Why do trees lose their leaves in winter but not in the south?

As far as I know the leaves on trees in the south stay on as there is not a need to protect the tree from the harsh winter weathers like in the north. As the seasons change it sends all the trees sap down to the roots to protect it from freezing and again in the spring send it back up to for the warm regrowth known for each spring! In the trees known as Maples this time of year is also known as the running of the sap..when they tap those trees for this syrup.