Pioneer spruce in Burwash Uplands
University of Alberta
As creeping saplings, they suck water and steal nutrients. They block the sun with their prickly needles. And their looming presence finally annihilates shrubby alpine competitors with spruce-like indifference.
OK, maybe this arboreal drama from the Kluane Ranges in southwestern Yukon Territory won’t make it as a matinee thriller. But a new innovative study by University of Alberta scientists found white spruce invading mountain tundra with surprising speed, pushing the tree line higher in elevation much faster than expected in response to climate change.
Once the summer warmth hit a certain level — the forest essentially pounced.
“The conventional thinking on treeline dynamics has been that advances are very slow because conditions are so harsh at these high latitudes and altitudes,” said Ryan Danby, from the UA Department of Biological Sciences. “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.”
The observations have strong parallels and lessons for Alaska, with its eastern border only a few miles west of the study site, Danby added in an email message.
“First, the effects of continued climate warming on vegetation will be significant. These changes are likely to have cascading effects on wildlife populations and how people use the land, so Alaskans and Yukoners need to begin thinking now about what future landscapes might look like.
“Secondly, our data shows that these changes can occur rapidly, but
that they can also be delayed. So we may not see change immediately, but that doesn’t mean it won’t occur.”
The findings add to a growing body of evidence that warming climate has already triggered dramatic changes in vegetation across the Far North. Alaska’s Arctic tundra has been greening, with shrubs spreading and growing taller. At the same time, Interior boreal spruce forests have been repeatedly stressed by dry springs and hot summers, with more insect outbreaks and forest fires. Two of the three top years for fire damage scorched Alaska in 2004 and 2005.
Figuring out what combination of factors stimulate tree growth, and which ones start to kill the forest, can be very complex, Danby said.
“We’ve done some experimental work to try an tease out the relative importance of some of these other variables, and the interactions between them,” he wrote in an email. “We set up 40 mini-greenhouses to artificially warm young white spruce at the treeline, just to see what would happen.”
The greenhouses warmed the air about 2 degrees C and the soil by 1 degree C. This lengthened the growing season, and mimicked the insulation of deep snow in winter.
“Our results from this experiment indicate that while temperature is important, and particularly soil temperature, its influence is partly contingent on the amount of snow and the rate of snow melt in spring,” he said. “So the positive effects of higher air and soil temperatures on plant growth may be negated unless there is enough snow to shelter seedlings during winter and/or unless the snow melts out quick enough for the plant to take advantage of the longer growing season.”
In the treeline study, Danby and David Hik, also on the UA science faculty, examined white spruce and willow at six sites ranging from 3,500 to 4,500 feet in elevation in the St. Elias Mountains foothills west of the Alaska Highway near Burwash Landing. Their field work lasted more than five seasons.
Using the data from tree rings. they tracked the lives of spruce trees and willow brush, from sprout to death, and reconstructed the shift of tree line vegetation over the past 300 years. The study, Variability, contingency and rapid change in recent subarctic alpine tree line dynamics, was published this month in the “Journal of Ecology.”
They found that 20th Century climate warming triggered a fast response in the mountain ecosystem. The tree line on south-facing slopes advanced as much 278 feet in elevation over a few decades. On north-facing slopes, the tree line didn’t move but the forest stands grew more dense — sometime as much as 65 percent.
Parts of the Burwash Uplands dominated by windswept tundra in the Goldrush era had basically transformed into a woodland between 1925 and 1945.
The mechanism for the rapid change was twofold, Danby explained. First — hot, dry summers triggered years of “extraordinarily high seed production.” Then successive years of warm temperatures nurtured seedling growth and survival.
This change will impact animals that use the uplands. As the tree line advances, the tundra habitat retreats and shrinks and becomes more fragmented.
“The problem is that in mountainous areas you can only go so high so they get forced into smaller and smaller areas,” said Danby.
Ryan Danby taking core samples
University of Alberta
Such changes could alter land use and distribution of sheep and caribou, and end up impacting the subsistence lifestyle of Alaska Natives and First Nations people, Danby explained.
It also creates positive feedback for even more climate warming — conifer trees like spruce absorb more sunlight as heat, and then in turn help heat the atmosphere that in turn fosters forest growth.
“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,” Danby said.