The problem of permanence for carbon sequestration in forests

Carbon sequestration is widely considered to be an effective climate change mitigation option. Forests store large amounts of carbon (C) and are thus proposed as a C banking resource. Forests, however, are subject to disturbances and tree mortality. Thus, it is important to understand the permanence of carbon storage in forests to better assess trade-offs between forest preservation, proforestation, and timber harvest. To assess disturbance rates, data were collected on California wildfires that occurred during 2016–2021. It was found that fire disturbed 0.27–4.3% of the total land area or 0.5–8.7% of the area excluding deserts and farms (i.e., for forest and shrublands). Disturbance was also modeled. For a 1% annual disturbance rate, only 37% of patches are > 100 years old at equilibrium. For a 2% annual rate, only 13.7% are > 100 years old. Data from three national parks in Canada confirmed that setting land aside does not prevent C losses. In contrast, C stored in wood products and subsequently in landfills is not subject to this magnitude of loss. While not all areas have disturbance rates as high as California, all forests experience disturbance and this needs to be considered when assessing climate benefits of C sequestration in forests.