Leaves take carbon dioxide from the atmosphere and use sunlight as an energy source to turn the CO2 and water taken in by the roots into glucose to provide food for the plant; this process is called photosynthesis. A chemical called chlorophyll helps make this process happen and chlorophyll is what gives leaves their green colour.
In winter, the days are too short and there is little sunlight for effective photosynthesis. Leaves are also quite delicate and prone to frost damage, so deciduous trees choose to shed their leaves and remain dormant until spring. In contrast, evergreen trees keep most of their leaves during the winter. They have special leaves, resistant to cold and moisture loss. Some, like pine and fir trees, have long thin needles. Others, like holly, have broad leaves with tough, waxy surfaces.
Evergreens may continue to photosynthesise during the winter as long as they get enough water, but the reactions occur more slowly at colder temperatures. But even evergreen leaves are prone to damage, from weather or insects, and will need replacing over time, but the tree does this gradually in intervals so that the trees always retain enough leaves to continue to function.
Frances Winder, conservation adviser, Woodland Trust