Why Does Snow Melt Around Tree Trunks First?

Jan 13, 2020

Snow and trees are plentiful in the Northwoods this time of year, which made the Masked Biologist select a Curious North question to answer as part of this week’s Wildlife Matters.

Sally Lippert submitted the following Curious North question: Why does the snow in my yard melt first around tree trunks? Do trees give off heat? Thanks for the question, Sally. In a word, yes. Trees definitely give off heat. However, it is not body heat like we generate. We are talking about radiant heat here. In fact, maybe it would be more accurate to say that the snow melts because the trees absorb sunlight. I can surmise that the trees you are talking about likely have brown bark, or at least not bright white bark (like paper birch). The lighter colored an object is, the more sunlight it reflects. Sunlight excites electrons around molecules, forcing them to temporarily shift valences, and when they drop back down they give off heat. Snow is white, at least the color we perceive is white (its actually clear but because it’s a crystal its prismatic and refracts the entire visible spectrum so we see white). Because it is white, it absorbs very little sunlight and therefore very little heat. A black item reflects almost no prismatic color, which is why you perceive it as black. There are actually light waves coming off of it, you just can’t see them, like infrared or ultraviolet.

Many of our Northwoods trees have bark that is some version of brown. Brown is a complex color; thinking back to elementary school, you have primary colors like red, yellow, and blue, and secondary colors like orange, green, and purple. Secondary colors don’t exactly have their own wavelengths, per se. Purple is a combination of red and blue wavelengths coming at you simultaneously. Brown would be a tertiary color, requiring a combination of three wavelengths. The color you perceive is darker because it absorbs many of the visible and invisible spectrum colors. It warms up, at least slightly, on the outside of the bark. Especially on the south and southwest side of the trunk, which gets more sunlight. The sun is lower in the sky, which means the rays have to get through more atmosphere to reach the tree, but it also means that the rays may be more perpendicular to the trunk than in the summer. And without leaves, if it is a deciduous tree like a maple or oak, the absorption is even greater. An evergreen can absorb a lot of heat when the branches are clear of snow, and melts the ground snow beneath it even faster, which is why animals often seek out a large pine or cedar tree to escape a storm or catch a few winks.

If the tree heats up enough, it can bring the living cells of the inner bark out of dormancy and cause problems. The cells can absorb water, refreeze, and burst. If the damage occurs just to the bark, this is often called sunscald or southwest injury.

However, especially in late winter or early spring, this effect can extend into the inner layers of wood; when the wood refreezes, the drier inner wood and wetter bark freeze at different rates and cause frost split. You have probably seen this on maple trees and not given it much thought. It presents as a vertical line with a mound of smoother bark tissue surrounding either side of it. Smoother-barked trees are more prone to this kind of damage because the layers of bark are thinner; consequently, it happens a lot to fruit trees. This is why some people wrap the trunks of their fruit trees. Well, that and because it helps reduce rodent damage under the snow line. I put white plastic tubes around the base of my fruit trees in winter to reduce damage from rabbits and to minimize the amount of heat the trunk bark absorbs to help minimize the risk of sunscald and frost split.

This bark activation process is important in spring. It is what brings the tree back to life, transporting nutrients from the roots to the chutes and leaf buds. It is also why we collect maple sap and make syrup while the snow is still on the ground too. However, if the winter has sunny days and lots of snow, it can really send mixed signals to these trees. Normally, though, the damage that’s caused is not enough to kill the tree. Thanks again for the great question Sally!