L11. Forest to Lumber
In the last lesson, we covered the status of our forest resources and some of the trends we’re seeing in our forests today. We know that wood is by far the most common bioproduct that we use - for lumber, for paper, for energy, and more. Over the course of the next several lessons, we’re going to talk about how trees are turned into products, and cover some important properties of wood.
We have all stood on (or have at least seen) a wooden deck. But how often do you stop to think about the properties of wood that made that deck possible? Have you ever wondered how that deck came to be? The journey that a tree takes to become lumber, and then a deck, is a fascinating one. And as with everything we've discussed in this course, there can be no resource consumption without environmental impacts. Each step of the lumber production process requires energy inputs, from forest thinnings to transportation to packaging. We will go into more depth with LCA of wood at the end of the unit, but for now, take a look at the following graphic to get you thinking about these various steps.
Life cycle stages for softwood lumber production (Puettmann 2012 Links to an external site.)
Harvesting
It all starts in the forest where the trees that will ultimately become our decking are logged. There are many different timber harvest methods, but they can be grouped into three main categories (definitions provided by the North Carolina Forestry Association Links to an external site.).
- Selection harvest - essentially a small-scale clearcut (1 to 5 acres in size) where groups of trees in a given area are harvested over many years so that the entire stand has been cut within 40 to 50 years. Imagine a checkerboard pattern where each year a different square is harvested (clear cut). Over a 40-50 year time period all squares would be harvested. This benefits wildlife by creating a patchwork of stands of varying ages, and provides a perpetual yield of timber. Single-tree selection is done when individual trees are ready for harvest, of low value, or in competition with other trees.
- Clearcutting - removes all the trees in a given area, much like a wildfire, hurricane or other natural disturbance would do. It is used most frequently in pine forests, which require full sunlight to grow, and in hardwood forests with yellow poplar, sweetgum, cherry, maple and other species that require full sunlight. It is generally utilized for the final harvest of mature timber. Clearcuts are an efficient way to convert unhealthy stands to healthy, productive forests because they allow forest managers to control the tree species that grow on the site through natural or artificial regeneration.
- Shelterwood harvest - mature trees are removed in two or three harvests over a period of 10 to 15 years. This method allows regeneration of medium to low shade-tolerant species because a “shelter” is left to protect them. Many hardwoods, such as oak, hickory and cherry, can produce and maintain seedlings or sprouts in light shade under a partially cut stand. A disadvantage to shelterwood cuts is that they require more roads to be built through the forest, and increase the risk of soil disturbance and damage to the remaining trees during harvesting.
Check out the video below by Vaagen Brothers Lumber that shows selection harvest in Washington state. As always, feel free to watch the video at 1.25x or 1.5x speed if you feel like the pace is too slow.
At the end of the video you can see the harvested roundwood being transported to the sawmill on a truck, and then unloaded in the log yard.
Milling
At the sawmill, the tree is cut so as to create the maximum amount of lumber. The image below shows some different sawing patterns that may be used. How they cut the log is a function of economics. Using the market value of different sizes of lumber and the specifics of the tree (diameter, defects, length, etc.) found by imaging, the computer determines the pattern of cut. Super cool.
The following video is a narrated tour of a sawmill located in South Carolina, where they're turning southern yellow pine into barn poles, utility poles, and lumber (some of which may then go on to be turned into decking!). As you watch the video, keep an eye out for how they make cutting decisions. Do they mention what they do with the waste like sawdust? Do they say anything about their energy usage? There will be no quiz questions on this video, so don't worry about memorizing the names of the machines. You will, however, need to watch it for the reflection question.
This mill is one of the more advanced sawmills in the South. You can see that there are many ways in which the process has been automated to ensure that it is efficient and that they are getting the maximum utilization out of each tree.
Typically about 52% of the material processed at an average U.S. mill becomes lumber. 36% is transferred to other facilities to become pulp for paper, fiberboard, particle board, etc. Often, the bark is sold as garden mulch and sawdust is sold to farmers for use as animal bedding. The remaining 11-12% (mostly the bark and other wood scraps) is recovered for energy production, some of which is shipped off for use by other industries and some of which is used on-site. The image below (taken from Dr. Howe's video in the last lecture) is a visual representation of this breakdown.
Wood debris being chipped and flowed into the back of a trailer to be transported and ultimately combusted to produce renewable energy.
Treatment & Drying
At the end of the Collum's Lumber video, we heard that the boards would go on to be dried in a kiln (huge oven) and then planed. Planers smooth all four surfaces, round the edges, and trim the pieces to their final dimensions. If you've ever worked with lumber, you may have noticed that a 2x4 (a board that is 2 inches thick, 4 inches wide, and any length) is not actually a 2x4. It's usually more like 1.5 inches by 3.5 inches. This is because the board is cut to the 2"x4" dimensions at the sawmill, and then shrinks when it is dried. In the next lecture we will talk more about moisture in wood, and explain why this shrinkage happens.
This board is sold as an "eight foot, two by four" and written 2"x4" x 8'.
There are several ways in which lumber is dimensioned. One measurement you'll hear is "linear foot", which just means you're not taking the width into account. If you bought 100 linear feet of lumber and laid the boards down end-to-end, they would stretch 100 feet. When you multiply the linear length by the width of the board you get the area (e.g. if a board is 8 linear feet by 2 feet wide, that's 16 square feet of wood). Another term you may hear in the lumber world is "board foot". This is a unit of volume equivalent to 144 cubic inches (in3), such as a board that's 12 in wide by 12 in long by 1 in thick. The board can have any dimensions as long as they multiply to 144 in3. See the image below.
Once it has been dried and planed, some lumber will go on to receive a pressure treatment to help it resist rot, fungus, and insects. Pressure treatment is performed by placing dried lumber into a closed cylinder, and then applying vacuum and pressure to force preservatives into the wood. You can recognize pressure-treated wood by its bluish-green tint.
After these final steps, the lumber is bundled according to the type of wood and the quality. The bundles are held together by steel or PET (plastic) strapping, and often wrapped in HDPE and LDPE (other types of plastic) laminated paper. The bundle will then be loaded onto a truck or train and shipped to a lumberyard for resale to customers, like us deck-builders.
Wrapped lumber bundles in transit to a lumberyard.
Lumber choices
We can now head to Menards and select which kind of wood we'd like to build our deck from.
The "Deck Accessories" and "Construction Lumber" sections of the Menards lumberyard.
Red cedar is popular for use as decking because of its rich color and natural weather resistance. The heartwood of cedar contains extractives (specifically tanins and oils) that make it naturally resistant to rot, decay, and insects - and a more expensive lumber choice (see image below). Redwood was historically used for deck building for these same reasons, but it is becoming an endangered species so the supply is now limited.
The most popular decking material today by far is pressure-treated (PT) wood. Most PT decking comes from southern yellow pine, which has no natural preservatives. PT wood is up to half the cost of cedar, costing only 60-99 cents per linear foot. (Remember that most plantation forests in the U.S. are southern pine plantations - do you think this explains the low cost?)
Here are the current prices of an 8 ft piece of cedar and PT decking at Menards.
We listed cedar and yellow pine as common decking materials, but there are many other species of trees growing on the timberlands of the U.S. Different species of trees have wood with different properties (strength, hardness, weight, color, grain, flexibility), which means they are used for different things. Softwoods are typically used for building construction, while hardwoods are used for furniture and other finer products because of their beauty and grain. Another example is that aspen is used on sauna benches and playground structures because one of its unique properties is that it doesn't easily splinter.
Pine is the most common wood used in construction, such as for framing new homes.
Some of the most common tree species in the US. and their uses are listed in the table below.
|
Tree Species |
Uses |
|
Aspen |
Woodchips, particle board, plywood, shingles, dimension lumber. Specialty uses: sauna benches and playgrounds. |
|
Basswood |
Hand carving, making crafts, veneer for plywood, furniture, pulpwood. |
|
Douglas Fir |
Lumber and plywood |
|
Lodgepole Pine |
Framing lumber, plywood, and railroad ties |
|
Northern Red Oak |
Flooring, furniture, millwork, crossties, mining timbers, veneer, pulpwood. Generally, the most important hardwood lumber species in the US. |
|
Ponderosa Pine |
Lumber, plywood, doors, furniture, etc. Considered the most commercially important western pine. |
|
Sugar Maple |
Flooring, furniture, millwork, veneer, and pulpwood. Also maple syrup! |
|
Sycamore |
Furniture parts, millwork, veneer in plywood, pulpwood, and biomass. Specialty products: butcher block. |
|
Yellow Birch |
Lumber and veneer in making furniture, plywood, cabinets, boxes, and interior doors |
|
Yellow Poplar |
Crates, toys, furniture, veneer plywood, pulpwood, etc. Extremely versatile. |
Environmental Impacts
We've talked about how biomass, particularly wood, is a uniquely renewable resource because it requires almost no energy to produce (plant growth is solar powered). Additionally, one study Links to an external site. found that nearly 80% of the energy required to produce softwood lumber comes from wood waste (a renewable source of energy). Thinking back to the life cycle graphic at the beginning of the lesson, wood waste is only a source of energy at the sawmill stage. Energy for forestry operations, harvesting machinery, and the trucks/trains that haul the logs to the sawmill/lumberyard is coming from fossil fuels.
The table below shows the energy consumed during the production of 1 cubic meter of dried, planed softwood lumber in the Northeast-North Central U.S. Energy consumption is expressed as the weight in kilograms of the raw materials used as fuel (including, coal, natural gas, oil, uranium oxide, and wood waste). You can see that the majority of the energy consumption occurs during wood production, not during forestry operations. You can also see that wood waste is used exclusively in the production phase.
Table 1. Energy in wood processing. Table is taken from Puettman et al. 2012 Links to an external site..
There are other impacts besides energy to consider. For example, while there is almost no waste produced in the milling process, there is still some waste produced from the packaging materials. The question we will explore later in this unit is how these impacts compare to those associated with the production of mineral-based construction materials like steel. We'll also compare wood decking to plastic-composite "maintenance-free" decking. As always, we will consider environmental impacts across the lifetime of these materials using LCA. Stay tuned!
In the next lesson, we’ll get into the specifics of how moisture affects wood and why we dry our lumber. We'll learn why dry wood is stronger, how exactly shrinkage/swelling happens, why boards warp, and why it all matters for our deck.