Predicted Future Changes in the Mean Seasonal Carbon Cycle: Impacts of Climate Change

Through photosynthesis, forests absorb significant amounts of CO₂ from the atmosphere while simultaneously releasing CO₂ back through respiration. The net carbon balance of a forest—whether it functions as a carbon sink (absorbing more CO₂ than it emits) or a carbon source (emitting more CO₂ than it absorbs)—depends on the relative magnitudes of these opposing carbon fluxes. The Mean Seasonal Cycle (MSC) provides a comprehensive view of the average carbon fluxes—Net Ecosystem Exchange (NEE), Gross Primary Production (GPP), and ecosystem respiration (Reco)—throughout the year.  

In this study, we assessed the ‘Three Dimensional–Coupled Model Carbon Cycle–Forest Ecosystem Module’ (3D—CMCC—FEM) ability to simulate key carbon fluxes. We validated the model against observed data and investigated whether the seasonal carbon sink/source dynamics patterns are affected under two climate change scenarios across five European forest sites. More specifically, daily observed meteorological (1997–2005) data for model validation come from the Fluxnet2015 Dataset, and future climate scenarios (2006–2099) are projected from three Earth System Models. These models are part of the Climate Model Intercomparison Project 5 (CMIP5) and are driven by two Representative Concentration Pathways (RCP), specifically RCP 2.6 and RCP 6.0. The five case studies selected to represent key European forest species are chosen for their presence in the Fluxnet network. These sites include: the temperate European beech (Fagus sylvatica L.) forests at Collelongo, Italy (IT—Col), and Sorø, Denmark (DK—Sor); the maritime pine (Pinus pinaster Ait.) forest at Le Bray, France (FR—Lbr); the boreal Scots pine (Pinus sylvestris L.) forest at Hyytiälä, Finland (FI—Hyy); and the temperate Norway spruce (Picea abies (L.) H. Karst) forest at Bílý Kříž, Czech Republic (CZ—Bk1). 

The model, validated under current climate conditions, confirmed the robust predictive ability in estimating NEE, GPP, and Reco across various forest species and climates. Under future climate scenarios, a consistent decline in forests C—sink capabilities is observed, with a more pronounced reduction under RCP 6.0. This decline is particularly pronounced in evergreen forests, which showed a greater decrease in NEE than deciduous forests. Finally, it was found that the number of days when evergreen forests act as C—sink increases over the years, with a forward shift of DoY to C—sink and a backward shift of DoY to C—source. In contrast, deciduous forests maintain a relatively stable number of C—sink (and C—source) days throughout the century (fixed DoY to C—sink or C—source). The DoY for deciduous forests remains constant, as the earlier onset of the growing season, driven by warming temperatures, is offset by an earlier increase in respiration. This indicates that over the long haul, deciduous forests demonstrate greater efficiency in utilizing photosynthates than evergreen forests.