Winter is Coming…to Minnesota Trees

by Jake Grossman

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Fall colors near Split Rock Lighthouse State Park, MN October 8, 2016
Even though southern Minnesota is hitting peak leaf color right now, University of Minnesota tree lovers were talking about the transition from summer to fall at the Midtown Farmer’s Market on October 1st. Just like us, trees need to prepare for the winter. Unlike the tomato plants in your garden, which die off and can be replanted from seed in the spring, or prairie grasses, which go dormant at or beneath the surface of the soil, trees have have to survive winter with their entire bodies in the freezing air. Some of them even keep their leaves all winter – and supporting all of that living tissue in a cold environment can be a big challenge!

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Jake Grossman, a PhD candidate at the University of Minnesota, explains how trees prepare for and tolerate cold Minnesota winters.

One of the most obvious ways that many trees prepare for winter is by shedding their leaves. During the spring and summer, green leaves, filled with nutrient-rich chlorophyll, make food through photosynthesis. As the days get shorter and colder, trees will suck up as much of the chlorophyll in their leaves as they can. This is a way of recycling nutrients, which can be used next year. The leaves are left without much green pigment, but with plenty of the anthocyanins and carotenoids that produce red, orange, and yellow colors. This is why leaves change color in the fall. The reds and yellows we see in October aren’t new pigments – they were there all along, hiding behind the green. After trees have taken as much out of their leaves, they let them “senesce,” or die off in a controlled way. This is different, and less damaging for the tree, than the freezing damage that you will observe if you leave your houseplants outside during a hard frost.

Minnesota’s trees are preparing for winter on the inside, too! To stay alive, they need to have a constant flow of water running from their roots to their crowns through the long, thin passages called xylem. These structures are like the veins and arteries of the tree, and they function like drinking straws: a bubble or interruption in flow can cause the whole xylem vessel to stop working. If water in the xylem freezes in the winter, it can create air bubbles (air is pushed out of liquid water when it freezes), which disrupt xylem flow. This is called “cavitation,” and must be avoided. Plants have many mechanisms to do so, including, in some species, the creation of natural antifreeze!

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Young scientists examine leaves and leaf litter under the microscope.

At Market Science, we want you to pay attention to how trees prepare for winter. So we asked visitors to become scientists and use a tool that many of us employ in our own research: “litterbags.” Fallen leaves are often called “litter,” so a litterbag is just a mesh bag filled with senesced leaves. Scientists interested in how quickly leaves decompose can put leaves of a known weight in a litterbag, leave them out in the world, and weigh the leaves after some time has passed. The rate of their decomposition can tell us about the leaves’ chemical composition and the environment where they were decomposing. Our visitors got to do this, too, by making litterbags filled with leaves from oak, pine, eastern red cedar, box elder, and basswood! These newly trained litter scientists will place their bags around their homes and yards and check them periodically to see how decomposition proceeds for their leaves of choice.

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Litterbag supplies: old leaves and empty mesh bags.

Thanks to everyone who came out to learn about the ways that MN trees prepare for winter. Two pieces of news for those who visited us:treefoliage

1. In case you were wondering, maples were the runaway favorite in our poll of favorite fall foliage, beating out birches, aspens, and oaks.

2. If you brought home a litterbag, keep an eye on it as we move through winter and spring. If you take a picture of it decomposing next year and send it to Market Science, you’ll be entered in a drawing to win a tote bag. Get in touch here.

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