If you plant to keep some tropicals and or decidous than a greenhouse is not needed. Most evergreen trees will do fine in places where it snows. Problem is that you dont want all that snow to pile in branch pads as it can break the branches. Even a hail storm can cause some damage and set the tree back as it tries to ramify (sp?). Best thing to do in the snow is to put a bit of shelter but a bit of frost is not a bad thing as long as you do not let the roots freeze dry. In saying this White pine love a bit of frost..
Joe