorb sunlight and re-emit it as heat into the atmosphere.
"That contributes to atmospheric warming, and it creates this spiral, what we refer to as a positive feedback," he said.
"That's the exact same process that's causing the rapid decay of the polar ice caps."
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