Contemporary forest carbon dynamics in the northern U.S. associated with land cover changes
Estimates of the spatial–temporal distributions of forest carbon (C) stocks subject to land use and cover changes is critical to greenhouse gas (GHG) estimation and reporting. Based on national forest inventory (NFI) and Landsat time series data, we applied matrix models to estimate and map spatial–temporal distributions of forest aboveground biomass (AGB) C, standing dead C, downed dead C, litter C, and soil organic C from 1990 to 2018 attributed to land cover changes and harvests in the northern United States (US). From predicted pixel-level maps, we found that all five forest C pools of northeast states and northern tier of Great Lake states had higher C density than other regions in the study area. We estimated that forest-related land cover changes reduced the forest C sink by 0.15 ton C ha-1 yr-1 (with a range of 0.12 to 0.18 ton C ha-1 yr-1) a accounting for 29% of forest C reductions over the study period. Forests remaining forests sequestered 2.38 Pg C (2.05 to 2.61 Pg C), hence the net forest sink of the northern US increased 1.73 Pg C (1.52 to 1.93 Pg C) during 1990–2018, which is an annual rate of 0.88 ton C ha-1 yr-1 (0.77 to 0.98 ton C ha-1 yr-1). Moreover, forest C was captured in harvested wood products by 0.33 ton C ha-1 yr-1. An uncertainty analysis with fuzzy sets suggested that the absolute uncertainties of land cover change and harvest impacts on standing dead C, downed dead C, and litter C were lower than 4.50 ton ha-1 during 1990–2018. In comparison, there were high uncertainties associated with estimates of soil organic C and AGB C densities at approximately 12–40 ton ha-1 in northern Michigan, Wisconsin, Minnesota, and Maine, New Hampshire, and New York. This study demonstrates methods for adhering to Intergovernmental Panel on Climate Change good practice guidelines for national GHG reporting and presents spatially explicit attribution of regional trends in C fluxes to particular activities and events. The resolved estimates from this analysis can be used to examine local and regional land use and cover change policies and practices in the context of C management in the northern US.