As we know, snow is not a good absorber of visible light as a high percentage of the sunlight that strikes snow is reflected back into space. But snow does absorb almost all of the infrared energy that strikes it. The bark of the trees absorbs the energy from the sun and emits infrared (long wave) radiation which the snow nearby absorbs. During this process this long wave radiation is converted to energy which in turn melts the snow near the tree trunk. This gradually creates the depressions in the snow that surrounds each tree. This time of year, most trees will have this ring of lesser snow amounts around the trunk, and now you know why.