How Timber Works

Tree Image Gallery

National Geographic/­Getty Images
Great forests like these once greeted newcomers to North America. See more tree pictures.

­When European settlers arrived in North America in the 1600s, they found vast forests stretching from the Atlantic Ocean to the Mississippi River. N­early 40 percent of the land north of Mexico lay beneath tree cover [source: Maycock]. Canada and Alaska held much of this forestland, but even below the Great Lakes, trees dominated the landscape.

While the primeval forests of the New World presented challenges, they offered a resource -- timber ­-- that would help the colonies grow and prosper. Wood harvested from forestland provided light and heat. It yielded homes and their furnishings. It became the building block of tools and machines. And it formed the framework of wagon and ship, enabling the efficient transportation of goods both across the country and across the sea.

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­Now, even though log cabins are a historical footnote and fossil fuels reign supreme, forests a­nd forest products remain critical natural resources. And not just in North America. Approximately 30 percent of the planet's land area is forestland, with Russia and Brazil holding the largest tracts [source: Food and Agricultural Organization of the United Nations]. Globally, the forest industry employs 13 million people and produces 123.6 million cubic feet (3.5 million cubic meters) of roundwood [source: Food and Agriculture Organization of the United Nations].

Like any global enterprise, the timber industry is complex and has spawned its share of idiosyncratic vocabulary and technology. This article will examine the world of trees -- from top to bottom, you might say -- to understand exactly how timber works. We'll focus on the forests of the United States, but the principles of planting, growing and harvesting trees is consistent regardless of location. So too are the major threats, such as deforestation, invasive species and wildfires.

Let's get started with a basic question: What's the difference between forest and timber?




The five major U.S. forestland regions
The five major U.S. forestland regions

­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.



Humans have relied on forest products for thousands of years. During the Pleistocene Epoch, about 1.5 million years ago, humans began using wood as a fuel to make fire. Today, wood remains the primary fuel for cooking and heating in many developing countries. This isn't the case in industrialized nations, which rely more heavily on fossil fuels.

In the United States, fuelwood accounts for only 7 percent of all timber consumption. Nearly all the rest falls under three categories:

  1. Lumber is responsible for 53 percent of U.S. timber consumption.
  2. Pulp and paper products consume another 32 percent.
  3. Wood-based composites, such as plywood and veneer, use another 7 percent.

[source: Alvarez]

Let's take a closer look at each category.

The word "lumber" refers to wood material that is finished square or rectangular in shape. That means it doesn't include roundwood, which is wood set in either a round or half-round shape, such as pilings, poles and posts. It does, however, encompass boards and planks, such as those typically used in construction -- two-by-fours, two-by-sixes, two-by-eights and so on. In fact, the construction industry uses about 50 percent of all the lumber produced in the United States [source: Bowyear]. The rest goes to make products such as crates, pallets and furniture.

Douglas fir
National Geographic/Getty Images
Douglas firs are in demand not only at Christmastime but constantly. The construction industry prizes their timber.

Furniture makers are especially fond of hardwood lumber. Hardwoods, such as oak, maple, walnut and mahogany, feature grain patterns that most people find pleasing. They're also very durable, making them ideal for cabinets and flooring. The furniture industry is the second-largest market for hardwood timber [source: USDA Forest Service]. Only the pallet industry uses more hardwood, primarily oak, which delivers extra strong wood to support heavy loads.

Very little hardwood goes into construction lumber. Softwoods, which include pine, larch, fir, Douglas fir, cedar, cypress and redwood, are preferred instead because they provide long, straight timber.

Pulp and Paper Products
Like all living things, wood is made up of cells. The cells function like rigid boxes that, collectively, make a tree strong. The walls of each box are made of two chemicals: cellulose and lignin. Cellulose is a tough but fairly flexible molecule. Lignin acts like glue to harden the cellulose and keep it from bending.

Humans discovered long ago that if they could separate the cellulose fibers from the lignin, they could make several useful products. The most important product based on wood fibers is paper. That material we write, draw and doodle on is made from a mass of softened fibers -- pulp -- that are formed into a mat, pressed and dried (we'll talk more about this in the Processing Timber section). Fiberboard is also made of cellulose fibers, but the fibers are bonded with an adhesive to make a dense product that can be used as a tabletop. Hardboard is even stronger. It's derived from pulp that has been pressed together under extremely high pressure.

Wood-based Composites
Composites combine wood with at least one other material. Plywood and particleboard are two common composites. Plywood begins as several thin pieces of wood known as veneers. The veneers are glued and pressed together to make a material that shrinks and swells less than lumber. Particleboard is similar to fiberboard in that wood products are mixed with an adhesive and then pressed together. However, the starting point of particleboard isn't pulp but rather sawdust or wood shavings. Oriented strand board, or OSB, is a type of particleboard that rivals plywood in strength and durability, making it a popular alternative in the construction industry.

­B­y now, it should be clear that no part of a tree is wasted, except for maybe a few stray leaves. But before wood can be transformed into a sheet of paper or the frame of a house, the timber must be harvested. That's the topic of the next section.



There are two basic approaches to harvesting timber. The first, commercial clear-cutting, focuses primarily on economic gain. In this approach, loggers remove all trees in a forested area, usually down to a small diameter, such as 6 inches (15 cm). The effects of clear-cutting can be devastating. When the harvest is complete, a once-beautiful forest can resemble a wasteland. More important, clear-cutting doesn't allow for sustainable forestry, which balances the perpetual growing and harvesting of trees with environmental protection and conservation.

The second approach to harvesting timber is based on silviculture -- the science of growing and harvesting trees for sustained yield. Silviculturists rely on several methods to harvest timber. One important method is shelterwood cutting, which uses partial cuttings over time to remove an entire forest, but gradually. In this way, desirable tree species naturally regenerate and grow into the new forest. Shelterwood cutting involves a series of two to four harvests occurring over a period of 10 to 20 years.

Foresters must analyze timberland carefully to determine which method of silviculture harvesting to use. Indeed, proper forest management involves balancing the economics of the harvest with the biology and ecology of the forest. Once these strategic issues are determined, foresters can survey timberlands to locate and estimate the volumes and grades of standing timber that meet their requirements. This process is called cruising.

Logger cutting down tree
National Geographic/Getty Images
You can see the cut this logger has just made in 120-foot, 800-year-old red cedar. This cut will set the angle of the fall.

­Next comes felling, or cutting down trees using a chain saw. To fell a tree, a worker makes four cuts: a top, bottom, back and felling cut. If you saw the tree after all those cuts but right before it fell -- not that we're suggesting that! -- it would look like it had two triangle-shaped bites taken out of it on either side. The goal is to leave a sufficient hinge of wood between the bottom cut and felling cut. This reduces tree kickback and provides greater control over where the tree will fall. When a team of loggers is working together, the tree faller usually shouts that "Timber!" warning that you're familiar with to alert fellow workers that a tree is about to fall.

Once a tree is on the ground, loggers remove its limbs and cut it into logs, a process known as bucking. Bucking involves making cross-sectional cuts, from the butt of the tree to the top. Next comes skidding, or moving logs from the forest to the landing area. Loggers skid timber by winching several logs to a tractor and dragging them through the forest along designated trails. Preplanned skid trails protect the forest floor by limiting soil compaction, which increases the soil's ability to grow trees in the future.

Risky Business
According to the U.S. Department of Labor, logging is the most dangerous occupation. A full-time logger has a better than 1 in 1,000 chance of getting killed on the job [source: Shaffer]!

­At the landing area, workers grade ­and sort logs by species. Then they load the logs onto trucks, which deliver the timber to its final destination. Pulp mills receive lower-grade logs, while veneer producers receive higher-grade logs. The rest of the timber either goes directly to sawmills or to concentration yards. Concentration yards sell and market logs to sawmills based on the mill's needs. Many sawmills specialize in a certain type of log and rely on concentration yards to collect and organize timber to meet their specifications.

­Finally, the timber is ready for processing. In the next section, we'll examine how a sawmill transforms a single log into the lumber stacked in your local lumberyard or home improvement store.



buttercup xylem and phloem
© Sun Kim
In this microscopic image of a buttercup, the primary xylem appear as large white cells in the tooth-shaped section toward the right-hand side.

Before we dive into the milling process, it might help to take a closer look at the structure of wood. We mentioned earlier that wood is made up of cells. Let's get a little more specific. Water-conducting cells are the key building blocks of a type of tissue known as xylem. These cells are long and hollow, like pipes, making them ideal transport systems to haul water from the roots to the top of the tree. Sugar-conducting cells are located outside the xylem. They form a second type of tissue known as phloem, which functions to carry sugars and other nutrients from the leaves to the rest of the plant.

In between the xylem and phloem lies cambium, a thin layer of stem cells. Cambium generates new xylem cells to the inside, new phloem cells to the outside. As new xylem and phloem cells are added, the tree grows thicker. That means the xylem closest to the center of a trunk is the oldest part of the tree. This is the heartwood, or the part of a tree that makes it so strong. The newer xylem, which lies just inside the cambium and still functions to carry water, is called sapwood. Outside the cambium is the bark. Bark contains new phloem cells and, on the very periphery, older phloem cells that become crushed as new tissue is laid down to the inside.

The heartwood and sapwood are the most useful parts of a tree and can be turned into paper, lumber, veneers and plywood. Bark can't be used to make paper or lumber, although it can be used for fuel and mulch. So, the first step in processing timber is separating the bark from the main part of the trunk. Logs are passed through a large, open-ended cylinder known as a debarking drum. Inside the drum, logs spin and rub against each other until all of the bark is removed.

A debarked log may be sent straight to the chipper if it's destined to become paper. The chipper reduces the log into small squares about 2 inches (5 cm) on each side and 0.25 inches (0.6 cm) thick. Then the chips are mixed with strong chemicals and heated in big pressure cookers known as digesters. Digesting separates the cellulose fibers from the lignin. The soft, wet fibers, now known as pulp, are blown from the digesters, washed and bleached to the proper shade of whiteness. From there, the fibers are mixed with water again and formed into large mats of pulp. These mats enter a series of rollers and presses that flatten the pulp into sheets and extract water. Finally, a starch solution is added to both sides before the final drying process.

Higher-grade logs don't go to the chipper, but to the sawmill. One log can become many different boards, planks and beams. The primary saw of a mill, the head rig, breaks down the log into all of its rough-cut pieces. Heartwood, because it's much older, often has more knots and is typically used for heavier planks or rectangular beams. Sapwood contains fewer knots and is ideal for a full range of boards and planks. After the head rig reduces the log, each piece of lumber passes through an edger, which removes irregular edges and defects, and then through a trimmer, which squares off the ends at standard lumber lengths. Finally, the lumber is sorted, stacked and dried in kilns.

­Next we'll l­ook at how forests are managed to ensure there's a renewable supply of timber for years to come.



Workers peeling logs in 1933
Photo courtesy K.D. Swan/USDA Forest Service
Workers peeling logs for building at Olympic National Forest in Washington, August 1933.

­Euro­peans coming to North America in the 17th century discovered a new world covered in forest -- about 1 billion acres (405 million hectares), according to some estimates [source: Alvarez]. But building a nation required land, lumber and fuel. By the end of the Civil War, logging and agricultural operations had cleared 25 to 30 percent of the original forestland [source: Alvarez]. Some worried that the 20th century would bring a "national famine of wood" and called for stringent conservation efforts. These efforts, combined with a decreased need for farmland, began to stem the tide. By 1920, forest acreage stopped declining, and it has remained relatively s­table ever since [source: Doyle].

In the United States, government ownership proved to be an effective way to conserve forests. Governments at all levels -- national, state and regional -- can own forestland, although the federal government holds the largest amount. All of this government-owned land, nearly 323 million acres (131 million hectares), is known as "public" forestland [source: Alvarez].

The agencies responsible for managing this land include the Bureau of Land Management, the U.S. Department of Agriculture (USDA) Forest Service, the Fish and Wildlife Service, and the National Park Service [source: USDA Forest Service]. Management of federal lands is influenced by federal legislation. For example, in 2003, President George W. Bush signed the Healthy Forests Restoration Act (HFRA), which contains a variety of provisions to protect federal forestland that is at risk of wildfire or insect and disease epidemics.

Of course, much of the U.S. forestland lies beyond the reach of federal legislation such as the HFRA. Private interests own 57 percent, nearly 428 million acres (173 million hectares), of U.S. forestland [source: Alvarez]. Private owners include individuals, families, private cooperatives and forest products companies.

Certification of Forests
A certified forest is one that adheres to strict sustainable forest management techniques. The United States maintains three forest certification programs: the Sustainable Forestry Initiative, the Forest Stewardship Council and the American Tree Farm System. You can read more about the Forest Stewardship Council in What is Forest Stewardship Council certification? The American Tree Farm System, the oldest program, certifies the forestry practices of family-owned and other nonindustrial, private landowners.

­Many forest companies participate in a program called the Sustainable Forestry Initiative (SFI), which provides a comprehensive system of principles, guidelines and performance standards that balance the perpetual growing and harvesting of trees with environmental protection and conservation. International Paper, the largest private owner of forestland, is a prime example. To adhere to SFI standards, International Paper regenerates every acre (0.4 hectare) harvested within two years by replanting, or within five years by natural reforestation [source: International Paper].

Other private owners are doing their part to protect forests as well. Many family forest owners are taking advantage of conservation easements. These easements restrict how a piece of property can be used, protecting the land against future real estate development, industrial use and many potential commercial uses. The right to enforce the restriction is given to a charitable organization or a government agency. These easements stay with the land, so even if it's sold, the restrictions remain.

­Through conservation efforts such as these, forests have been able to satisfy the increasing demand for lumber, paper and other wood products. But numerous threats remain. Find out what they are next.



Gypsy moth caterpillars
Karen Kasmauski/Getty Images
A U.S. Department of Agriculture laboratory that is growing thousands of gypsy moth caterpillars. These bugs are ultimately used in an anti-gypsy moth spray.

­Although the standing inventor­y of hardwood and softwood tree species in the United States has increased continually over the last 50 years, foresters remain concerned about deforestation -- the destruction of forests or a severe reduction in their overall health. Most people are familiar with the issue as it relates to tropical rainforests, but it's a serious problem in forests globally. You can read more about the causes and effects of deforestation in How Deforestation Works, which focuses primarily on human impact.

Unfortunately, human activity isn't the only danger to forests. There are three other important causes of tree mortality: insects, diseases and invasive plants.

Several native and nonnative insects have wreaked havoc on U.S. forests (see table, below). One of the most devastating is the gypsy moth, a Eurasian native that arrived in North America in the late 19th century. In its larval form -- a caterpillar -- it consumes the leaves of hardwood trees. It prefers oak but will feed on many different species of trees. Since 1930, the gypsy moth has defoliated more than 80 million acres (32 million hectares) of forests in the East [source: USDA Forest Service]. Several eradication strategies have prevented the insects from spreading to western states.


Tree Species Targeted


Hemlock wooly adelgid

Eastern and Carolina hemlock

Half of hemlock forests across 11 states infested

Western spruce budworm

Western conifers, including Douglas firs, white firs, blue spruces and western larches

The most widely distributed and destructive defoliator of western coniferous forests

Mountain pine beetle

Lodgepole, ponderosa, sugar and western white pines

Caused tree death on more than 3 million acres (1.2 million hectares) in 2005

Southern pine beetle

Loblolly and shortleaf pine

Affected 2.7 million acres (1.1 million hectares) in 2004

Sources: Alvarez, USDA Forest Service

Aside from insects, diseases, too, can take their toll on forests. Sudden Oak Death, or SOD, is a relatively new but serious threat to American forests. The disease, which is caused by the pathogen Phytophthora ramorum, was first reported in California in 1995. Since then, it has killed 1 million oaks, primarily coast live oak, California black oak, shreve oak and tan oak [source: Alvarez]. It also has infected several other types of plants, such as rhododendron. Scientists are trying to decipher the biology of the disease, but they still don't know how it spreads. One concern is that SOD will infect oak forests of the eastern United States.

Lastly, about 1,400 invasive plant species pose significant threats to American forests, according to the USDA Forest Service. Most aren't native to the United S­tates and have no natural predators. As a result, these plants grow uncontrollably, overwhelming and displacing other plants. Mile-a-minute weed is a great example. Introduced from Asia, this climbing vine grows unchecked across the Mid-Atlantic. It prevents natural forest regeneration by crowding out seedlings, which jeopardizes future timber harvests. It also destroys habitats, mostly in the forest shrub layer, in which birds and mammals live.

­Controlling these threats and others, such as wildfire, is vitally important, as is decreasing our consumption of wood and paper. If we don't, we risk losing more biologically diverse forestland and the timber resources we rely on for the hundreds of wood-based products we use every day.



Related HowStuffWorks Articles

More Great Links


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