Besides their many other classifications, trees are grouped according to whether or not they shed their leaves each fall. Those with leaves that fall off yearly are called deciduous while those with leaves that stay on are called evergreen. As daylight diminishes in fall, the leaves of deciduous trees are no longer needed and are shed, allowing the trees to survive winter. Leaves, or needles, remain on evergreens, which are better equipped than deciduous trees to withstand cold.
The leaves of deciduous trees are generally wide, exhibit parts of the trees' vascular system in the form of veins and grow on individual stems. Examples include the leaves of the sugar maple tree (Acer saccharum), which is hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8. Conversely, the needles of narrow-leaf evergreens are slender, coated in a thick, waxy, protective substance and grow in clusters or dense rows along stems. The leaves of broadleaf evergreens resemble those of many deciduous trees, but their thick, leathery texture and sticky sap make them capable of withstanding severe winter temperatures.
Evergreens also differ from deciduous trees in how they use sunlight to manufacture food, a process called photosynthesis. Tree leaves require abundant sunshine to trigger a chemical reaction that changes water molecules and carbon dioxide into starches. The process continues longer each year in evergreens than deciduous trees because leaves remain on evergreens all year. The extended period of photosynthesis causes evergreen trees to extract more water and nutrients from soil and also to release them more slowly into the air for longer periods of time than deciduous trees.
Examples of broadleaf evergreens include the southern magnolia (Magnolia grandiflora), which is hardy in USDA zones 7 through 9. It can grow 80 feet tall, and the top of its large leaves are dark green while the underside is a paler green or grayish-brown. Another broadleaf evergreen is the black mangrove tree (Avicennia germinans), hardy in USDA zones 9 through 11 and growing 30 to 50 feet tall with a 20- to 30-foot spread. The American holly (Ilex opaca) is a broadleaf evergreen that is hardy in USDA zones 5 through 9 and grows to a maximum height of 50 feet. The American holly is distinguished by its bright-red berries that appear in fall and last through winter.
Examples of narrow-leaf evergreens include the Scots pine (Pinus sylvestris), which is hardy in USDA zones 3 through 7. Used extensively as a Christmas tree, Scots pine grows in a wide variety of climates and is valuable as an individual specimen tree and grown in rows as windbreaks. The white spruce (Picea glauca) is a narrow-leaf evergreen that grows to 60 feet tall with a 15-foot spread, and its cones hang from dense branches bearing whitish-green needles; it is hardy in USDA zones 2 through 6. Canadian hemlock (Tsuga canadensis), another narrow-leaf evergreen, grows best in cool northern areas; it is hardy in USDA zones 3 through 7.