## 3 Results
### Simulated and Observed Daily Carbon Fluxes  
Simulated and measured daily carbon fluxes for 2015-2017 of the study site DE-HoH are shown in Figure 2. The model can reproduce the measured fluxes at daily time scale and correlates with an R\({}^{2}\) of 0.82 for _GPP_ and of 0.77 for _R\({}_{co}\)_. _NEP_ correlates with an R\({}^{2}\) of 0.65 (see Figure A2 in Appendix C). In the simulation, we obtain a mean value of 16.6 Mg C ha\({}^{-1}\) a\({}^{-1}\) for _GPP_ and 13.2 Mg C ha\({}^{-1}\) a\({}^{-1}\) for _R\({}_{co}\)_. Therefore, the forest is a carbon sink with a mean _NEP_ of 3.4 Mg C ha\({}^{-1}\) a\({}^{-1}\) (Figure 2). Mean simulated annual values deviate from observed values by 14% for _GPP_ and 17% for _R\({}_{co}\)_. The observed RMSE of daily _GPP_ is 9 Mg C ha\({}^{-1}\) a\({}^{-1}\) and the RMSE of daily _R\({}_{co}\)_ is 5.8 Mg C ha\({}^{-1}\) a\({}^{-1}\). Simulation results slightly underestimated observed values. However, the magnitude of seasonal and daily fluctuations could be reproduced with our simulation. Measured as well as simulated _GPP_ reaches maximum daily values of around 70 Mg C ha\({}^{-1}\) a\({}^{-1}\) in summer time. This seasonal course can also be observed in _R\({}_{co}\)_. During summer time _R\({}_{co}\)_ reaches maximum values of around 45 Mg C ha\({}^{-1}\) a\({}^{-1}\). During winter time, when the forest is nearly leafless, the forest releases carbon (\(-\)3.9 Mg C ha\({}^{-1}\) a\({}^{-1}\)), and gains carbon in summer (10.8 Mg C ha\({}^{-1}\) a\({}^{-1}\)).  
Figure 2: Daily simulated and measured carbon fluxes for the years 2015–2017 at the forest Hohes Holz (DE-HoH). The observed data as black lines, simulated carbon fluxes as green line for (**a**) _GPP_ (**b**) _R\({}_{co}\)_ and (**c**) _NEP_. Measured temperature, the simulated and measured soil water content, and the photon flux density are in the Appendix A (see Figure A1). For 1:1 plot see Figure A2 in the Appendix C.  
In measured data, daily fluctuations are less compared to simulation results. Fluctuations of simulated carbon fluxes were mainly driven by temperature and water stress as well as by the available incoming light. Non-optimal meteorological conditions have stronger impacts on simulated carbon fluxes whereas measured carbon fluxes show less fluctuations on the daily timescale. One reason might be that sub-daily time scale processes such as stomatal closure to prevent transpiration on days with high temperatures are not directly included in the model. These processes could have influence on daily fluctuations due to a buffer effect for water or temperature stress. On the other hand, the reliable implementation of stress functions, especially the short-term temperature stress response of trees, is partly uncertain.  
The strongest differences between simulated and measured values can be observed in spring time at the start of the growing season. During this season, simulated \(GPP\) and \(NEP\) are higher and fluctuate more than the EC-derived carbon flux. In winter time (October-March), measured \(GPP\) (3.7 Mg C ha\({}^{-1}\) a\({}^{-1}\)) differs from model simulations as simulation results show no productivity during the dormant phase in winter time (further discussion on this issue see Appendix E).  
\(R_{eco}\) could be reproduced with model simulation throughout the year. Deviations between modelled and measured fluxes were mostly observed in summer time on days with high temperatures, low soil water content or on bright days. On these days' simulated respiration is often lower than the measured respiration.