When other trees shed their leaves and grow barren during the cold winter months, pine trees and other evergreens stay lush and green, making them a potent symbol of the persistence of life. Control over water loss explains why pine trees stay green.
Leaf drop protects deciduous plants from dehydration during the winter months. Water supplies underground may freeze in the winter and become unavailable to roots. At the same time, however, sunny winter days cause plants to lose water through their leaves. Eventually, the plant dehydrates and, lacking water for basic life functions, dies.
Pine trees possess adaptations that help them conserve water without shedding all of their leaves at once. Needles contain fewer pores through which they lose water, and a thick, waxy coating prevents further loss. The large number of evergreen needles lets them capture a lot of sunlight with relatively little risk of water loss.
Because evergreens keep their leaves, they do not need to expend energy each year to regrow a completely new set of leaves. According to the U.S. Department of Energy's Newton program, this means that evergreens can exist in areas where poor soils do not provide them with the resources for constant leaf regeneration.