Growing plants for food is called agriculture; growing trees for human use is silviculture—and the two things have a great deal in common. Wood is a plant crop that must be harvested just like any other, but the difference is how long trees take to grow, often many years or even decades. How wood is harvested depends on whether trees are growing in plantations (where there are hundreds or thousands of the same species, generally of similar age) or in mature forests (where there’s a mixture of different species and trees of widely differing ages).
Planted trees may be grown according to a precise plan and clear-cut (the entire forest is felled) when they reach maturity. A drastic approach like that makes sense if the trees are a fast-growing species planted specifically for use as biomass fuel, for example. Individual trees can also be selectively felled from mixed forests and either dragged away by machine or animal or even (if it makes economic and environmental sense) hauled upward by helicopter, which avoids damaging other nearby trees. Sometimes trees have their bark and small branches removed in the forest before being hauled away to a lumber yard for further processing, though they can also be removed intact, with the entire processing done offsite. It all depends on the value of the tree, the growing conditions, how far away the lumber yard is, and how easy the tree is to transport. Another interesting form of forestry is called coppicing, which involves removing long, thin, low-growing branches from trees such as hazel and willow in a careful and respectful way that does no long-term damage.
A freshly cut tree is a bit like a sponge that comes presoaked in water, so it has to be completely dried out or seasoned before it can be used. Dry wood is less likely to rot and decay, it’s easier to treat with preservatives and paint, and it’s much lighter and easier to transport (typically, half a freshly felled tree’s weight may come from water trapped inside). Dry wood is also much stronger and easier to build with (it won’t shrink so much) and if a tree is destined for burning as firewood (or an energy crop), it will burn more easily and give out more heat if it’s properly dried first. Typically wood is dried either in the open air (which takes anything from a few months to a year) or, if speed is important, in vast heated ovens called kilns (which cuts the drying time to days or weeks). Seasoned wood is still not completely dry: typically its moisture content varies from about 5–20 percent, depending on the drying method and time.
Preserving and other treatment
In theory, wood might last forever if it weren’t attacked by bugs and bacteria; preservatives can greatly extend its life by preventing rot. Different preservatives work in different ways. Paint, for example, works like an outer skin that stops fungi and insects penetrating the wood and eating it away, but sunlight and rain make paint crack and flake away, leaving the wood open to attack underneath. Creosote (another popular wood preservative) is a strong-smelling, oily brown liquid usually made from coal-tar. Unlike paint, it is a fungicide, insecticide, miticide, and sporicide: in other words, it works by stopping fungi, insects, mites, and spores from eating or growing in the wood.
Different kinds of treatment help to protect and preserve wood in other ways. It’s a great irony that wood can be used to build a fine home that will last many decades or burn to the ground in minutes. Wood is so plentiful and burns so well that it has long been one of the world’s favorite fuels. That’s why fire-protection treatment of wooden building products is so important. Typically, wood is treated with fire retardant chemicals that affect the way it burns if it catches fire, reducing the volatile gases that are given off so it burns more slowly and with greater difficulty.
There’s a big difference between a tree and the table it might become, even though both are made from exactly the same wood. That difference comes mainly from skillful cutting and woodworking. How much cutting a tree needs depends on the product that’s being made. Something like a utility pole or a fence post is not much more than a tree stripped of its branches and heavily treated with preservatives; that’s an example of what’s called roundwood. Trees need a bit more work in the sawmill to turn them into lumber, timber, or sawnwood (the three names are often used interchangeably, though they can be used with more specific meanings). Flat pieces of wood can be made from trees by cutting logs in two different directions. If you cut planks with the saw running in lines parallel to the length of the trunk, you get plainsawn (sometimes called flatsawn) wood (with ovals or curves on the biggest flat surface of the wood); if you fell a tree, cut the trunk into quarters, then slice each quarter into parallel planks, you get quartersawn wood (with the grain running along the biggest flat surface in broadly parallel stripes).
See how attractive those patterns look? Not surprisingly, wood that’s destined for furniture and other decorative uses has to be cut much more thoughtfully and carefully with regard to what’s called its figure. This is the way a particular tree is cut to show off the growth patterns it contains in the most attractive way in the final piece of wood. The figure can also depend on which part of a tree is used. Wood cut from near the stump of a tree will sometimes produce a more attractive figure than wood cut from higher up.
Other wood products
Roundwood and sawnwood are what you might call natural wood products, because they involve using cut pieces of tree more or less in raw form. There are many other ways of using trees that involve greater amounts of processing. Some woods are very rare and expensive, while others are cheap and plentiful, so a common technique is to apply an outer layer of expensive and attractive wood to a core of cheaper material. Veneer is a thin decorative layer applied to cheaper wood made by turning a log against a blade, much like peeling an apple. Using veneer means you can get an attractive wooden finish at much lower cost than by using a solid piece of expensive wood. Plywood is made by taking layers of wood (or plies) and gluing them together with an outer coating of veneer. Typically each ply is placed at 90 degrees to the one underneath so the grains alternate. That means a piece of plywood is usually much stronger than a piece of the natural wood from which it’s made. Laminated wood is a weaker kind of plywood in which the grain of each layer runs in the same direction. Particle board (often called chipboard) is made by taking the waste chips, flakes, and sawdust from a mill and forcing it under high pressure, with glue, in a mold so it sticks together to make planks and panels. Low-cost and self-assembly furniture is often made this way. Fiber-board is similar, but made with wood-pulp fibers instead of wood chips and sawdust. Hardboard is a thin sheet of wood made from wood fibers in much the same way. source