How do trees survive winter?

Trees go through a process similar to hibernation called dormancy, and that's what keeps them alive during the winter. Dormancy is like hibernation in that everything within the plant slows down. Metabolism, energy consumption, growth and so on. The first part of dormancy is when trees lose their leaves.

Dormancy is like hibernation in that everything within the plant slows down. Metabolism, energy consumption, growth and so on. The first part of dormancy is when trees lose their leaves. They don't make food in the winter, so they have no use for masses of leaves that would require energy to maintain.

This brief video explains it well:

More at The Guardian.