Climate change affects Carbon cycle of forests cohorts but not homogeneously

Forest age plays a critical role in the distribution of carbon (C) stocks and fluxes in different forest ecosystems, as it is influenced by replacement ecosystem factors such as forest fires, logging, or insects. Large amounts of C stored for decades or centuries can be rapidly released into the atmosphere following a disturbance. Therefore, the net accumulation of C in forest ecosystems fundamentally depends on the age of the forest, which represents the time elapsed since the disturbance. To investigate the impact of forest age on net primary production (NPP) and total carbon woody stocks (tCWS) in a suite of observed and virtual forest stands, we used an already-validated biogeochemical-based forest growth model in three even-aged European forests across a latitudinal gradient, specifically: a European beech stand in Denmark, a Scots pine stand in Finland, and a Norway Spruce stand in Czech Republic. The model was forced by climate outputs of five Earth System Models under four representative climate scenarios (including one "no climate change" scenario as a benchmark) to simulate the effect of climate change on 11 age classes (from 12 to 140 years) of forest stands. We find out, with notable exceptions, that the production peak was for middle-aged class forests (42-70 years old), while carbon pool sizes increased with age with different trends in the three sites, but not linearly. Indeed, preliminary results show significant differences between the three sites and for the three species. Beech forest has an expected behavior under climate change scenarios, increasing NPP as atmospheric CO2 concentration increases (following the so-called "CO2 fertilization effect") as much for younger age classes as for older ones. At the same time, Norway spruce seems already at its optimum for climate, so even a small variation in environmental parameters leads to a drastic decrease in productivity and stocks. Finally, the Scots pine has an intermediate behavior. NPP and tCWS decrease with climate change compared with the NCC scenario, but increasing temperatures and atmospheric CO2 concentration lead to an increase in tCWS up to the RCP 6.0 and then a relative decrease under RCP 8.5. Age trends in C-cycling and stocks are evident in all three sites, showing that a better understanding of how forest age interacts will significantly improve our fundamental knowledge of the terrestrial C-cycle.