Instead of flat leaves that dazzle us in the fall with their vivid coloring, some trees--the evergreens--have needles instead. The trees are called "evergreens" because the needles stay green, and the tree retains its needles, year-round. These prickly needles serve an important purpose, enabling the tree to survive.
An evergreen is a tree that does not lose its needles all at one time. The evergreen tree typically drops its needles as they age, growing new ones in their place to maintain coverage year-round, unlike deciduous trees, which drop their leaves and become completely bare in the winter. Some common evergreens are pines, spruces and firs.
The function of evergreen needles is simple: they don't absorb water--their job is to help retain it. Once trees have drawn water up from the soil they need to keep it and be able to use it to meet their daily needs. Trees can lose precious water reserves through the evaporation process that takes place in the leaves or needles. Needles, due to their thinner and more narrower shape, lose less water than flat, wide leaves do, since leaves make an easier target for the sun to draw water from.
Trees get their water from the soil. But 99 percent of the water that is absorbed is eventually lost through the evaporation process that occurs in the trees' leaves or needles. An evergreen tree is better able than other trees to control that water loss because it has needles instead of leaves. The evaporation process is slowed. This is why evergreen trees can survive in some areas and conditions that other trees cannot. They lose less water, so even if water is in short supply in the soil, they will still keep most of what they are able to soak up.
Not all needles on evergreen trees are the same size, color, length or shape. For example, pine trees have clusters of needles grouped together, but spruce and fir trees have individual needles. The color and length of needles vary from one tree to another and are more a result of environmental conditions than serving any specific purpose, since all the needles (regardless of size) still help retain moisture.
If you are one of the millions of people who bring a real tree into your home for the Christmas holiday, you more than likely bring in an evergreen--needles and all. But you might want to forgo the Blue Spruce tree due to its stiff and sharp needles, which can be quite painful when you're handling the tree, according to Ohio State University.