I’ve noticed this phenomenon lately and I’m at a loss to explain it completely. We’ve had a good deal of snow this month. In the last two weeks, we’ve had none, and the air temp has been around freezing, lately, a little above. As the snow melts away, I’ve seen many trees with rings - about the diameter of their crowns - at their bases, where the snow is gone. Bare ground. My first hypothesis was simply that these trees, which were evergreens, had blocked the snow in the first place, so little accumulated there and as it started to sublime and melt, the ring would form. But lately, I’ve seen many trees, some decidouous saplings, and some not much more than 8 foot sticks, with the same types of rings around their bases, and that can’t have been from blocking the snowfall, because there was only a stick there. I believe that there are some chemicals that keep the water in the root cells from freezing. Could they be providing warmth the ground? Could there be enough organic decay to raise the temp enough to melt the snow? Any WAG’s dopers?
The dryads come out and sweep their front porches after it snows. 
Plants, being alive, have metabolisms that generate waste heat. This is a lot less waste heat than what is generated by you or I, but it exists nonetheless. If the temperature is right at freezing, the “body heat” of a tree may be sufficient to raise the temperature right at its base a degree or two above freezing.
There are even some plant species that throw more heat than average - usually mountain or tundra species.
No facts, but I can speculate.
The amount of organic material within the drip=zone of trees tends to be around 8X higher than in the surrounding area. There’s a lot of variation within that figure but you can usually expect it to be significantly higher. Organic material could be contributing to this effect in two ways.
Firstly as the ground begins to thaw the soil microbes start to become active and begin in digesting that organic material. That decay process generates heat which in turn warms the soil further still and makes decay occur even faster and so forth. The trees themselves will be literally feeding this process as they break dormancy. Trees have a relationship with some soil microbes and feed them sugars, and the digestion of those sugars will in turn generate heat.
Secondly organic material tends to make soil more insulated. With the less heat being transferred from the soil to the snow the air temperature will tend to melt it faster. It’s equivalent to leaving an ice cube in the open on a frozen block of wood or a frozen block of metal. The ice on the wood will tend to melt faster because there’s less heat transfer form the melting ice into the wood than there is into the metal.