Dormancy is like hibernation in that everything within the plant slows down. This includes energy consumption and growth. After the leaves have fallen off, a chemical produced by the tree (ABA) signals the tree to suspend it growth, which prevents cells from dividing. This stalling of growth and saving of energy reserves can be very beneficial to a tree in the winter months. This is very similar to the concept of animal hibernation.
Roots grow and react to weather and climate differently than the above ground portions of a tree. Root activity is periodic, with the period of maximum growth occurring in late spring and early summer. Distinct bursts of growth also occur in early fall. Root growth thru the rest of the growing season is at a lower rate. When the soil temperature is below freezing roots go into a dormancy period similar to the above ground parts. However, tree roots seem to always be in a permanent state of readiness and are able to start to grow again if soil temperatures rise above freezing.
Not all species of tree can be successful in a Wisconsin winter. Our tree species have distinctive evolutionary traits that other milder climate trees are lacking. Some of the adaptions include cold protecting leaf bud scales, waxy needles, thicker more insulated bark along with a complex interaction with the timing of bud and leaf opening in the spring.
It is possible to force a tree to evade dormancy if you keep it inside and with a stable temperature and light pattern. However, this is usually bad for the tree. It's natural for trees to go through dormancy cycles, and the lifespan of the plant is dramatically decreased if the tree is not allowed to go dormant for a few months. Trees have winter dormancy for a reason, and it's best to just let them run their course as nature intended.