Anyone who drove north of Whitehorse during the summer of 1999 probably noticed the large numbers of trees with reddish--brown needles. The lodgepole pine trees were not dead yet, but they were showing the signs of unusually dry conditions.

Ed van Randen, the Forest Health Forester with the Department of Indian Affairs and Northern Development, first noticed the problem in June. When he took a closer look, he found that climate -- not bugs or disease -- was making the trees sick, but he says that it's difficult to put a label on the exact cause.

"One of the hardest things to diagnose is abiotic damage, and what is driving it," he says. Van Randen explains that changes in climate can cause three types of damage -- winter damage, drought damage and red belt -- and the effects of all three can look very similar.

In subarctic climates, low winter temperatures are enough to cause winter damage to trees. Winters when the weather fluctuates dramatically from very cold to above average temperatures can make matters even worse for trees.

Trees exposed to sun or wind can lose moisture rapidly from their needles. If their roots are still frozen, they are unable to take in new moisture from the soil to replace what they have lost, so the dry needles die and turn red.

Van Randen says that when the needles on many trees quickly turn red all at once, the disease called red belt is the first suspect. Red belt was first identified in Alberta on the eastern side of the Rockies, an area where chinook winds and winter temperature inversions are common.

Red belt gets its name from the fact that the affected trees are usually found at the same elevation, and thus form a band, or belt, of reddish-brown trees running across a hillside or slope. Usually all of the buds on a tree are not killed by red belt, so the tree recovers after a few years.

North of town, the affected trees grow at different elevations, so van Randen does not think that red belt is the prime suspect. He thinks the trees are probably suffering from drought damage.

Drought damage occurs after several years of lower than average rainfall, and usually shows up first in trees on drier sites such as sandy soils or southern exposures -- exactly where lodgepole pine are commonly found. With drought damage, the needles at the extreme ends of the tree usually die first. Needles usually die starting at the top of the tree and spreading down, and at the tips of the branches and spreading inward.

"The trees can lose every needle, but they will retain the bud at any cost," he says. "If it's a severe drought they might not be able to retain the buds, but normally these events are not severe enough to remove all of the trees."

Van Randen says that all of these problems are caused by a lack of moisture, and lodgepole pine are more susceptible than spruce to unusually dry conditions. Lodgepole pine do not have extensive root systems and they often grow close together in dense stands so they compete for water. They tend to grow on sandy soils that do not retain moisture well.

Young trees are more likely to die off than old trees as their root systems are not as developed. When trees are stressed in this way by climatic conditions, they are also more vulnerable to infestations by insects. Van Randen says one piece of good news is that the spruce beetles killing trees further west near Haines Junction will not spread into these weakened trees as they only attack spruce trees.

In addition to the obvious problems with the trees north of town, van Randen says that quite a few trees around town are showing classic winter damage. While drought will affect the entire tree, winter damage usually shows up on the side of the tree most exposed to the drying effects of wind and sun.

If homeowners notice this sort of damage on trees on their property, "all you can do is water and put in fertilizer stakes," he says. "It is natural for trees to have a life span just like we do. For young trees watering and fertilizing may help, but it may not for older ones that may be at the limit of their reserves."

Van Randen says that it is too early to say whether the trees north of town will die or eventually bounce back. "I've done a few surveys and it did not look like the buds were doing very well," he says.

And with the effects of global warming already being recognized, is this an ominous sign of future problems for northern forests? Van Randen says that he is an optimist. "Forests can adapt to changes in the climate. The question is whether the speed of warming will outdistance the ability of tree species to respond genetically. You rarely see 100 percent of the trees killed off by these conditions."

For more information on Yukon forests, contact Forest Resources at the Department of Indian Affairs and Northern Development at 667-3350.