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U.S. Forest Carbon Data: In Brief

CRS Report – U.S. Forest Carbon Data: In Brief Updated July 26, 2022: “Introduction: The flux—or flow—of carbon dioxide (CO2) and other greenhouse gases into the atmosphere is the dominant contributor to the observed warming trend in global temperatures. Trees, however, store (sequester) CO2 from the atmosphere, accruing significant stores of carbon over time. Trees also release some CO2 back into the atmosphere (e.g., emissions). This process is known as the forest carbon cycle. The forest carbon cycle starts with the sequestration and accumulation of atmospheric CO2 due to tree growth. The accumulated carbon is stored in five different pools in the forest ecosystem: above ground biomass (e.g., leaves, trunks, and limbs), below ground biomass (e.g., roots), deadwood, litter (e.g., fallen leaves and stems), and soils. As trees or parts of trees die, the carbon cycles through those different pools, specifically from the living biomass pools to the deadwood, litter, and soil pools. The length of time carbon stays in each pool varies considerably, ranging from months (litter) to millennia (soil). The cycle continues as carbon flows out of the forest ecosystem and returns to the atmosphere through several processes, including respiration, combustion (e.g., fire), and decomposition. Carbon also leaves the forest ecosystem through timber harvests, by which it enters the product pool. This carbon is stored in harvested wood products (HWPs) while they are in use but eventually will return to the atmosphere upon the wood products’ disposal and eventual decomposition, which could take several decades or more. In total, there are seven pools of forest carbon: five in the forest ecosystem and two in the product pool (HWPs in use and HWPs in disposal sites). Carbon is always moving through the pools of forested ecosystems. The size of the various pools and the rate at which carbon moves through them vary considerably over time. The amount of carbon sequestered in a forest relative to the amount of carbon released into the atmosphere is constantly changing with tree growth, death, and decomposition. If the total amount of carbon released into the atmosphere by a given forest over a given period is greater than the amount of carbon sequestered in that forest, the forest is a net source of carbon emissions to the atmosphere. If the forest sequesters more carbon than it releases into the atmosphere, the forest is a net sink of carbon…”

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