Great question — answering as the UphillTech team:
Much of it is due to conifers -- especially spruce and fir shedding snow away from the trunk. I routinely see trees with 1/3 of the snow under their branches as in the open. A spruce in winter appears nearly black. Sunlight enters, and bounces around a lot before escaping. Most is absorbed during a bounce. So the crown of the tree is warmer than the surrounding air. There is some warm radiation between the lower branches and the ground. The space under the tree cannot radiate readily on a clear night into space. So the temps at night aren't as cold. My experience is that wells down to bare earth don't start to form until late winter or early spring. I don't think that temps have to be above...
Full answer by Sherwood Botsford: https://outdoors.stackexchange.com/a/24609 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
