Modelling above and below ground carbon dynamics in a mixed beech and spruce stand influenced by climate

Tree growth and carbon dynamics are important issues especially in the context of climate change. However, we essentially lack knowledge about the effects on carbon dynamics especially in mixed stands. Thus, the objective of this study was to test the effects of climatic changes on the above and below ground carbon dynamics of a mixed stand of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) by means of scenario simulations. To account for the typical tree interactions in a mixed-species stand a spatial explicit tree growth model based on eco-physiological processes was applied. Three different climate scenarios considering altered precipitation, temperature, and radiation were calculated for an unthinned and a thinned stand. The results showed significant changes of above and belowground biomass over time, especially when temperature and radiation were increased additionally to decreased precipitation. The reduction in biomass increments of Norway spruce were more attenuated above than below ground. In contrast, the results for beech were the opposite: The belowground increments were reduced more. These results suggest a shift in the species contribution to above and belowground biomass under dryer and warmer conditions. Distinct effects were also found when thinned and unthinned stands were compared. A reduced stand density changed the proportions of above and below ground carbon allocation. As a main reason for the changed growth reactions the water balance of trees was identified which lead to changed biomass allocation pattern.