## 3 Results
### Productivity and Autotrophic Respiration across Species Derived from Model Simulation  
The distribution of the overall simulated productivity across the four tree species corresponds with the observed species occurrence at the study site, with _Fagus sylvatica_ and _Quercus petraea_ as the dominant tree species in _GPP_ and respiration (Figure 4b, for respiration see Figure 3). In the case of _Fagus sylvatica_, simulated productivity is 7.4 Mg C ha\({}^{-1}\) a\({}^{-1}\) with an average A\({}_{GPP}\) of 0.55 Mg C m\({}^{-2}\) a\({}^{-1}\). Mean simulated respiration is around 4.4 Mg C ha\({}^{-1}\) a\({}^{-1}\) which results in an A\({}_{\rm R}\) of 0.33 Mg C ha\({}^{-1}\) a\({}^{-1}\). For _Quercus petraea_, a slightly lower productivity (6.8 Mg C ha\({}^{-1}\) a\({}^{-1}\)) but an equal respiration of 4.4 Mg C ha\({}^{-1}\) a\({}^{-1}\) were derived. A\({}_{\rm R}\) of _Quercus petraea_ is nearly the same to that of _Fagus sylvatica_, whereas the A\({}_{GPP}\) is lower (0.5 Mg C ha\({}^{-1}\) a\({}^{-1}\)). The mean simulated productivity and respiration of _Betula pendula_ and _Carpinus betulus_ are much lower compared to _Fagus sylvatica_ and _Quercus petraea_. They contribute only up to 14% of total productivity and  
Figure 3: Measured and simulated major carbon stocks and fluxes in a temperate mixed deciduous forest in Germany (DE-HoH). All units are Mg C ha\({}^{-1}\) for stocks or Mg C ha\({}^{-1}\) a\({}^{-1}\) for fluxes. Blue values indicate measured carbon fluxes with the EC method. Black values indicate simulated values, and arrow sizes are related to the magnitude of the flux.  
16% of respiration, whereas _Betula pendula_ has the least share of productivity (3.2%) and respiration (4.9%) at this study site. This order can also be seen in productivity and respiration per BA (for respiration see Appendix D and Figure A3) based on model simulation results. _Carpinus betulus_ has an A\({}_{CPP}\) of 0.3 Mg C m\({}^{-2}\) a\({}^{-1}\) and an A\({}_{\rm R}\) of 0.19 Mg C m\({}^{-2}\) a\({}^{-1}\) whereas _Betula pendula_ has an A\({}_{CPP}\) of 0.34 Mg C m\({}^{-2}\) a\({}^{-1}\) and an A\({}_{\rm R}\) of 0.32 Mg C m\({}^{-2}\) a\({}^{-1}\) (Figure 4d).  
Virtual Experiments: The Influence of Species Composition and Forest Structure on Forest Productivity  
Simulation results of the species experiments show that the productivity for medium-sized trees is almost identical for all four monoculture settings (Figure 5a). For each monoculture setting, independent of the species, productivity of medium sized trees is around 12 Mg C ha\({}^{-1}\) a\({}^{-1}\). Compared to the inventory data setting, the productivity shows  
Figure 4: Simulated annual mean productivity _GPP_; (**a,b**), as well as productivity per basal area BA; (**c,d**) across three tree size classes and species. The bars indicate the simulated temporal fluctuation throughout the simulation time and simulation runs. Due to the normalization of the index (values between 0.2–0.6), the variability in (**c,d**) seems large due to small changes in _GPP_ and basal area. The productivity index is defined in Section 2.6.  
a mean increase of 13.5%. Without large trees (forest structure experiment) productivity of medium-sized trees increased for each simulation run compared to the simulation setting in the species experiment with large trees (Figure 5b). The forest structure experiment based on _Quercus petraea_ benefits the most and shows an increase in productivity of 17.1% compared to the species experiment with _Quercus petraea_ (with large trees). _Fagus sylvatica_ and _Carpinus betulus_ show an increase of around 12%, whereas the simulation based only on trees of the species _Betula pendula_ show nearly no increase in productivity (2%).