The global carbon cycle is the process by which the element carbon moves between the air, land, ocean, and Earth’s crust. The movement of increasing amounts of carbon into the atmosphere, particularly as greenhouse gases, is the dominant contributor to the observed warming trend in global temperatures. Forests are a significant part of the global carbon cycle, because they contain the largest store of terrestrial (land-based) carbon and continuously transfer carbon between the terrestrial biosphere and the atmosphere. Consequently, forest carbon optimization and management strategies are often included in climate mitigation policy proposals.
The forest carbon cycle starts with the sequestration and accumulation of atmospheric carbon due to tree growth. The accumulated carbon is stored in five different pools in the forest ecosystem: aboveground biomass (e.g., leaves, trunks, limbs), belowground biomass (e.g., roots), deadwood, litter (e.g., fallen leaves, stems), and soils. As trees or parts of trees die, the carbon cycles through those different pools, 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, 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 the products 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).