A Generic Spinup Procedure to Initialize European Forest State Variables in Land Surface Models

Forest structure is shaped by forest management practices, land-use changes, and natural disturbances, including droughts, fires, storms, and insect outbreaks that drive species-specific and size-specific mortality. By modifying the carbon-water-energy exchanges with the atmosphere, it influences a stand's capacity to buffer against or succumb to extreme weather events, which in turn determines the long-term stability of the terrestrial carbon stocks. Given the importance of forest structure for forest land sink, land surface models are moving toward explicit representations of forest structure and management strategies. We present a new procedure to initialize forest diameters over Europe and document its implications for simulations of future forest carbon sinks. The simulated diameters for each grid cell covered by forests are initialized toward the diameter from a forest inventory. To this end, a 300-year semi-analytical spinup was carried out to bring the soil carbon pools into equilibrium. A lookup table with the simulated diameter and plant functional type as its entries was built by clearcutting all forests followed by a 200-year simulation over Europe. For each grid point, the year associated with the simulated diameter that is the closest to the observation is selected, enabling the production of new initial state files over Europe. The new initialization procedure makes the initial state of forest more realistic and therefore significantly modifies the evolution of the forest carbon sink. The method could be further extended to initialize other forest state variables such as height or aboveground biomass.