By Claire Eamer
In 1998, the forest around Fox Lake, northwest of Whitehorse, was on fire.
“I was living in Whitehorse in 1998,” says forest ecologist Jill Johnstone. “I remember experiencing all of the smoke than hung around the area - lovely sunsets - but I didn’t actually see the fire burning.”
Since then, the University of Saskatchewan researcher has seen plenty of the Fox Lake burn. She’s been leading a study to see what kind of forest is growing back on the 45,000 hectares swept by the fire. Is it the same white-spruce-dominated forest that grew there before 1998, or has something changed?
So far, the answer is coming down on the side of change, and the culprit seems to be a warmer, drier climate. South-central Yukon is already one of the driest parts of the boreal forest, Johnstone says, but recent warming means the region has become even drier. Tree-ring studies show that white spruce have been experiencing widespread and increasing drought stress over the past couple of decades.
Mature trees can survive that kind of stress for years, which means the forests don’t appear to be changing dramatically. When the forest is disturbed by something like the Fox Lake fire, however, the effects of the stresses are revealed. What grows back might not be the same kind of forest.
Johnstone and her colleagues were able to reconstruct the pre-fire forest fairly precisely, not just from records and from the unburned forest around it, but also from what the fire left behind.
“The green wood of the tree stem and branches do not tend to burn, because it is too wet,” she explains. “We can then identify tree individuals to species after a fire, based on cone, bark, and branch morphology.”
Tree rings allowed the researchers to tell how old the trees were, and how long ago the forest last burned. Some of the trees were well over 200 years old, but the largest number were about 120 years old in 1998. The researchers deduced that most of the site was occupied by a forest that had grown up after a fire around 1870 to 1880.
That pre-fire forest was heavily dominated by white spruce, with only a sprinkling of aspen. Much the same kind of forest grew in the valley bottoms and on the slopes above, no matter which direction the slopes faced. The forest Johnstone found emerging in the 2005 and 2008 field seasons - seven and 10 years after the fire - is quite different.
Her team looked at three different areas: slopes facing northeast, slopes facing southwest and lowland areas. Spruce were coming back most strongly on the cool north-facing slopes. On the warmer and drier south-facing slopes and in the lowlands, aspen seedlings outnumbered the spruce seedlings. The young trees are barely a decade old, but Johnstone says that, given our level of knowledge about boreal forest succession, that’s old enough to tell a story.
“If we know what species colonized in the first decade after a fire, we can do a pretty good job of predicting what the stand will look like in 50 or 100 or 150 years.”
What the Fox Lake stand will look like is the forested areas around the Takhini River, says Johnstone. There, a 1958 fire destroyed a huge expanse of spruce forest, and most of it hasn’t returned. In 20 years, that’s what the Fox Lake burn will look like, she predicts.
“The valley lowlands and south-facing slopes will be dominated by open aspen stands, with some sparsely treed, grass-dominated areas. Young spruce forests will be present on the north-facing slopes, and there will be areas of spruce growing up under the aspen in moister parts of the valley bottom.”
It’s a dramatic change, with implications for wildlife habitat, land-atmosphere energy exchange, and patterns of forest productivity and carbon storage, as well as human land uses. But it’s a change we’re likely to see happening more and more often, Johnstone says, and not just in the Yukon.
In fact, Johnstone has been talking about the results of her Fox Lake work in Saskatchewan, where the southern margin of the boreal forest is also suffering from hotter, drier conditions, and where she says the same pattern of change is likely.
“We can expect to see these marginal forests responding first and fastest to ongoing climate change,” she says.
Johnstone will be talking about her work at the Fox Lake burn in a Yukon Science Institute lecture at the Beringia Centre in Whitehorse on Sunday, February 20, at 7:30 p.m. and at the Kluane Park Visitor Centre in Haines Junction on Monday, February 21, at 7:30 p.m.
For more information about Canada’s forests, go to http://canadaforests.nrcan.gc.ca. For information about Jill Johnstone’s work, go to http://homepage.usask.ca/~jfj445.
The Your Yukon column is co-ordinated by the Northern Research Institute at Yukon College. This column is sponsored by the CircumArctic Rangifer Monitoring and Assessment Network. A full list of funders and all past articles are available at www.taiga.net/yourYukon.