Critical land change information enhances understanding of carbon balance in the U.S.

Large-scale terrestrial carbon estimating studies using methods such as atmospheric inversion, biogeochemical modeling, and field inventories have produced different results. The goal of this study was to integrate fine-scale processes including land use and land cover change into a large-scale ecosystem framework. We analyzed the terrestrial carbon budget of the conterminous U.S. from 1971 to 2015 at 1-km resolution using an enhanced dynamic global vegetation model (DGVM) and comprehensive land cover change data. Effects of atmospheric CO2 fertilization, nitrogen deposition, climate, wildland fire, harvest, and land use/land cover change (LUCC) were considered. We estimate annual carbon losses from cropland harvest, forest clearcut and thinning, fire, and LUCC were 436.8, 117.9, 10.5, and 10.4 TgC yr(-1) , respectively. Carbon stored in ecosystems increased from 119,494 to 127,157 TgC between 1971 and 2015, indicating a mean annual net carbon sink of 170.3 TgC yr(-1) . Although ecosystem net primary production increased by approximately 12.3 TgC yr(-1) , most of it was offset by increased carbon loss from harvest and natural disturbance and increased ecosystem respiration related to forest aging. As a result, the strength of the overall ecosystem carbon sink didn't increase over time. Our modeled results indicate the conterminous U.S. carbon sink was about 30% smaller than previous modeling studies, but converged more closely with inventory data.