Since the (non-) compliance of climate objectives is widely discussed, more attention has been shifted towards the multiple functions of forests. In this context, one of the highly-debated topics is the extent to which environmental and climate effects contribute to forest growth and thus also to the carbon sink. This work adds a crucial part to this discussion by implementing Austrian forest inventory data, and two different approaches for the estimation of the annual carbon fluxes into a stock & flow diagram, realised in the program Vensim®. The so-called residual sink and simultaneously environmental- and climate effects are being determined by calculating the difference of the carbon fluxes. The first approach for the determination of carbon fluxes is an inventory-based one, the second approach is the carbon flux-bookkeeping model of Houghton et al.. The latter reflects only LULUCF- (Land Use, Land-Use Change and Forestry) effects, whereas the inventory-based model includes all types of influences. Model simulations demonstrate the continuous increase in the forest area, in the biomass density, and also in the logging rates of the Austrian forests in the period between 1961 and 2010. Further results show that the residual carbon sink in the Austrian forests amounts on average to 4,07 TgC/year and to 1,8 TgC/year. The size of the residual sink depends on the type of simulation, whether a constant or a dynamic standing crop at harvest time has been assumed. When considering dynamic carbon densities, a smaller part of the residual sink is attributed to environmental and climate effects. A dynamization of static (biomass growth-) models is highly recommended for future approaches, especially to overcome various problems resulting from the assumption of a constant standing crop at harvest time.