The soil freezes in the North during the cold-weather months. Any water in that soil turns to ice and thus becomes unavailable to plants. Even snow-melt on a warm day may not reach down to the roots. Your evergreen trees (and deciduous ones, too) can succumb to winter damage in this icy "desert."
You can do something to protect them, however. Part of the solution lies in watering evergreen trees in autumn.
I say watering is "part" of the solution because you cannot simply irrigate willy-nilly, whenever you feel like it. I tell you specifically when to water trees in fall in the following FAQ.
Following this tree watering regimen is especially important for the evergreen trees, which, although their growth slows down, do not enjoy quite the same period of dormancy that helps protect deciduous trees. Evergreen trees keep their foliage during the winter months. That foliage is continually giving off moisture throughout the winter -- moisture that is not readily replenished in the virtual desert conditions discussed above. You can promote the health of your evergreen trees by allowing them to enter the winter season well-watered.
Also water evergreen trees during the "January thaw" and other warm periods that pop up unexpectedly in winter. Every little bit helps.