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Climate change is affecting human communities and ecosystems worldwide. The increasing concentration of carbon dioxide and other greenhouse gases in the atmosphere is a major driver of climate change. Some of these emissions are associated with human activities. To reduce the negative effects of climate change, it is critical to reduce carbon dioxide and other greenhouse gas emissions. Fortunately, trees naturally sequester (or take up) carbon dioxide and store it for long periods of time, keeping it out of the atmosphere. According to the USDA Forest Service, the trees in the United States sequester about 800 million metric tons of carbon dioxide annually.
How do forests store carbon?
When plants and trees conduct photosynthesis, they take carbon from the atmosphere and turn it into sugars and other organic compounds. Trees use most of these sugars to keep their cells alive, which releases some carbon back into the atmosphere through respiration. The rest of the carbon-based sugars are used to build wood, branches, roots, and leaves. These parts of the tree store carbon for different amounts of time and are called carbon pools because they contain about 50% carbon by dry weight. The carbon stored in wood can last for decades or centuries, keeping it out of the atmosphere. When roots, leaves, or branches fall off, or when trees die, animals, fungi, and bacteria break down the material. This returns some carbon to the atmosphere and moves the remaining carbon to the soil. Wood may take many decades to fully decompose, depending on the tree species and the environment in which it is found. Although less than half of a tree’s biomass ends up in harvested wood products, these products still store carbon. When harvested wood is used for energy, or to make building materials that reduce the use of fossil fuels, it can have more climate benefits than just storing carbon. This is called substitution.
For example, the coastal mountain areas of Oregon and Washington are home to large mature trees, most of which can live for well over 200 years. These trees have been sequestering carbon for centuries and storing it in their trunks, branches, leaves, and roots. Even after the trees die, much of the carbon remains stored in their biomass, gradually releasing some carbon back into the atmosphere and some into the soil carbon pool. Alternatively, if harvested, that biomass can be used to create long-lived wood products.
How does carbon storage evolve as forests age?
Not all forests sequester carbon at the same rate or have the same level of carbon storage. Carbon sequestration and storage depend on tree species, the number of trees in a forest stand, their age, growing season, and local climate. Forest carbon stocks change as forests age, especially after major events like clearcutting or wildfires. Young forests typically have more trees, but they have not stored as much carbon as older forests. While not all young trees survive to maturity, they still contribute some of their stored carbon to the soil carbon pool.
Mature forests are made up of medium-to-large trees that are close to their maximum height. These forests store more carbon than younger forests because they have more biomass. It is important to know that large trees keep growing as fast, or faster, than smaller trees, so they continue to sequester carbon from the atmosphere. However, at the stand level, carbon sequestration tends to slow down as forests get older. This happens because more trees die, and dead wood decomposes. Although large trees still hold a lot of carbon, the total amount of carbon added to older forests slows down as tree deaths balance out new growth.
Net Primary Productivity (NPP) can help explain how forest age influences carbon storage at the stand level. NPP represents the difference between carbon gained through photosynthesis and carbon losses through mortality, respiration, decomposition, and herbivory. As a forest stand matures, its overall NPP declines. While individual mature trees continue to sequester carbon at rates comparable to younger trees, the overall NPP of the stand decreases because of increased carbon losses from tree mortality and the decomposition of dead wood. As a result, less carbon is being sequestered in mature stands, but there is more overall carbon compared to young forest stands. It is also important to keep in mind that different tree species have different NPP rates during their lifetimes. For example, Douglas-fir reaches its peak NPP faster and has a much higher NPP than species like Oregon white oak.
Forest soil carbon
When wood, branches, roots, and leaves die and fall, invertebrates help to break them into smaller particles. Once the particles are small enough, microbes can consume the organic matter. When microbes break down organic matter, they respire some carbon back into the atmosphere. This process is known as decomposition. The carbon that is not respired through decomposition can remain stored in the soil for decades to centuries. Like trees, not all forest soils sequester carbon at the same rate or have the same level of carbon storage. Differences in climate, soil characteristics, mineralogy, and vegetation cause this variation.
For example, temperate forests in coastal areas of Oregon, Washington, and Alaska experience a mild, wet climate. This allows plants and trees to grow for most of the year, which provides consistent inputs of carbon to both the aboveground biomass and the soil, allowing for significant carbon storage in both pools. Meanwhile, the dry interior mixed-conifer forests east of the Cascade Mountains have a drier, more arid climate. These dry conditions speed up decomposition and limit plant growth, reducing the amount of carbon stored in the soil compared to the coastal forests.
How much carbon is in the Northwest Climate Hub region?
Forests in the Northwest Climate Hub region have some of the highest carbon densities in the world. These forests are highly productive, making them more efficient at sequestering and storing carbon than other temperate forest systems. Young trees in the region quickly take up and store carbon dioxide. However, they do not store as much carbon as mature trees. Mature trees store large amounts of carbon in their biomass and soil.
Forest carbon stocks are likely to shift as the climate changes. Longer growing seasons and changes in fire regimes will influence forest carbon stocks. Understanding how forests in the Northwest Climate Hub region store and cycle carbon is necessary to anticipate future changes in forest carbon stocks.
