Snow clearly melts more quickly around tree trunks, but why? Photo: www.faculty.luther.edu
Question: There’s a sugar maple forest behind my house. I’ve noticed that it’s the first place where the snow completely melts away each spring. I have trees elsewhere in the yard and the same thing happens: the snow melts more quickly around the trunks than in the open spaces of the yard. Could it be that trees give off heat when they wake up in the spring?
Answer: That would certainly seem logical, but the phenomenon has undergone serious scientific studies and no, there is no indication that tree trunks produce their own heat.
In the case of trees growing isolated from others, the effect is created by the sun’s rays heating the darker-colored bark faster than the white snow all around. (White snow reflects light and thus absorbs little heat.) Thus, on sunny days, the trunk is warmer than its immediate environment (snow), often enough to melt it.
So much for individual trunks, but why does snow often melt faster in a forest than in the open? That’s because there is usually less snow to start with!
Not all the snow that lands on branches ends up on the ground. Photo: forums.odforce.net
Less snow tends to accumulate on the ground in a forest than in an open location. This is because a more or less important part of the snow falling in a forest first settles on the branches. Some of this snow does end up falling to the ground, especially when the branches move in the wind, but another portion, being more exposed to the sun given its elevated position, melts and reaches the ground in the form of water or even evaporates and never reaches the ground at all.
So, there is less snow accumulation in a forest and thus the snow often disappears more quickly in the spring … although any snow drifts formed by snow blown into the forest by the wind will often actually be the last places to lose their snow cover.
This phenomenon is particularly evident in a coniferous forest, whose dense needles often retains more snow than the bare branches of deciduous trees. There, even less snow reaches the ground. Under such circumstances, there is often less than a third of the accumulation compared to open areas nearby. But even in dense deciduous forests, snow accumulation is less than elsewhere.
Flowers that Produce Heat
That’s not to say that other plants don’t give off enough heat to melt the snow.
Skunk cabbage (Symplocarpus foetidus) flowers melt their way out of the snow. Photo: Wikimedia Commons
In the same sugar maple forest, there may be skunk cabbages (Symplocarpus foetidus), an aroid whose large brownish purple flower bud begins to emerge from the ground in late winter while the ground is still deeply covered in snow. It soon begins to produce heat (this is called thermogenesis) and melts the snow all around. This heat production is due to an exothermic chemical reaction (the oxidation of lipids) and can result in the inside of the inflorescence being 15 to 30 °C warmer than its surroundings.
Actually, the skunk cabbage’s heat production is not that unusual. Many aroids (members of the Araceae family) are thermogenic. In their case, the goal is not to melt snow, because most aroids are of tropical origin, but to attract pollinators. The heat the flowers produce helps diffuse their generally quite foul aroma into the surrounding air so as to better attract pollinators.
Beetles pollinating aroid flowers on a spadix… and doing a bit of mating while they’re at it. Photo: naturesdepths.com
Even the skunk cabbage produces its heat not so much to melt the snow (that’s only a happy consequence), but primarily to attract pollinators, diffusing its malodorous aroma into the still frigid air all around. It has even been theorized that its pollinators—flies and small beetles that awake particularly early in the season—also take advantage of the heat of the inflorescence to hang around for a while and warm themselves up before having to face the cold air all around in their search for other flowers. Indeed, some of these insects even mate in the warmth of the skunk cabbage’s blooms!
Winter aconites (Eranthis hyemalis) melting their way through the snow. Photo: www.landmorphology.com
Among the other thermogenic plants (thermogenic species are found in some 10 botanical families) is a small spring-flowering bulb of the Ranunculaceae (buttercup family) which is often offered in garden centers in the fall: the winter aconite (Eranthis hyemalis). It’s believed its flower buds melt the snow so they can get early access to pollinators, although I don’t believe this has been confirmed scientifically.
Richard Hamel’s reply: I’m not sure your explanation really holds water. If it’s the heat of the sun on the bark that melts the snow, why is it that the snow melts all around the trunk, even on the north side that receives no sun at all?
Answer: Yes, it’s true that snow does melt all around the trunk and, believe it or not, scientists have studied that too. (What can I say? They’re just innately curious!) They discovered that, on sunny days, the bark starts to heat up on the sunny side, but then that the heat is slowly transmitted in all directions by conduction, even to the north side, thus melting the snow all around. On cloudy days and at night, though, the bark is about the same temperature as the surrounding snow, so no melting occurs.
If this reasoning still seems suspicious to you, do note that snow melts more quickly around inanimate objects too, not just living trees. A circle of melting snow will form around telephone poles, fence posts and even pots left outdoors all winter.
So, no, trees don’t carry out thermogenesis; it’s the heat of the sun that melts the snow around their trunk.