To melt snow, there must be a net gain of energy by the snow. This energy gain can either melt the snow or sublimate the snow, causing it to go from the frozen water phase directly to a gas in the atmosphere.
All objects gain energy from their surroundings, while at the same time losing energy to their surroundings. In this situation, the main energy-exchange mechanism is radiant heat.
An important source of radiant heat is the sun, or solar radiation. Tree trunks are dark and so absorb much of the sun’s energy that falls on them. Trees also lose radiant energy by emitting radiation outward from their tree trunk and into the environment. This radiant energy is much less energetic than solar energy and is often referred to as terrestrial radiation.
Snow is very reflective of solar energy. It is bright white because it reflects about 90% of the sun’s visible light that falls on it. Snow is also an excellent reflector of UV light from the sun.
Snow is also a very good absorber of infrared energy, which is the type of thermal radiation emitted by objects with temperatures observed on Earth.
In an open area, snow does not gain much energy from the sun because of its high reflection, nor does it gain much infrared energy from the atmosphere because the atmosphere emits relatively small amounts of infrared energy.
On the other hand, snow around the base of trees absorbs much of the energy emitted by the tree trunk near the ground. Thus, the energy gains of snow around a tree trunk are greater than in the area away from the tree.
The increased energy gain is often just enough to increase sublimation of the snow around the tree. This leaves holes or depressed layers of snow around the tree trunk.