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finding 9.3 : key-message-9-3
Annual harvest removals from forestry operations in select regions decrease forest carbon stocks, but this decline in stocks is balanced by post-harvest recovery and regrowth in forestlands that were harvested in prior years. Removal, processing, and use of harvested biomass causes carbon emissions outside of forests, offsetting a substantial portion (about half) of the net carbon sink in North American forests (high confidence).
This finding is from chapter 9 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.
Description of evidence base: Recent trends in natural disturbance rates indicate that the strength of net forest uptake has diminished across much of North America. Net loss of forest carbon stocks from land conversions also reduces sink strength across the continent, with carbon losses from forest conversion exceeding carbon gains from afforestation and reforestation. These findings are supported by 1) national inventory reports of greenhouse gas emissions and removals in the forestland category in Canada (ECCC 2016), Mexico (INECC/SEMARNAT 2015), and the United States (U.S. EPA 2018); 2) atmospheric inversion models (Peylin et al., 2013); 3) syntheses of forest inventory and land-change data (Pan et al., 2011); 4) measurements of forest-atmosphere carbon exchange with eddy covariance (Amiro et al., 2010); and 5) ecosystem process models (Sitch et al., 2015).
New information and remaining uncertainties: Intensively managed forests are among the most well understood ecosystems in North America. Decomposition dynamics associated with harvested wood products are less well understood, however, and changes in forest use and climate may alter these dynamics in the future. Furthermore, basic understanding of carbon flux and stock dynamics following disturbance is still limited, with some studies suggesting a substantial impact to fluxes (Edburg et al., 2011) and others reporting a more muted response (Moore et al., 2013; Reed et al., 2014). Predictions of future disturbance trends are hampered by limited understanding of disturbance interactions from legacies of flammability, host species presence and absence, and active management responses such as fuel reduction treatments or preemptive and salvage logging.
Assessment of confidence based on evidence: The carbon balance impacts of harvesting are well observed and well understood thanks to a wide range of observations that are compiled, analyzed, and reported in detailed accounts.
ProvenanceThis finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation
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