Most Yukoners are familiar with the Takhini Burn, a swath of aspen forest that fills the valley floor west of Whitehorse.

A major fire swept through the area in 1958, burning away the spruce trees that once dominated the forest there, and aspen has held sway ever since.

The Takhini Burn is an unusual forest for this part of the world, looking more like a landscape from northern Alberta than the Yukon. But one forestry researcher thinks that in a warmer world, such aspen parklands might well become a more common sight in the Yukon.

For her doctoral degree, Jill Johnstone studied the interactions between fires, forest and climate. Now working on a post-doctoral research fellowship, Johnstone is trying to determine how climate change might affect the boreal forest.

Right now many scientists predict that the warmer, drier conditions produced by climate change could make fires more frequent, disrupting the normal patterns of succession.

More than any other forest on the planet, the boreal forest depends upon fire for renewal and typically burns every 50 to 150 years. But if fires occur more frequently, the pattern of succession could change.

"Without climate change, when a forest burns down the expectation is that it eventually will regenerate to what it was before," says Johnstone, and in this part of the world that is most often spruce.

Typically fast-growing species such as aspen and pine are first off the mark after a fire, but eventually give way to slower-growing species like spruce. With time the spruce grow taller than the aspen or pine, which begin to die out in the shade of the taller trees.

But if the forest burns more frequently because of climate change, that pattern could change.

On test plots in the Yukon and Alaska, Johnstone has been studying forests that have burned at different times in the past. Her research has shown that a forest's future is pretty much determined by what happens within the first ten years after a fire.

Spruce will usually re-establish themselves after a fire if enough of their seeds survive to kick start a new generation. Black spruce -- as well as lodgepole pine -- depend upon heat from forest fires to melt the resins sealing shut their cones, allowing them to burst open and release their seeds.

But if the fire cycle speeds up and forests burn at younger ages, the spruce will have had less time to produce seed for the next generation.

Johnstone has spent a lot of time looking at what happens right after a fire; and how the severity of a fire helps determine which trees will re-establish themselves.

When severe fires leave behind only bare mineral soil, aspen have a better chance of establishing themselves. The common assumption used to be that the aspen regenerated from roots that survived the fire, but -- after stomping around many a burned stand of forest -- Johnstone does not think that is the case.

"One very large surprise is that I found aspen can come in by seed; they don't just come in by asexual regeneration, or what we know as suckering."

"To have asexual regeneration you need to have some aspen present, so if I could not find evidence for aspen in the pre-fire stand and there is no root source, the logical explanation is that it came in by seed."

"People have hypothesized that live roots can be present when there are no live trees, but I think that is implausible."

The upshot of all of this is that climate change could favour some species over others, and aspen trees might be the winners if the North continues on a warming path.

"The best analog for the Yukon might be Alberta; that may be where the Yukon is headed if conditions become drier with less permafrost. I think more aspen parklands are a real possibility."

White spruce could be the big losers in this scenario as they release their seeds every year, and forest fires often burn early in the summer before the seeds are mature. If fires occur more frequently, the chances are even slimmer that enough white spruce seeds will survive to generate a new spruce forest.

"White spruce in general does not do well when fires burn more frequently," says Johnstone. "Scientists have already documented drought-stress in Alaskan white spruce. Could they become extinct in some areas? It is a question."

Johnstone stresses that in general the boreal forest tends to be a very stable ecosystem, but it too has changed over time and 6-10,000 years ago, deciduous trees like aspen and birch also dominated the landscape.

The problem will be tracking the cause of the changes as forests evolve slowly over long periods of time. "These processes are such slow ones and occur over decades or centuries, while we tend to want to look at things on an annual scale."

Jill Johnstone is based at Yukon College while she is working as an NSERC (Natural Sciences and Energy Research Council of Canada) post-doctoral research fellow. She can be contacted at (867) 668-8881 or jjohnsto@yukoncollege.yk.ca.