Sentence:  First, <WSD>tree wells</WSD> are formed when the canopy of a tree prevents snow from accumulating at the base. <WSD>Tree wells</WSD> are noted by a deep ring of much shallower snow around the trunk, with walls of snow just beyond the protection of the canopy. 
Meaning of the word marked with <WSD> tags:  A hole or depression in the snow around the base of a tree caused by the tree's canopy preventing the accumulation of snow.