Area and Growth Contributions to Dynamics of Forest Biomass Carbon Stocks in Northeast China

Understanding the spatial and temporal dynamics of forest biomass carbon stocks (BCSS) and their regional driving mechanisms is of great geographical significance. However, the relative contributions of forest area and forest growth (i.e., enhanced carbon density) to BCSS dynamics in Northeast China remain unclear. Here, we used long-term forest inventory data from 1973 to 2018, combined with the refined continuous biomass expansion factor and Forest Identity methods, to quantify forest area and growth contributions to the spatial and temporal dynamics of BCSS across Northeast China. Results indicate that forest area declined from 39.07 Mha during 1973−1976 to 31.85 Mha during 2014−2018 in the region, while carbon density increased from 41.89 Mg C/ha to 49.11 Mg C/ha over the entire study period. Forest BCSS decreased from 1636.77 Tg C to 1564.10 Tg C, with an average carbon loss of 3.08 Tg C/yr. Heilongjiang accounted for 239.0% of the total regional carbon loss, while Jilin and Liaoning mitigated 72.4% and 66.6% of this loss, respectively. In Heilongjiang, natural forests functioned as a carbon source driven by areal reduction, whereas planted forests served as a carbon sink, with areal expansion and forest growth contributing 38.0% and 62.0%, respectively. In Jilin, forest growth dominated the carbon sink for all forests (141.9%) and natural forests (274.3%), while areal expansion contributed more than forest growth for planted forests (52.0% vs. 48.0%). In Liaoning, forest growth was a larger contributor to the carbon sink than areal expansion for all (63.3% vs. 36.7%), planted (62.0% vs. 38.0%), and natural (75.4% vs. 24.6%) forests. During 1973−2018, the relative contribution of forest area to BCSS change declined from 132.1% to 29.7% in the whole region, whereas that of carbon density increased, making it the dominant driver of the carbon sink after 2009. Both planted and natural forests exhibited rising carbon density (up to 2.030%/yr and 1.343%/yr, respectively), indicating that forest growth has increasingly governed the carbon sink across the region.