Field-Calibrated Stem-Carbon Models and a Scale-Transfer Diagnostic for Interpreting Satellite Biomass in European Beech Forests of the Central Rhodope Mountains, Greece

Satellite biomass products require locally relevant field information, but destructively measured reference data are rare in Mediterranean mountain beech forests. We used 60 felled Fagus sylvatica L. trees sampled in 2023–2024 in the Central Rhodope Mountains, Greece, to develop local stem-carbon models and an exploratory field-to-satellite scale-transfer diagnostic. Smalian stem volumes were converted to dry matter and carbon using a basic wood density of 0.58 Mg m−3 and a carbon fraction of 0.47. Log-linear models were evaluated by leave-one-out cross-validation (LOOCV) and bootstrap resampling. Mean stem carbon was 0.165 Mg C tree−1 (95% confidence interval: 0.116–0.221). The diameter at breast height (DBH) plus height model performed best (LOOCV coefficient of determination R2 = 0.963, root mean square error RMSE = 0.041 Mg C tree−1, and mean absolute percentage error MAPE = 14.4%). Satellite outputs from the European Space Agency Climate Change Initiative (ESA CCI) Biomass v6 were clipped to the three forest complexes. The Forest Information System for Europe (FISE) threshold masks had no local pixels, so the diagnostic used the European Forest Genetic Resources Programme (EUFORGEN) × WorldCover forest footprint. Its verified mean 2020 living-biomass carbon density was 71.94 Mg C ha−1. With a literature-based stem fraction of 0.65–0.85, this corresponds to 36.25–47.40 Mg stem C ha−1; the central value is 41.83 Mg stem C ha−1, or about 253 sample-mean-equivalent reference stems ha−1. Because the 2020 map predates the 2023–2024 field campaign, these equivalents are exploratory scaling quantities, not evidence of temporal agreement, measured stand density or pixel-level validation.