The question then is how do trees survive below zero, when the water within them must freeze solid?

They go dormant in the winter like a bear hibernates. There are still nutrients in the soil. The roots have some protection, particularly the deeper ones, and even the shallow ones may have sufficient snow cover to offer some insulation and warmth.

To survive, a tree must begin to prepare for this journey when the days shorten, late in the summer season. There are many changes in the leaves, stems and roots that begin to take place. We are all looking at the breathtaking colours of the leaves, while the trees undergo miraculous transformations. The first is actually a change in the cellular membranes that take place as the tree prepares for the cold weather. The membranes become more pliable, actually allowing water to flow out of the cell into spaces between the cells. This water places pressure against the outer walls of each cell, and as the cells have less water inside them they actually shrink. The tree also stores additional sugars within its sap by converting starch to sugar, essentially adding antifreeze to its own body. This makes the water inside the tree freeze at a lower temperature or on milder days not at all. This also brings the freezing to a later time of year, and a shorter number of days.1

The bark provides an insulation layer to the cells. Bark provides fairly watertight barriers for most tree tissues. The bark also helps protect the tissues from abrasion and physical damage.So a combination of elements protects our trees through our cold winters.

1.http://www.simcoemuskokatreeservices.ca/blog/2013/01/how-do-trees-survive-in-our-cold-canadian-winters/