Increases in carbon dioxide (CO2), and other pollutants in the atmosphere known to affect global climate, has caused some people to become interested in carbon capture and sequestration technology. Fortunately, one of the best carbon capture systems already exists: trees and forests. According to the US Forest Service, America's forests sequester over 800 million tons of carbon a year, which is roughly 12% of the US annual emissions (depending on the year). Forests sequester or store carbon mainly in trees and soil. During the process of photosynthesis trees pull carbon out of the atmosphere to make sugar, but they also release carbon dioxide back into the atmosphere through decomposition. Carbon and other gases within forests are captured and released on a cycle. Forest management is able to influence these cycles and enhance carbon capture.
Trees
Trees are without a doubt the best carbon capture technology in the world. When they perform photosynthesis, they pull carbon dioxide out of the air, bind it up in sugar, and release oxygen. Trees use sugar to build wood, branches, and roots. Wood is an incredible carbon sink because it is mostly made of carbon (about 50%), it lasts for years as a standing tree, and takes years to break down after the tree dies. While trees mainly store carbon, they do release some carbon, such as when their leaves decompose, or their roots burn sugar to capture nutrients and water.
Let's look at a real example. A white oak can live for 200 years; all that time it is pulling carbon out of the air and storing it. After several outbreaks of anthracnose, the tree dies, but it takes decades for the tree to rot. While it is slowly breaking down, the rotten tree is still keeping carbon out of the atmosphere. If the white oak were to be harvested and made into long-lived wood products, the carbon stored in the wood product is often preserved for a longer time than if it were left to decompose in the forest.
Forests capture and store different amounts of carbon at different speeds depending on the average age of the trees in the stand and the number of trees in the stand. Young forests have many trees and are excellent at capturing carbon. Young trees grow quickly and are able to pull in carbon rapidly. Not every small sapling becomes a large tree due to competition for light, resources, and growing space, but when they die and decompose, little carbon is released. The trees that remain continue to grow and sequester more carbon as the forest matures.
Established or mature forests are made up of "middle-aged trees," which are medium to large, healthy, and have a large root system. Middle-aged trees grow slower than young trees, but the amount of carbon sequestered and stored is relatively greater. Some large trees occasionally die, but they are quickly replaced by younger trees that take advantage of the new space. Since more trees are growing compared to those that are dying, the overall net productivity (how many trees grow versus how many die) is positive and carbon capture is enhanced.
Old-growth forests have a more fixed, or less dynamic, carbon cycle within live and dead trees and the soil. In old-growth forests, large trees dominate by shading out small saplings, so recruitment of young trees and net productivity is near zero. Still, the carbon is well contained within the big trees, slowly rotting logs, thick leaf litter, and soil. Large individual trees may take up as much carbon as an individual middle-aged tree, but since there are fewer trees in an old-growth stand, the rate of carbon sequestration is slower.
Soils
The carbon that is sequestered in forests comes in many forms. For example, forest soils contain plant roots, leaf litter, and other dissolved organic material. The amount of carbon stored in forest soils is variable, and how much carbon soil can sequester is dependent on many local factors like geology, soil type, and vegetation. In some forests, like in Canada by the tundra, the soil holds more carbon than the trees, but in other forests, like the rainforest, the soil holds relatively little carbon and the trees store more carbon. This is because some soil types, like clay soils, can bind up a large amount of carbon, whereas sandy soils are not able to bind much carbon. Soils with more organic material (bits of wood, decaying leaves, or dead creatures) can store more carbon because organic material easily binds loose carbon molecules and the organic material itself is stored as carbon. Soils that are frozen for a good part of the year or have a high-water table can also store large amounts of carbon because decomposition is slow.
Carbon Cycle and Climate Change Mitigation
Like all things natural, the carbon in forests eventually gets released into the atmosphere through the process of respiration, decomposition, and combustion. The rate at which these processes occur can vary across regions and forest types. Many people are interested in using forests to help mitigate climate change. Forests that grow quickly and store carbon for long periods of time are well suited for this goal.
The Amazon rainforest is often considered a good place for carbon sequestration and storage because it is full of big trees that grow rapidly. But research has found that the carbon moves in and out of tropical systems very quickly compared to temperate zone forests. Whole trees rapidly decompose in the hot humid climate and the soils can be low in organic matter. Also, the near-constant rain helps to break down organic material and wash away soil and nutrients. So while tropical forests are good at capturing carbon, they are also just as good at releasing the carbon in a short time frame.
In contrast, the spruce forests of Alaska in the northern hemisphere are excellent at carbon storage and serving as carbon sinks. The spruce can grow very large, and decomposition is very slow due to the cold. Also, the carbon in the soil is often locked up in the permafrost. The rate of carbon capture in spruce forests, however, is very slow. Cold temperatures and limited growing season mean that forests grow very slowly. Changes in global climate have also led to losses in the permafrost, releasing the carbon in the soils.
Forests in temperate climates, such as North America, offer an ideal middle-of-the-road solution. The trees grow relatively quickly and many species are long-lived. Decomposition in temperate regions occurs at a slower rate compared to tropical regions, and soils in these regions represent up to 50% of the carbon within a forest. This means temperate forests have great potential to serve as an effective carbon sink and provide long-term carbon storage.
Management Strategies
Natural forests not only provide carbon capture services, but also many other benefits (e.g., wildlife habitat). This is why the most important strategy is to keep forests as forests. When forests are converted to other types of land uses, carbon is released and the land loses its potential to store carbon, as well as other ecosystem services.
Forest management can help encourage trees to sequester more carbon, by changing the age structure and tree density in the stand (i.e., number of trees per acre). The protocols used in a carbon offset project are often based on sustainable forestry and are designed to increase total carbon gains over time.In stands where the trees are many different ages, there is continuous recruitment of younger trees, but older trees also remain and help hold carbon for long periods.
To help improve growth and tree regeneration, some trees can be removed using single tree or group selection harvesting methods. However, removing individual trees can disturb the soils in the local area. These soils also hold carbon and frequent disturbance over time can turn soils from a carbon sink to a carbon source. To help prevent soil disturbance in these stands it is useful to extend the rotation period. For example, a hardwood forest that has been traditionally thinned every 10–15 years could be thinned every 20–25 years, so the soils have time to recover between entries.
The best way to enhance carbon capture without cutting the existing forest is to increase forest cover. This can be done by planting old fields with a mix of native tree species or restoring old mine sites. Clear-cutting, however, resets the forest's age and in some cases can accelerate carbon capture by introducing younger and faster-growing trees. Climate benefits also occur when timber products displace the use of other products that require the use of fossil fuels (e.g., plastics).
Controlling invasive plant species is another important strategy for enhancing carbon capture. While many non-native/invasive plant species can grow rapidly they are not a good carbon sink. Invasive species also disrupt native ecosystems, change the makeup of the local soil microbes, and prevent tree regeneration, all of which interfere with a forest's ability to sequester carbon. Native trees and plants are adapted to thrive in local conditions and tend to function better as carbon capture mechanisms. Native plants also provide other important benefits such as wildlife habitat.
There are several other best practices landowners can adopt today to enhance carbon storage in trees and soils. When harvesting, it is important to reduce damage to the soil. This can be done by putting slash on skid trails, not harvesting in the rain, harvesting in the winter, and using forwarders instead of whole-tree skidding. Harvesting trees that are slow growing can also contribute to carbon sequestration. Instead of letting mature trees die and decompose, they can be removed and cut into products like 2x4s, flooring, or cabinets which go into homes and buildings and that could be around for centuries. The Liberty Bell is a great example of how high-quality wood products can help store carbon. The wooden yoke of the Liberty Bell is made from American elm harvested in the 1770s. Instead of decomposing in a forest centuries ago, the carbon in that wood is still around today holding up the Bell.
Harvesting is considered sustainable when decisions are based on silvicultural knowledge and follow a long-term management plan. Professional foresters are important for helping owners meet multiple management objectives while maintaining the value of their stands. Forests that maintain their value are more likely to remain as forests in the future when ownership changes.
Closing Remarks
Forests are an important carbon sink, since both trees and soils are able to store large amounts of carbon for a long time. However, carbon management is not just about deciding which trees to cut, but also where harvesting and planting occurs on the landscape. It is useful to maintain a mix of tree ages and forest types with a focus on young and established forests, as these forests capture and sequester the most carbon. However, this does not mean old-growth forests should be sacrificed to create more young forests. This could release large amounts of carbon, and a new forest would take decades to sequester as much carbon as currently stored in the old-growth forest. The key is to use planning and management strategies that help capture additional carbon while minimizing losses of stored carbon. Professional foresters can help you understand the potential of your land and forests for enhancing carbon capture through forest management while maintaining the value and health of your forests.
