Pine trees are evergreens. They keep their thin leaves, usually call needles, year-round. Like smaller plants, pine trees are self-sustaining organisms and use photosynthesis to create their own food.
Photosynthesis is the process in which sunlight, water and carbon dioxide are taken in by the pine tree and converted into sugars and starches to be used as food energy. Oxygen, a waste product of the process, is released into the atmosphere.
Chlorophyll is the chemical that makes photosynthesis possible. It is found in the pine needles and is also the reason the needles look green.
Pine trees use the energy they produce in a specific order. First they make sure all parts of the tree receive nutrients, then the tree produces new hair-like roots, leaves and reproductive structures. Branches and the main root system are extended; wood is added to the trunk, branches and the root system; and, finally, some pines make an antipest chemical for protection.
Pine trees are able to photosynthesis in winter because the cells in the needles are protected by self-made antifreeze that is carried in the tree sap. This is what gives pine trees their distinctive smell. Conifers do lose some needles during the winter, so photosynthesis is not as efficient as during the summer.
As pine trees age, the rate of photosynthesis decreases. Older trees tend to have less foliage, so there is not as much available chlorophyll to help with the process. Older trees also have more carbon energy stored in their tissues and don't need to photosynthesize as rapidly as younger trees.