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by Kevin Strauss
As the snow piles up on our driveways and sidewalks, it also piles up on our forest trees. While we can just shovel off our driveways, trees need other adaptations to survive the cold and snow.
Two of the most prominent evergreen trees in our area are the white spruce and the balsam fir, and they are built for life in the northland. Northern Minnesota is actually near the southern limit for these two trees which grow far into northern Canada. Living in the northland is no easy task, so these trees have some special adaptations (or “tools”) to help them survive the winter cold and snow. The most obvious adaptation that these trees have is their shape. The pyramidal “Christmas tree” shape of these trees help them shed snow, so it doesn’t build up on their branches and cause them to break under the weight. What’s more, balsam firs actually benefit from snow piling up on their lower branches. If snow bends fir branches to the ground, those branches could sprout roots and begin to grow a whole new (cloned) fir tree.
While these trees remain green all winter, they aren’t actively growing all winter. If spruces and firs were to open the pores on their needles to take in carbon dioxide for photosynthesis, they would lose so much water vapor that they would dry out and die. With all of the water in the soil frozen, these trees are living in the equivalent of the Sahara Desert during the winter here. So instead, these trees keep their needle pores closed and go dormant during the coldest part of the winter. The waterproof waxy coating on their needles also keeps them from losing water during the winter.
Spruces also have another defense against the cold: intercellular freezing. When the temperatures drop below -40 degrees F, many other trees have ice crystals forming in their cells. When it freezes, water expands and plant cells, like water pipes, expand and rupture when they freeze. While broken water pipes can flood your basement, broken cells can kill a tree.
Spruces avoid this problem by releasing some liquid into the spaces between their cells. There the liquid freezes without harming the tree’s cells. This adaptation allows a white spruce to survive temperatures as low as -60 degrees F.
The presence of these trees with their thick foliage and lower branches are important for northland birds and mammals as well. When the cold winds blow, chickadees and nuthatches seek shelter in the branches of a spruce. When the snows pile up, snowshoe hare take shelter under the tent-like canopy of the balsam fir.
So the next time you are out for a walk, keep an eye out for these amazing sub-zero survivors and shelters of the woods.
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