uncertain times for the treeline

According to the climate modellers, warming temperatures are likely to result in movement of the treeline both farther north and farther up the sides of the mountains.

By Claire Eamer

According to the climate modellers, warming temperatures are likely to result in movement of the treeline both farther north and farther up the sides of the mountains.

Are the modellers right?

The answer, says Queen’s University researcher Ryan Danby, is yes. And also, no. As well as maybe, partly, and sometimes. Basically, it’s complicated.

Danby has been studying treeline ecology in the Ruby Range of southwestern Yukon for more than a decade. There’s a real possibility that treeline will advance, he says, and it could happen quite quickly.

But understanding the ecological processes involved in treeline change isn’t simple. It requires examining the question at multiple scales, from the landscape down to individual trees.

Treeline isn’t really a line, he explained in a Yukon Science Institute talk earlier this week. It’s the boundary zone between forest and tundra, where trees survive at the edge of what’s possible for their species. A small change in conditions can tip the balance in either direction and transform the landscape, tree by individual tree.

The change confronting the Yukon is not small, and it’s already happening. All models for climate change show the magnitude of change and its impacts strongest in northern regions, Danby said. In North American terms, the northwest – including the Yukon – will feel those effects even more strongly than other parts of the North.

To see what can result from a temperature increase of several degrees – as projected for the Yukon over the next few decades – we need only look at the past, Danby said. Studies in Russia show that a few thousand years ago, when the temperature was a bit warmer, treeline was hundreds of kilometres farther north, reaching the shores of the Arctic Ocean in places.

That kind of advance in the Yukon would mean a major loss of tundra habitat, both in the northern part of the territory and on the upper slopes of mountains. And loss of habitat affects the animals that live there, from insects up to caribou.

Forest expansion could also accelerate climate change, Danby said. Forest landscapes are darker than tundra landscapes, especially in winter when the tundra is covered in snow. Light colours reflect the sun’s heat back into the atmosphere, while dark colours absorb it, so forest landscapes are warmer and contribute to increasing air temperatures. A study in Alaska suggests this effect could be as important as carbon dioxide in contributing to further global warning.

So will treeline change? Is it changing already? And what causes the change?

First, Danby looked at the landscape level, at where trees give way to tundra. Using satellite images so detailed that he could see individual trees, he mapped the distribution of trees in the Ruby Range and found, not to his surprise, that elevation was a major factor in tree distribution. In the mountains, higher elevation generally means lower temperatures during the growing season. However, Danby said, mapping showed that sunny, south-facing slopes can have dense forest, while north-facing slopes at the same elevation have only a few scattered trees. Also, slopes showing bare rock, indicating a thin layer of topsoil, had fewer trees.

“Elevation is important, but it’s not omnipotent,” he concluded. Solar radiation and soil are also factors.

Next, he looked at the ecotone scale – that is, within the broad band of mountainside where the forest peters out and tundra takes over. That’s where he found confirmation that changes are taking place.

Danby was able to compare aerial photographs taken in the region in 1947 and 1989, and they showed trees growing more thickly in the lower reaches of the treeline zone and extending further up the slopes in 1989.

“But it’s not enough to know there’s been change,” he said. “We need to know why.”

Temperatures have increased in the region during that time, but understanding how the trees respond to temperature and what other factors might account for the change in treeline requires a shift down in scale, to the individual plant.

One of the reasons the Kluane region is a good place to study treeline, Danby said, is that you can stick to one species of tree. “In the Kluane region, by some quirk of biogeography that no one has really put a finger on, white spruce is the only conifer in the landscape.”

Still, even one species offers plenty of complications. White spruce can reproduce from seed or from shoots that grow out of their root systems. They need different environmental conditions at different times in their lives. And they grow in different shapes and densities in different places.

Danby has examined tree rings from more than 2,000 individual trees, counted trees and seedlings in plots up and down the slopes, examined the record left by dead trees, and experimented with tiny, topless greenhouse-like structures that raise the temperature around individual small trees to see how they respond to heat.

Through the photographs and tree rings, he has identified a dramatic increase in tree density on south-facing slopes, beginning around 1920 when temperatures began to climb in the Kluane region.

“It’s really quite remarkable. That change happened across the entire ecotone.”

Whether that kind of change in the cards for the Yukon’s future depends on the precise mix of factors that determine whether a tree thrives or fails, and Danby is still working on that problem. For more information about his continuing research in the southwest Yukon, contact ryan.danby@queensu.ca.

This column is co-ordinated by the Northern Research Institute at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at www.taiga.net/yourYukon.