LegiStorm

Summary:

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). Carbon is always moving through the pools of forested ecosystems (known as carbon flux). 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 that forest releases 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. If the forest sequesters more carbon than it releases into the atmosphere, the forest is a net sink of carbon. These forest carbon dynamics are driven in large part by different anthropogenic and ecological disturbances. Anthropogenic disturbances are planned activities, such as timber harvests, whereas ecological disturbances are unplanned, such as weather events (e.g., hurricanes, droughts), insect and disease infestations, and wildfires. Generally, disturbances result in tree mortality, causing the transfer of carbon from the living pools to the deadwood, litter, soil, and product pools, and/or eventually to the atmosphere. If a disturbed site regenerates as forest, the carbon releases caused by the disturbance generally are offset over time. If, however, the site changes to a different land use (e.g., agriculture), the carbon releases may not be offset. The U.S. Environmental Protection Agency (EPA) measures forest carbon annually using data collected by the Forest Inventory and Analysis Program in the U.S. Forest Service. According to EPA, U.S. forest carbon stocks contained 58.7 billion metric tons (BMT) of carbon in 2019 across the seven pools, the majority of which was stored in soil (54%). The aboveground biomass pool stored the next-largest portion of forest carbon stocks (26%). The pools' relative size varies considerably across U.S. forests, however. EPA estimates that, for the forest carbon flux, U.S. forests were a net sink of carbon, having sequestered 221 million metric tons (MMT) of carbon in 2018—an offset of approximately 12% of the gross annual greenhouse gas emissions from the United States for the year. The net sink reflects carbon accumulation on existing forestland and carbon accumulation associated with land converted to forestland within the past 20 years. Within the carbon pools, most of the annual flux is associated with aboveground biomass (58%). In general, the annual net flux of carbon into U.S. forests is small relative to the amount of carbon they store (e.g., 221 MMT of carbon is 0.3% of the 58.7 BMT of total carbon stored in U.S. forests in 2019). There are three primary strategic approaches for optimizing forest carbon sequestration and storage: (1) maintain and increase the area of forestland, (2) maintain and increase forest carbon stocks, and (3) increase the use of wood products as an alternative to more carbon-intensive materials or as a fuel. In many cases, optimizing carbon sequestration and storage may compete with other forest management objectives and require tradeoffs. As such, the applicability of each approach will vary, depending on existing site characteristics and other objectives. In addition, each of these approaches comes with varying levels of uncertainty related to effectiveness and potential for co-benefits. All of these considerations are in the context of the uncertainty related to the future effects of changing climatic conditions on forests broadly.