From my readings, one foot of snow is what many people report as being the amount necessary to get them moving downward. Obviously, as noted above, there seems to be some variation in different population.
They love Western Red Cedar and hit it hard all winter and spring. As others have stated in the past, they have diets similar to goats. They will eat just about anything if they feel inclined. When things get really tough, they even eat the bark off of trees.
Extended periods of severely cold weather will kill a lot of BT deer, which thankfully is a rarity in the BT range. Long periods of wet rainy mid-30 degree weather may kill a substantial percentage of the fawn population over the course of the winter. We don't see them too often because the carnivores/omnivores clean them up pretty quickly.