Country-scale forest carbon modeling to support national greenhouse gas inventories in Croatia and Slovakia

Abstract Forest ecosystems are important carbon sinks and play a crucial role in reaching carbon neutrality. This study presents a reproducible workflow of using the biogeochemical ecosystem model Biome-BGCMuSo to support national greenhouse gas (GHG) inventory by including CO 2 emissions and removals from carbon pools lacking explicit carbon stock change (CSC) estimates within the Land Use, Land-Use Change and Forestry (LULUCF) sector. We estimated forest carbon stocks and their temporal dynamics in Croatia and Slovakia using the Biome-BGCMuSo v6.2 model. Our approach included (1) country stratification based on bioclimatic zones and soil characteristics; (2) simulations of representative stands of each stratum; and (3) upscaling results to the country scale using stratum area. The model was driven by the ERA5-Land meteorological dataset and site-specific soil characteristics, optimized eco-physiological parameters for dominant tree species, and country-specific business-as-usual management. Simulated live aboveground wood carbon stocks were validated against national growing stock data, while carbon stocks in deadwood, litter, and soil and annual Net Ecosystem Exchange (NEE) were evaluated against published ranges. After applying correction for relative stocking, the model showed moderate predictive performance of national growing stocks with normalized RMSE of 44% and normalized BIAS of 9% for Croatia, and normalized RMSE of 41% and normalized BIAS of 2% for Slovakia, while maintaining NEE, and carbon stocks in deadwood, litter and soil pools within the plausible literature ranges. Time series of simulated carbon stocks during the reporting period (1990–2023) were upscaled to the country level and used to estimate national CSC factors for deadwood, litter, and soil pools. Applying model-derived CSC factors increased previously reported national CO 2 removals by 1.6% in Croatia and 2.1% in Slovakia. This study strengthens the methodological completeness of national GHG inventories under the LULUCF sector.