To many people, "forest" and "timber" are interchangeable terms. But to a forester, someone who studies the science of managing forest resources, the two words have very different meanings. A forester looks at any large area of land covered with trees and calls it a forest. He or she might even get more specific and say that the land needs to be at least 1 acre (0.4 hectares) in size and contain at least 10 percent tree cover. By this definition, forestland accounts for about 750 million acres (304 million hectares) in the United States and about 766 million acres (310 million hectares) in Canada [sources: Alvarez, Canadian Forest Service].
Scientists find it convenient to classify forests by the type of trees they contain, which is often related to their geographic location. For example, tropical rainforests are those that grow near the equator, in warm, wet climates, and that contain predominantly broadleaf evergreens (see How Rainforests Work for more information). Boreal forests, on the other hand, are filled with needleleaf evergreens -- spruce, fir and pine -- that grow well in the cold winters of northern latitudes. As the map shows, there are five major forestland regions in the United States.
But for the sake of simplicity, it's often convenient to consider just two regions: the eastern hardwood region (east of the Mississippi River) and the western softwood region (west of the Mississippi River). Hardwoods include oaks, gums, maples, hickories and walnuts. Softwoods include pines, cedars, spruces, hemlocks, true firs, Douglas firs and redwoods.
More Than Wood
Forests do much more than provide wood. One important function of forestland is carbon sequestration, the trapping of carbon so that the buildup of carbon dioxide (the principal greenhouse gas) in the atmosphere is reduced. They also contribute to biodiversity by providing habitats for a variety of birds, mammals, reptiles, amphibians and insects. Finally, forests play an important role in the hydrologic cycle, soaking up large amounts of rainfall and filtering water as it passes through the soil to become groundwater.
Not all of the trees in forests are available and suitable for exploitation and use as commercial products. In fact, this is the primary difference between forestland and timberland. Timberland is a forest that is capable of growing 20 cubic feet (0.6 cubic meters) of commercial wood per acre (0.4 hectares) per year [source: Alvarez]. According to some estimates, approximately two-thirds of the nation's forestland, about 502.5 million acres (203 million hectares), can be classified as timberland [source: USDA Forest Service].
Another defining quality of timberland is its sustainability. Commercial timberlands can be used repeatedly as long as the net annual gain -- the amount of timber grown each year minus the amount of timber harvested -- remains positive. In 2006, the net annual gain of U.S. timberland was 9.6 billion cubic feet (271.8 million cubic meters), more than four times higher than the net annual gain in 1953 [source: Alvarez]. That's good news for the forest products industry, which looks to use wood as a raw material to make thousands of products, from building materials to paper to wood-based chemicals.
In the next section, we'll look at timber products in more detail.