Climate impact on mean annual cycle and interannual variability of CO2 fluxes in European deciduous broadleaf and evergreen needleleaf forests: insights from observations and state-of-the-art data-driven and process-based models

The impact of climate on the annual cycle and interannual variability of CO2 fluxes is assessed in European evergreen needleleaf forests (ENFs) and deciduous broadleaf forests (DBFs) using observations from 19 sites, alongside outputs from process-based and data-driven models. All models capture the temporal phasing of CO2 fluxes, including a shorter sequestration period in northern rather than southern Europe, a more pronounced annual cycle for DBFs than for ENFs in central Europe and strong interannual variability across sites. However, they generally underestimate both the magnitude of CO2 sequestration and its interannual variability compared to observations. Regarding the annual cycle, all datasets indicate enhanced CO2 uptake from late spring to early fall, with a stronger climate-CO2 flux coupling in northern and central Europe than in southern Europe, where seasonality is less pronounced. At the interannual timescale, the climate does not show a significant influence on observed and modelled net ecosystem exchange (NEE) when correlations are computed using monthly anomalies across all months combined. This apparent lack of relationship conceals meaningful seasonal patterns. In winter and fall, NEE tends to be positively correlated with temperature, soil moisture and vapour pressure deficit (VPD). In spring, NEE shows negative correlations with temperature and VPD but a positive correlation with soil moisture. The summer pattern is reversed compared to the spring pattern. In the observations, these relationships are noisy in both time and space, suggesting strong site-specific effects. In contrast, the models exhibit more structured and spatially coherent patterns with strong correlations, which may reflect an exaggerated response to climate forcing despite underestimated magnitude in CO2 flux interannual variability.