Ring around the tree trunk

It's March 5th, and here in central Pennsylvania the 2+ feet of snow that fell a few weeks ago has now melted down to about 6 inches of crystallized white stuff.  It was beautiful for a while -- now it's mostly a dirty slippery crusty coating keeping spring at bay.

But it's also curious.  Now that it has been melting in earnest, there are rings around a lot of trees, rings of no snow.

Here's what's happening in our backyard, from wide rings, to just-getting-started rings, to no ring at all.

Trees aren't warm-blooded creatures so why is the snow melting around the trunks?  Where is the heat coming from? 

These trees are all within fifteen feet of each other, so it's not likely that there's something about the soil, or proximity to some heat source that explains the difference.  

Time to ask the internets.

It turns out that there are plants that do produce a lot of heat, though it's apparently not for the purpose of melting snow.  Voodoo lilies and skunk cabbage are the two examples that come up again and again when you google exothermic plants, plants that give off heat.  Both of these plants smell rotten -- to people, at least.  They flower early in the spring, even through the snow, and the heat they produce apparently helps to dissipate the odor and attract (non-human) pollinators.  How this happens is well-known, but not relevant to our question today.  

But I did stumble across a beautiful description of this by a skunk cabbage-loving botanist (see the link for lovely drawings).  

A couple of times I've been lucky enough to see spathes growing up through a thin layer of ice, the ice melted around the spathe in a circular form. This is an indication of skunk cabbage's remarkable capacity to produce heat when flowering. If you catch the right time, you can put your finger into the cavity formed by the spathe and when you touch the flower head, your finger tip warms up noticeably. Biologist Roger Knutson found that skunk cabbage flowers produce warmth over a period of 12-14 days, remaining on average 20° C (36° F) above the outside air temperature, whether during the day or night. During this time they regulate their warmth, as a warm-blooded animal might!

And, plants respire.  That is, they convert sugars into energy to fuel metabolic processes, growth during the growing season, and just staying alive in winter, and some of that energy will be released as heat.  The roots respire during winter, too.  Even if it's not much heat being released, it may be enough to melt snow when the ambient temperature is high enough.  

Plus, here in Pennsylvania we have no deep permafrost, and tree roots go deep, and make a solid connection to the trunk above ground, and sap may be running this time of year, too.  So it is reasonable that as they emerge from the ground, trunks are just a touch warmer than freezing.  One would, however, need an explanation for those trees in the same area and of the same species, that don't have a melt-ring at their base.  With no leaves to create shade, are they nonetheless located to get less sunlight than their ringed neighbors?

Probably a best, or plausibly better answer is that trees have low albedo or reflecting power, which means that they absorb more energy from the sun than does the surrounding ground, and thus are slightly warmer.  We're guessing here, but perhaps younger trees, with thinner bark, release more of that energy than older, thick-skinned trees, which could explain why it seems that the snow melts more readily around smaller, younger trees.  The idea receives considerable credence in the observation that there are melt rings around other posts and poles, not just trees.  The daylight may not only be absorbed by them, but reflected downward to the snow at their base.

Whatever the true answer, clearly spring is on its way.