Sustainable Firewood: Recycling Atmospheric Carbon

Wood is a renewable fuel because young trees grow up to replace those harvested for fuel. That’s a simple enough statement, but there is much more to consider when you look into the details.

The use of wood as a heating fuel does not contribute to global warming and climate change the way fossil fuels do. When oil, gas and coal are burned, the carbon they contain (which was absorbed from the atmosphere by plants millions of years ago) is oxidized to carbon dioxide (CO₂), the main greenhouse gas. In effect, the combustion of fossil fuels releases ancient carbon, thereby increasing the atmospheric concentration CO₂.

Wood is about half carbon by weight but its use as a fuel releases a far smaller net amount of carbon dioxide per unit of heat than the fossil fuels. This is because trees absorb CO₂ as they grow so that much of the carbon released in combustion is absorbed by young trees. When trees mature, die and fall in the forest and decompose there, about the same amount of CO₂ is emitted as would be released if they were burned for heat. In other words, decomposition (rot) is a slow form of oxidation whereas combustion in a wood stove or furnace is fast oxidation, with heat as a by-product. Considered over the normal forest regeneration period of about fifty years, the net release of CO2 during combustion is almost balanced by the absorption of CO₂ by young trees. In heating our houses with wood, we are simply tapping into the natural carbon cycle in which CO₂ flows from the atmosphere to the forest and back. Therefore, when wood is burned as a substitute energy source for fossil fuels, a net reduction in GHG emissions results.

The Keeling Curve, named for Charles David Keeling of the Scripps Institute of Oceanography, was based on accurate measurements of atmospheric CO₂ in isolated Hawaii, starting in 1957. The Curve is old news now that global warming and climate change are widely discussed by scientists, policy makers and the public, but it does offer a useful insight into the role of forests in exchanging atmospheric gases and influencing their concentrations. The ‘sawtooth’ annual pattern of change in CO₂ concentration is the result of the massive absorption by trees and plants during the summer growing season in the Northern Hemisphere, which has far larger productive forest cover than the Southern Hemisphere. The small inset chart shows the CO₂ concentration falling during the summer and rising to a peak in spring. The lesson from this pattern is that forests act like the lungs of the earth, with each breath taking a year to complete. The healthier the forests are, the more CO₂ they can store and the more influence they can have on stabilizing the greenhouse effect. Those who live in the forested areas of the Northern Hemisphere may have noticed the explosion of growth that occurs in spring after winter dormancy. That spring and summer growth spurt is built largely using CO₂ absorbed from the air and it shows up on the Keeling Curve.

The actual reduction in household CO₂ emissions by using wood instead of fossil fuels cannot easily be estimated with precision. However, a rough estimate can be made in the case of wood substituting for the use of fuel oil. The CO₂ emission factor for fuel oil is 22.384 pounds per gallon. Therefore, the burning of a standard 200 gallon tank of oil would release 4477 pounds, or over two tons of CO₂.

Although it is often said that the combustion of wood from sustainable sources is CO₂ neutral, this is not strictly true. There are fossil fuel inputs to firewood production (chainsaws, splitters, trucks) and the combustion of wood releases some methane and carbon monoxide which are greenhouse gases but which are not absorbed by trees as they grow. Soot is also a product of wood combustion that has an impact of global temperatures. To account for those fossil fuel inputs and other greenhouse gas emissions we’ll assume that wood is only about 75 percent CO₂ neutral. The reduction in CO₂ emissions for a household that displaced the use of one 200 gallon tank of oil through the use of wood fuel is slightly less than two tons. That would mean that the owners of a house of modest size that previously used two tanks of fuel oil each winter for heating would, if they switched 100 percent to wood heating, cut their GHG emissions by around 3.4 tons.

Note that, while in this calculation we have estimated and included the fossil fuel inputs to firewood production, we have not accounted for the fossil fuel inputs to fossil fuel production, which are far larger than those for wood. Since the average energy return on energy invested for oil produced in or imported to North America is less than half of that of firewood, the real reduction in greenhouse gases by substituting firewood for two tanks of fuel oil could be twice as high as the estimated 3.4 tons.

Sustainable human activity meets the needs of the present without compromising the ability of future generations to meet their own needs. Woodlots that provide an annual yield of firewood can conform to this prescription of sustainability.

Forests can be used for aesthetic, recreational and industrial purposes while sustaining their essential qualities and health. Forest sustainability usually means that the integrity of the site and its soil is preserved or enhanced over time and that the diversity of native plant and animal species is maintained in perpetuity. In practical terms, effective management can be summarized this way:

Sustainable woodlot management consists of uneven-aged selective harvesting, thinning of dense stands and removal of poorer quality trees, while leaving seed trees of all present species and ages, and some standing dead trees to provide wildlife habitat.

Whew, that is a dense sentence, but it says almost everything you need to know about sustainable forest management.

On the fuelwood production front, the main goal is to ensure that our forests are harvested sustainably. While North America has many regions with highly productive forests, a woodlot can be ruined by over-harvesting or highgrading, which means taking the best and leaving the worst.

Many landowners have been maintaining their woodlots following this prescription for generations. In 1993, forest ecologist Ole Hendrickson wrote “the forms of wood energy use that have evolved in rural North America provide important but neglected models of sustainable development.”

The careful work by generations of farmers and other woodlot owners, visible in healthy woodlots along highways and rural roads, provides the stewardship model that others can follow. Unfortunately, some farmers have maximized short term profit by clear cutting their woodlots and converting the land to cash crop production. And while many woodlot owners understand and practice sustainable forest management, others exploit the resource for short-term profit. Some companies and individuals have made a practice of buying large parcels of unused forest land, highgrading them and then reselling the depleted parcels. These profiteers do meet the definition of woodlot owners, but they do not maintain ties to the lands they buy and sell.

The selection of trees for harvesting should take into consideration all aspects of the site including the slope, soils and age range of all trees in the immediate area. To do this work correctly, a woodlot owner needs specialized training and experience. Alternatively, the owner can contract with a professional forester to evaluate the woodlot and mark trees for harvesting.

It has long been said that a healthy, well-managed woodlot can yield half a cord of wood per acre per year forever – one full cord being a pile eight feet long, four feet wide and four feet high – and that a ten acre woodlot could sustainably produce enough firewood each year to heat a house. Although that guideline is old and not very precise, it still holds true. In fact, it takes a lot less than five cords of wood, and therefore less than a ten acre woodlot, to heat a new, modest-sized, energy-efficient house using a modern wood stove. There is some evidence that carefully designed and built small houses can be heated with as little as 1.5 cords of firewood.

Each spring when the leaves open on the trees where you live and begin to inhale carbon dioxide, you might give some thought to their role as the lungs of the earth and the many ways we use them to sustain us.