On the assessment and initialization of European forest state variables in Land Surface Models

Forest structure is shaped by forest management practices, land-use changes and forest disturbances including droughts, fires, storms and insect outbreak. It plays an important role in climate by modifying the carbon-water-energy exchanges with the atmosphere, and affects the capability of forests to undergo future disturbances in a changing climate. Given the importance of forest structure for the climate, land surface models are moving towards 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-years semi-analytical spinup was carried out to bring the soil carbon and nitrogen pools into equilibrium until the European forests were clearcut. Then, a 150-years biosphere simulation over Europe was performed to build a look-up-table of simulated diameters. 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 is expected to have significant influence on the evolution of the forest carbon sink. In this work, we will assess the effect of the initialization procedure on the simulated land carbon sink and we will evaluate the representation of the diameters in the ORCHDEE LSM. The method could be further extended to initialize other forest state variables such as height or aboveground biomass.