Trees Prepare for Winter Cold
by Kevin Strauss

It had to happen. After the colorful reds and yellows of autumn leaves in September and October, the deciduous (broadleaf) trees drop their leaves and get down to the business of preparing for winter. Losing their leaves is only one of the most obvious things that aspens, birches and red maples do to get ready for winter.

It might surprise some people that trees aren’t really preparing for winter’s cold, they are actually reacting to the shorter days of fall and winter and the dry conditions of the upcoming season.
With the sun setting around 5:00 this time of year, rather than the 9:00 sunsets of the summer, aspens and birches just can’t make enough sugar to make photosynthesis worthwhile, so they shut down the factory.
What’s more, winters are dry in the northwoods. Now you might be thinking that we get lots of snow up here, but snow isn’t water and trees can’t use snow or ice for their water supply. Instead, many trees seal up their stomata (breathing pores) or drop their leaves and go dormant during the winter. By comparison, at the height of winter in January, the Superior National Forest can have a humidity drier than the 25 percent humidity of the Sahara Desert, so it’s no wonder that trees go dormant.

Evergreens don’t drop their needles and you might think that they keep growing all through the winter, but you’d be wrong. Evergreens like white spruces, balsam fir and white pines need water to live and grow too, so on cold days, they go dormant. The advantage of being an evergreen is that whenever we have a short winter warm-up and some of the snow melts, the evergreens can open their breathing pores and make some sugar on short notice. Evergreens also start growing earlier in the spring than broadleaf trees and can continue growing later into the fall than their broadleaf neighbors.

With the leaves off of the broadleaf trees, this is also a good time to notice the difference between trees with opposite branching and those with alternate branching. Opposite-branching trees like maples, ashes and dogwoods have branches (and leaves in the summer) that emerge across from each other, paired on a branch. Alternate-branching trees like birches, aspens and many other species, have unpaired branches that seem to alternate on the trunk. Knowing if a tree has opposite or alternate branching make it a lot easier to identify trees that have dropped their leaves.

The next time you take a walk outdoors, notice the differences in the leafless trees in your neighborhood.

The Ely Timberjay