A lot of people refer to all evergreens as “pine trees,” but as you go further north this is less and less likely to be so. In North America pines (Pinus) dominate the evergreen forests of southeastern United States on the Piedmont and in the coastal lowlands. In the mountains and further north the spruces (Picea) and the firs (Abies) are much more common with the boreal forests (taiga) consisting largely of these two genera, especially in the eastern half of North America. (All three genera are part of the pine family Pinaceae.)
Spruces and firs are readily distinguished from each other when they have cones, as spruce cones (like those of pines) hang below the branches, while fir cones stand erect above the branches like candles. Whereas pine and spruce cone scales open to release their seeds at maturity, fir cones disintegrate to release seeds. Trees may not produce cones for the first 15 or 20 years of their lives, sometimes longer.
In eastern North America there are only two species of fir: balsam fir (A. balsami) and Fraser fir (A. fraseri). The latter is present only in disjunct populations at high altitude in the southern Appalachians. In the same region there are three species of spruces: red (P. rubens), black (P. mariana), and white (P. glauca). The red spruce is the most widespread in the east (ranging furthest south), while the black spruce ranges further north. The white spruce is more widely distributed in the West compared to the other two species.
These taxa are all monoecious: they have both male and female cones on the same trees. The male cones release pollen in the late spring and it is borne on the wind to female cones. After fertilization the cones mature through the summer and are ready to release seed in late September or October.
All these firs and spruces (except the black spruce) are grown for use as Christmas trees and their boughs are also cut for making Christmas wreaths and other holiday decorations.
You can tell the difference between spruces and first by rolling the needles in your hand. Spruce needles are round and fir needles are flattened. They are arranged differently on the branches too. Picea needles are attached to branches in a spiral pattern all the way around, while Abies needles are attached similarly, but the needles themselves twist at the bases to produce two double-ranked rows of needles on either side of a branch. Furthermore, while spruce needles appear attached to small pegs emerging from the branches, the point of attachment for fir needles is flattened or even depressed.
The ecology of these trees is similar: they require shorter, relatively cool summers and cold winters. As a result they often grow together to create a boreal forest. Of the five, P. mariana can tolerate the wettest habitat and is often found in and near bogs, where the soil is saturated and low in free oxygen. P. rubens and A. balsami are the next most tolerant of moisture, growing right down to sea level in Maine and the Canadian Maritime provinces, where their branches can be hung with bearded moss (Usnea spp.), a form of lichen.
As lumber pine, spruce, and fir are often treated indistinguishably as SPF. None of them tolerate moisture or insect infestation well, so they can only be used for indoor purposes. Balsam fir is regarded as the least advisable to use as lumber, and it is most often used to make pulp and as the layering for plywood. The “fir” that is regarded as superior to spruce or pine for decking is Douglas fir (Pseudotsuga), which grows in western forests and is one of the more important lumber-producing trees in North America.
In contrast to fir (Abies), spruce is a valuable timber tree. In addition to being used for framing lumber and pulp wood, sruce is often used for instrument making, particularly the sounding boards of stringed instruments. When it is used for this purpose it must be from very old, slow-growing trees (very tight grained) and it is referred to as “tonewood.”