Forest Atlas Of The United States

Forests and the Carbon Cycle

Forests–through photosynthesis–store (sequester) carbon from the atmosphere with carbon becoming part of plant mass or eventual wood products. When trees die, carbon continues to remain in the forest ecosystem and cycle through dead trees (or wood products), downed dead wood, forest floor, soil organic carbon, and/or eventually to the atmosphere through decay or combustion.

Plants are the lungs of the Earth, shaping the atmosphere which sustains us today. Every natural process and human activity eventually results in gases being added to or removed from the atmosphere. A century of fossil fuel burning has raised international concern over levels of carbon dioxide that could substantially alter Earth’s climate. Forests remove carbon dioxide from the atmosphere and store it in long-lived pools (like trees and soils) and products (like furniture and building materials), so forests may play a critical role in climate change mitigation. The cycle of carbon atoms from the atmosphere through forest ecosystems and back to the atmosphere is highly complex and varies over both time and spatial scales.

Across highly productive forests (for example, the central hardwoods of the Eastern US), large quantities of carbon are sequestered on an annual basis. Forest products, like the pages of a book, offer an alternative approach to storing carbon over longer time periods. By contrast, disturbance events like wildfires and hurricanes may be important for forest health, but they also release large quantities of carbon dioxide. Forest carbon currently stored in trees and soils dwarfs annual fossil fuel emissions, and there are concerns about the stability of these pools under a changing climate.

One of the first steps in the forest carbon cycle is the initial conversion of carbon dioxide through photosynthesis into living forest tissue. Many areas of the US, such as the central hardwoods of the East, are densely forested regions with high levels of carbon sequestration into living biomass. However, given the complexity of the forest carbon cycle the question still remains what will eventually come of this initial storage.

Although carbon may reside in many long-lived forest pools for centuries (e.g., soil), there are opportunities to combine society’s need for forest products (e.g., housing materials) with the need to sequester atmospheric carbon. Pulpwood harvests are common across the eastern US and although not as long-lived as sawtimber products, they offer an opportunity to provide social benefits while maintaining a level of ecosystem services.

The combustion of living and dead biomass in forests results in the immediate emission of gases such as carbon dioxide. Across the Western US, wildfires occur annually ranging in size in excess of 100,000s of acres. The emissions associated with these “mega-fires” can easily exceed a several million tons of carbon dioxide.

The complex interplay between forest sequestration of carbon and emissions to the atmosphere is difficult to quantify on global scales. Nonetheless, there is no doubt that forests play an important role in climate change mitigation, and that forest conservation and management serve multiple, complimentary objectives. These include the production of forest products, providing a livelihood for people, provisioning environmental services, creating habitat for wildlife, and supplying clean fresh water.

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FOREST CARBON STOCKS AND FLUXES WITH THE ATMOSPHERE. Forests alter the amount of carbon dioxide in the atmosphere, removing (otherwise known as sequestering) it as trees grow and returning carbon dioxide to the atmosphere as they decay or burn. Annual forest sequestration currently exceeds emissions from decay and fire combined. Within the US, this net sequestration offsets approximately 15 percent of annual fossil fuel carbon emissions. This dynamic is expected to continue, but climate change may alter drought and fire frequency, and forest conversion to other land uses may reduce the amount of forest land. These changes may result in forests emitting more carbon than they remove. Carbon Absorbed Sequestration: Carbon is removed from the atmosphere (sequestered) by forest ecosystems through photosynthesis, the process where living plants convert light into stored sugars from carbon dioxide and water. Forest carbon emissions in the United States are small compared to sequestration rates so forests are net accumulators of carbon. Changing rates of wildfire and land conversion could put this valuable benefit at risk. Carbon Stored Stocks: Forest carbon is stored in live trees, understory vegetation, and roots. Over time, this carbon is transferred to forest products, dead wood, and soils. Fossil fuel reserves are storing carbon removed from the atmosphere by plants millions of years ago. It may be surprising, but forests in the United States store nearly 30 percent of the carbon in America's coal, oil, and natural gas reserves. Carbon Released Emissions: Carbon emissions into the atmosphere are dominated by fossil fuel combustion for electricity and transportation. Forest carbon is emitted slowly through the decay of dead biomass or rapidly through combustion during wildfires. Carbon has always cycled between the atmosphere and plants, but the release of fossilized carbon since the Industrial Revolution has serious implications for the earth's climate.

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