Evergreens stay green all year because they have tough and leather-like leaves that are not prone to drying out like the leaves of other trees. Evergreens have these leaves because they are often located in cold climates and have to endure cold winters.
Evergreens, unlike other trees, are able to continue performing the photosynthesis process despite the cold temperatures. They can do this at or below freezing levels and have no trouble getting the energy to their leaves, as long as they have water. Evergreen trees store water and energy throughout the year to ensure they will be able to survive the dry parts of the winter.
Evergreen trees will occasionally go dormant in extreme temperatures, when there is no usable form of water for them to survive. Snow will sometimes help to keep them hydrated throughout the winter, but it is often not enough for them to thrive.
The needles on an evergreen tree are its best defense against the cold temperatures and lack of usable water in the winter. Unlike other leaves, the leaves and needles of an evergreen tree are rolled up into tight tubes, which enable them to conserve water that has been left over from the growing season. They can store water in their leaves for the entirety of the winter and will be able to use it in the event that there is no water available from their environment.