As the world seeks ways to cut atmospheric carbon dioxide (CO2) — the greenhouse gas produced by burning fossil fuels — science says managed forests will play a key role.
Trees are the most powerful concentrators of carbon on Earth. Through photosynthesis, they absorb CO2 from the atmosphere and store it in their wood, which is nearly 50 percent carbon by weight.
You might be surprised to learn young forests outperform old growth in carbon absorption. Although old trees contain large amounts of carbon, their rate of absorption has slowed to a near halt. A young tree, although it contains little fixed carbon, pulls CO2 from the atmosphere much faster.
While cutting down an old tree results in a net release of carbon, new trees growing in their place can more than make up the difference. Wooden furniture made in the Elizabethan era still holds the carbon fixed hundreds of years ago.
The relationship between trees and greenhouse gases is simple enough on the surface. Trees grow by taking carbon dioxide from the atmosphere and, through photosynthesis, converting it into sugars. The sugars are then used as energy and material to build cellulose and lignin, the main constituents of wood.
When a tree rots or burns, the carbon in the wood is released to the atmosphere. Active forest management, such as thinning, removing dead trees, and clearing debris from the forest floor very effectively reduces the number and intensity of forest fires. And the removed wood can be put to good use for lumber, paper and energy.
Accounting provides a useful metaphor to discuss forests and carbon sequestration. Old growth forests often have a large “balance” of carbon that has built up over time in wood and soil. They don’t add much new carbon because they decay at about the same rate they grow.
In financial terms, this is like a company with many assets that operates on a break-even basis. Young forests have a smaller balance of carbon compared to old forests but accumulate carbon rapidly. In that sense, they are like an emerging, very profitable company with few assets that is growing rapidly.
The effect of forests on the global carbon cycle can be boiled down to these key points:
On the negative side, the most important factor influencing the carbon cycle is deforestation. This results in a permanent loss of forest cover and a large release of CO2 into the atmosphere. Deforestation — primarily where tropical forests are permanently cleared for agriculture and urban settlement — accounts for about 20 percent of global CO2 emissions, says a United Nations-World Meteorological Organization Intergovernmental Panel on Climate Change report.
On the positive side, planting fast-growing trees is the best way to absorb CO2 from the atmosphere. Many countries with temperate forests have seen an increase in carbon stored in trees in recent years. This includes New Zealand, the United States, Sweden and Canada.
Plus, using wood sustainably reduces the need for nonrenewable fossil fuels and materials such as steel and concrete — the very causes of CO2 emissions in the first place.
The good news is that U.S. forests are net carbon sinks, since annual growth exceeds annual harvest. We are experiencing an increase in forested land: Forests are being re-established on land previously used for agriculture.
Catastrophic wildfires are uncommon in managed forests, whereas millions of acres of unmanaged forests burn every year due to excessive build-up of dead trees and woody debris.
Every wood substitute, including steel, plastic and cement, requires far more energy to produce than lumber. More energy usually translates into more greenhouse gases in the form of fossil fuel consumption or cement production.
Some activists would have us believe using wood is bad for forests. Yet we are the world’s largest per-capita wood consumers, and North American forests cover about the same amount of land as they did 100 years ago.
According to the United Nations, our forests have expanded nearly 10 million acres over the past decade. This is precisely because we use a great deal of wood, which sends a signal to the market to grow more trees to meet demand. This is a win-win situation for both the economy and the environment.
One of the best ways to address climate change is to use more wood, not less. Wood is simply the most abundant, biodegradable and renewable material on the planet.
It is hard to imagine a more all-purpose, environmentally friendly act than contributing to the number and variety of trees growing throughout the world. In the age of climate change, Johnny Appleseed takes on a new meaning.
Patrick Moore, a co-founder of Greenpeace, is chairman and chief scientist of Greenspirit Strategies Ltd. in Vancouver, Canada.