All last winter, there was a hanging tree in the forest near Wolf Creek. This year there will be another.

It's not a reversion to frontier justice -- just part of an experiment to determine the effects of the forest canopy on snowmelt and runoff.

The tree itself is the thing that hangs. It's suspended from a tower festooned with instruments to record information like temperature, wind speed, and the amount of snow falling.

The cable supporting the tree contains a device to track the tree's changing weight over the winter, as snow comes and goes from its branches.

The men responsible for the hanging tree are Dr. John Pomeroy and research support technician Newell Hedstrom of Environment Canada's National Hydrology Research Centre in Saskatoon. And it's not the first tree they've treated this way.

For several winters they have run a similar experiment at a southern boreal forest site in Saskatchewan's Prince Albert National Park, testing stands of jackpine and black spruce. In the southern Yukon's Wolf Creek Research Basin, they're using white spruce, the dominant tree in the surrounding northern boreal forest.

The point of the experiment is to determine how much of the snow that falls over a winter eventually reaches the forest floor and contributes to spring runoff, and what factors affect the process. By gathering information from one tree in the forest, they hope to increase their understanding of the whole forest.

In a spruce forest, Hedstrom and Pomeroy estimate, up to 70 percent of the winter's snow is intercepted by the forest canopy. About 35 to 40 percent of the annual snowfall never makes it to the ground. It sublimates, or transforms from ice to water vapour, directly from the tree branches.

Hedstrom says they'll know more after a second winter of monitoring the Wolf Creek forest.

"The first year of any project such as the suspended tree is often used to sort out problems with instrument design and sampling technique," he says. "It looks as though last winter was a success and the data from one season is enough to begin analysis, but two seasons would be better."

A variety of factors affect how much snow is trapped in the forest canopy and how long it stays there. The shape and density of the tree branches and needles have an obvious effect. So do wind, which can jostle the snow loose, and temperature, which affects how strongly the snow crystals stick together and how stiff the supporting tree branch is.

In fact, the complexity of the factors affecting the snow load in the canopy is the reason behind the hanging tree experiment. Pomeroy and Hedstrom have used information from the suspended trees in Prince Albert National Park to develop a computer-based model that describes the snow-trapping behaviour of the forest as a whole.

"The Wolf Creek experiment will validate its use and provide more insight on its application in a variety of environments," says Hedstrom. They're also running a similar experiment near Inuvik.

Understanding how the forest canopy affects water quantity and runoff patterns is important because of the changes going on in Canadian forests. Processes such as climate change, reforestation, logging, fire, and vegetation succession could have dramatic impacts on the hydrology of the forest and the land and water downstream. The model derived from the hanging trees could help predict those impacts.

If the ultimate product of the experiment sounds a bit abstract, the process of collecting the information is quite the opposite. In fact, setting up a suspended tree experiment can be quite messy, says Hedstrom.

"It's often very difficult to cut a tree and get it to the ground without some of the branches breaking off," he says. "It's also quite tricky to estimate the weight of a tree while it's standing. Some trees have been very heavy and difficult to handle once they have reached the ground."

"Finding a tree to volunteer is probably the largest problem," he jokes, "but can you blame them?"

For more information about this and other research in the Wolf Creek Research Basin, check the Wolf Creek website at www.taiga.net/wolfcreek.