Most people think it is the extreme cold which keeps the black and white spruce you find along the tree line at bay. However, they are able to withstand temperatures down to minus 90 degrees F (-68 C). Actually, wind and permafrost limit tree growth. Notice how the trees on the picture to the left like one-sided Christmas trees pruned to stand in a tight corner! It is the harsh, bitter and plentiful wind with its sharp crystals of ice, that cuts and kills trees. If you look at a map of the northern hemisphere, you will notice that the tree line is not a straight line around the globe. It fluctuates in large depending the force of COLD winds.
Permafrost is the name for ground which is permanently frozen. Through out the arctic tundra, it is usually within 3 ft (1 m) of the top layer of soil and goes up to 3,250 feet (1,000 m) deep. The farther north you are, the closer to the surface the permafrost creeps. As a result, the trees are forced to anchor themselves with roots so shallow that eventually they are too weak to prevent the tree from tipping over!
The most important factor though is the strength of the sun Ė The very short summer. A tree needs a certain amount of warm summer days of at least 50 F (10 C) to survive or actually to be able to grow. If temperatures do not rise this high, not even the hardiest of trees can complete their annual growth cycle and will die.
That explains why the arctic tree line is a broad boundary sometimes many miles wide unlike the sharp alpine tree line you will find on most mountain sides.
Especially when you travel on a dogsled -- you are going at just the right pace to notice how the trees gradually get fewer and smaller and smaller. Some trees are only knee high and maybe 80 years old, but they have not had much time each year to grow! Finally, the trees surrender and disappear.