While deciduous trees such as the oak and maple lose their leaves every autumn, evergreen trees like the pine tree remain green throughout the year. Unlike deciduous trees, pine trees do not hibernate in the winter and retain their needles.
Pine trees do not lose their needles because the tree produces a substance similar to antifreeze in its sap that allows them to keep their needles. This antifreeze protects the cells in the needles from getting frost damage.
Some of the distinctive smell of the pine tree comes from this substance that prevents freezing. It also helps the needles on the trees to continue the photosynthesis process that has ceased in the deciduous trees around them.
Pine tree needles do drop, but as a part of the tree's natural cycle. Each type of pine is different, but pine needles live on either a two or five year cycle in which they will change color and drop off at the end.