It’s all about soil temperature.  Deciduous trees become dormant in winter (along with their evergreen brethren) as the temperatures drop with the leaves.  Below grade however, it is a whole different story.  Roots will function in a soil temperate environment above freezing even if it’s brutally cold outside.  How is this possible?  Snow is a good insulator and years with deep cold and no snow can challenge a tree’s ability to survive – especially urbanized trees absent of the soft duff layer found in the forest (a reason to mulch).