Christmas trees are of many different species, but all have certain things in common. Unless they are the artificial kind, made of aluminum or plastic, Christmas trees are conifers, that is, cone-bearing trees. Also, because tradition demands a certain shape for Christmas trees, only certain species of conifers are commonly used. Fir and spruce are the classic Christmas-tree form: branches growing in tiers along a vertical trunk, the lowest branches longest, tapering gradually to the topmost branches which are shortest. In nature, this has a function: the topmost branches do not shade the lower branches, so the whole tree gets as much sunlight as possible.
All the popular Christmas tree species also have needle-shaped leaves. This, too, has a biological function, since most species of fir and spruce naturally grow in cold climates. When the ground is frozen in winter, trees cannot extract water from the soil; so those which keep their leaves in winter need to minimize water loss. The narrow needle shape and waxy coating of fir and spruce leaves releases far less water than, say, a broad maple leaf. This is also why needle-leaved evergreens become more and more predominant the further north you go, or the further up a mountainside. The longer the snowy season, the less time there is for a broadleaved tree to take up sunlight. By having needle leaves, fir and spruce can keep their leaves through the snowy winter, and thus continue to take up sunlight year-round, when the broadleaf trees must go dormant.
This is a general pattern. On the other hand, one of the most important Christmas tree producing regions is the Pacific Northwest, between the Cascade Mountains and the Pacific Ocean. This is a comparatively mild climate, where winters are more usually rainy and humid, and frozen ground rare. Nevertheless, here, too, the broadleaf trees lose their leaves in winter, and so the needle leaved conifers can continue taking up sunlight through the winter, storing up energy for spring growth, while the broadleaf trees must use such energy as they have left over from the previous fall. This may be why the needle leaved conifers grow to such towering heights in the Northwest, while the broadleaf trees are only of average size.
Shelton, Washington bills itself as “Christmastown, USA,” because its Christmas tree farms ship trees all over the nation. Most of these Christmas trees are Douglas-fir (Pseudotsuga menziesii), which is not a true fir, but a representative of a group of trees found only in the northern half of the Pacific Rim, and in some ways intermediate between fir and hemlock. A few Christmas tree farms around Shelton also produce noble fir (Abies nobilis), a true fir with the characteristic upturned needles. In either case, the trees are planted in even-aged stands in rows, and are sheared to cause them to grow fuller. These farms most nearly resemble the shrub-sapling stage of forest regeneration, and are home to those species of wildlife which prefer open areas, such as flycatchers and swallows, and those which prefer the brushy interface between forest and open land, such as black-tailed deer. The main difference between a Christmas tree farm, and a very young timber farm, is that the timber farm will be allowed to grow into woodland and then forest; while the Christmas tree farm will be kept in this youngest stage by harvesting the trees while they are still small enough for indoor decoration.
Once a Christmas tree is cut, it begins slowly to die. This process can be considerably slowed by keeping the base of the tree in water. A cut Christmas tree is for practical purposes no different from a cut flower in a bouquet; the base of the stem will be callused over, and in order for it to take up water properly, this callused base should be cut off just before setting it in the water. Cutting at an angle is preferable, as this opens a larger area of the vascular pipes which take up the water.
On a cut Christmas tree, there is of course no harm in trimming the topmost upright point, called the leader. But on a living Christmas tree, whether a large one with roots bundled in burlap, or a small one in a pot, the leader is the main growing point and it should not be cut. Cutting the leader may not kill the tree outright, but it will disfigure future growth. Either the tree will have no top, and the side branches will grow outward while the tree stays the same height, or, in some cases, one side branch will manage to turn upwards and become a new leader, but with a crook in the trunk at that point. In the Northwestern forests, one can often see tall Douglas-firs with a conspicuous crook in the otherwise perfectly straight vertical trunk. This marks a point where, far in the tree’s past, its leader was broken off.
The most popular Christmas tree species – fir, Douglas-fir, spruce, and Scotch pine – cannot survive indoors for an extended time. The indoor environment tends to be very dry and warm compared to the conditions of their natural habitats, especially since they are sensitive to the seasons, and accustomed to winter conditions during Christmastime. So if you have chosen a live Christmas tree of one of these species, it is best to keep it outdoors until just a few days before Christmas, and return it to the outdoors soon afterward.
An exception to this is the so-called Norfolk Island pine. This is not a true pine, but a representative of an unusual group of trees found only in the southern half of the Pacific Rim. Norfolk Island is in Polynesia, and the Norfolk Island pine cannot survive in a frosty climate. It is best kept indoors year-round as a houseplant. Like the other Christmas tree species, however, Norfolk Island pine also has a single leader which should not be cut.
With a little understanding of Christmas tree biology, the natural Christmas tree, whether cut or live, can be an enjoyable part of the holiday season.
