Constraining Hourly to Decadal Forest Carbon Fluxes with European National Forest Inventories

Accurate monitoring of forest CO₂ sequestration is essential for the European Union’s mission to achieve carbon neutrality. However, current estimates of the European forest carbon sink vary greatly depending on the methodology used. In particular, forest dynamics, such as harvest, disturbances and regrowth, are poorly captured by current approaches. Inverse modelling based on observations of atmospheric CO₂ is a key approach to constraining global and regional carbon budgets and quantifying the forest CO₂ uptake. CO₂ inversions capture hourly-to-seasonal variability in surface CO₂ fluxes well, such as during droughts. Nevertheless, due to the rapid mixing of atmospheric CO₂, inverse modelling systems require additional information from local observations to capture the effects of forest dynamics over longer timescales and across space. National forest inventories (NFIs) provide a promising and underused observational data stream of aboveground biomass, biomass increment, and forest demography across space and time.

Here, we integrate forest demography data from NFIs with a biosphere model to constrain European-wide land carbon fluxes from hourly to decadal scales. Our approach combines the Simple Biosphere Model 4 (SiB4) with European forest inventory data and the forest resources model EFISCEN-Space. Including forest demography in our simulations leads to an average increase in the total European land sink strength of roughly 50 TgC yr^-1 over 2000-2020, compared to a baseline simulation without demography. This additional uptake is mainly attributed to managed forests in Central Europe. Climate extremes such as the 2018 and 2022 droughts introduce additional variability in European forest carbon fluxes, which we show both spatially and temporally in our optimised net ecosystem exchange (NEE) fluxes. Finally, we evaluate the simulated CO₂ mole fractions from our modelling system across the network of European atmospheric monitoring sites. This analysis marks an important step towards including NFIs as an additional constraint in our inverse modelling system for the carbon cycle, CarbonTracker Europe. With this development, we aim to bridge atmospheric and ground-based data in estimating the European forest carbon sink.