Why do the leaves fall off in the autumn? Part II

Despite all the rain- and snowfall, the winter is a relatively dry season with cold dry air. It actually works to dry laundry outdoors cold, clear winter days. The water freezes in the garment and then evaporates without passing through the liquid state. We have, in other words, a fairly high vapor pressure on the garment surface and the same would apply to the leaves. The tree would have a hard time trying to retain moisture and at the same time not able to take up new liquid (the ground's frozen) with dehydration as a result.