Application of the 3-PGS model to assess carbon accumulation in forest ecosystems at a regional level

In this study we assessed carbon sequestration by Italian forest ecosystems at a regional level. We applied a monthly time-step process-based model (3-PGS), coupled with a modified soil respiration model, to predict both gross primary production (GPP3-PGS) and net ecosystem production (NEP3-PGS). To evaluate the general reliability of model estimates, we compared, at five different forest sites, monthly and annual GPP3-PGS, NEP3-PGS, and predicted total ecosystem respiration (TER3-PGS) with averages of monthly and annual eddy covariance (EC) measures of GPPEC, NEPEC, and TEREC. A strong correlation was found between annual GPP3-PGS and annual GPPEC (r2 = 0.77, RMSE = 1.28 Mg C·ha–1·year–1), and monthly (r2 = 0.85, RMSE = 35 g C·m–2·month–1), as well as between NEP3-PGS and annual NEPEC (r2 = 0.76, RMSE = 0.21 Mg C·ha–1·year–1), and monthly (r2 = 0.78, RMSE = 18 g C·m–2·month–1). The TER3-PGS also showed a high correlation with annual TEREC (r2 = 0.93). Furthermore, a sensitivity analysis showed that GPP3-PGS was highly sensitive to the satellite greenness index (normalized difference of vegetation index) and to the vapor pressure deficit. With general confidence in the models, we established a 30 year average meteorological grid of 8 km × 8 km resolution across Italy and created a map representing annual NEP3-PGS across Italian forests, based on the remotely sensed CORINE Land Cover forest classification.