Evergreen is a term applied to plants that do not lose their leaves at the end of the growing season. Plants that do lose their leaves are known as deciduous. It is not the cold that causes trees and shrubs to lose their leaves but the threat of desiccation. Because of the low humidity of the air, a tree would dry out if it retained its unprotected leaves. When the ground freezes, a tree is unable draw more water through its roots.
Deciduous plants shed their leaves annually to conserve water.
The leaves of evergreen trees and shrubs have a thick, often waxy covering that prevents the loss of water. The leaves or needles remain alive and on the plant throughout the winter. Because they retain their leaves year-round, evergreens, including pines, firs, spruces, cedars, hemlocks and hollies, are critical to wildlife for winter cover. The berries, seeds and needles provide important foods for resident birds and the few mammals that venture out in the winter sun. Mammals, such as white-tailed deer, chipmunks and gray squirrels, feast on seeds and needles.