Modeling carbon and water fluxes in agro-pastoral systems under contrasting climates and different management practices

Grasslands are worldwide spread ecosystems involved in the provision of multiple functional services, including biomass production and carbon storage. However, the increasingly adverse climate and non-optimised farm management are threatening these ecosystems. In this study, the original semi-mechanistic remotely sensed-driven VISTOCK model, which simulates grass growth as limited by thermal and water stress, was modified and integrated with the RothC model to simulate the ecosystem fluxes. The new model (GRASSVISTOCK) showed satisfactory performance in simulating above-ground biomass (AGB) in dry matter (d.m.) and fractional transpirable soil water (FTSW) along Alps (AGB, RMSE = 85.39 g d.m. m⁻²; FTSW, RMSE = 0.21) and Mediterranean (AGB, RMSE = 136.84 g d.m. m⁻²; FTSW, RMSE = 0.13) grasslands. Also, GRASSVISTOCK was able to simulate the net ecosystem exchange (NEE - RMSE = 0.03 Mg C ha⁻¹), the gross primary production (RMSE = 0.04 Mg C ha⁻¹), the ecosystem respiration (RMSE = 0.04 Mg C ha⁻¹) and the evapotranspiration (RMSE = 1.44 mm), where these observations were available (Alps). The model was applied under present and two climate datasets characterised by temperature increase and precipitation decrease (+2 °C temperature, -10 % precipitation) and reference or enriched CO₂ concentration (394 vs. 540.5 ppm) scenarios. The results showed that, while changes in temperature and precipitation alone had a negative impact by increasing NEE (+0.69 Mg C ha⁻¹) and decreasing total biomass (-0.20 Mg d.m. ha⁻¹) in the reference CO₂ scenario, the enriched atmospheric CO₂ concentration partially smoothed the NEE trend (+0.27 Mg C ha⁻¹) and increased total biomass (+0.60 Mg d.m. ha⁻¹) compared to the present period. It is concluded that the GRASSVISTOCK model represents a first step towards an integrated tool for estimating the performance of the agro-pastoral systems in terms of biomass production, water and carbon fluxes, in the face of ongoing climate change.