## 4 Discussion
### The Carbon Fluxes of Mixed Temperate Forests  
Carbon fluxes of terrestrial ecosystems are highly dependent on management practices and environmental factors such as climate and forest structure. Carbon fluxes of mixed forest with a diverse species composition, e.g., with both evergreen and deciduous trees, are a result of competition between different growth strategies leading to different carbon dynamics. While in Europe temperate humid deciduous forests have typical _GPP_ values of around 13.8 \(\pm\) 0.5 Mg C ha\({}^{-1}\) a\({}^{-1}\) and mean \(R_{exo}\) values of 10.5 \(\pm\) 0.6 Mg C ha\({}^{-1}\) a\({}^{-1}\) (mean biomass:108.8 \(\pm\) 56.7 Mg C ha\({}^{-1}\), mean age: 75 \(\pm\) 50), evergreen forests (mean biomass: 149.34 \(\pm\) 135.6 Mg C ha\({}^{-1}\), mean age: 91 \(\pm\) 141) have higher productivity (17.6 \(\pm\) 0.5 Mg C ha\({}^{-1}\) a\({}^{-1}\)) and respiration rates (13.4 \(\pm\) 0.6 Mg C ha\({}^{-1}\) a\({}^{-1}\)) [9]. The study of Anderson-Teixeira (2021) reported higher productivity values of 19.5 \(\pm\) 5.94 Mg C ha\({}^{-1}\) a\({}^{-1}\) and respiration values of 15.6 \(\pm\) 7.7 Mg C ha\({}^{-1}\) a\({}^{-1}\) for mature temperate broadleaf forests (defined as \(\geq\)100 years old, biomass 121 \(\pm\) 34 Mg C ha\({}^{-1}\)) around the world [44]. Europe-wide studies reported typical values for _GPP_ of 11.1 \(\pm\) 3.5 Mg C ha\({}^{-1}\) a\({}^{-1}\) based on EC measurements and site studies (12 \(\pm\) 0.8 Mg C ha\({}^{-1}\) a\({}^{-1}\)) [45]. Simulated _GPPs_ with process-based ecosystem models such as LPJ-DGVM, Biome-BGC and ORCHIDEE reach mean values from 9 \(\pm\) 2.3 up to 11.8 \(\pm\) 2.9 Mg C ha\({}^{-1}\) a\({}^{-1}\). These Europe-wide estimates include less productive biomes such as Mediterranean, boreal as well as temperate semiarid evergreen forests [9; 45], which could explain the lower values compared to temperate humid forests such as DE-HoH. Furthermore, the study site DE-HoH is located in an area with a high nutrient supply, which might lead to a higher productivity compared to other forests with equal species mixing due to favourable growing conditions. Other potential factors are climatic conditions. An evaluation of a global carbon flux dataset has shown that climatic conditions alone are responsible for 71% of global _GPP_ variability, with increasing productivity along with increasing temperature and precipitation [9]. _NEP_ show less correlation with meteorological drivers. Only 5% of the global variability can be explained with temperature and precipitation patterns [9]. _NEP_ derived for EC sites was estimated to be around 3.1 \(\pm\) 0.3 Mg C ha\({}^{-1}\) a\({}^{-1}\) for temperate humid deciduous forests with higher values for evergreen forests (+0.87) [9]. For eddy flux sites in Germany, _NEP_ was estimated to be between 4.8-5 Mg C ha\({}^{-1}\) a\({}^{-1}\) for beech stands (unmanaged, uneven-aged mixed speech stand in mature state (0-250 years old) as well as a managed even-aged beech stand of _Fagus sylvatica_, 70-150 years old) [46] and between 4.4-6.98 Mg C ha\({}^{-1}\) a\({}^{-1}\) for a spruce forest [47]. A worldwide study estimated a value of 3.09 \(\pm\) 1.9 Mg C ha\({}^{-1}\) a\({}^{-1}\) for mature temperate broadleaf forests (defined as \(\geq\)100 years old) [44]. These findings go along with the obtained values in this study and support the observation that European, as well as German forests, continuously accumulate carbon in their biomass and act as carbon sinks [5; 9]. Simulation results from this study for \(R_{h}\) are close to literature ranges for similar biomes with slightly lower values compared to literature: values for humid temperate forests are between 3.8-4.2 Mg C ha\({}^{-1}\) a\({}^{-1}\) for \(R_{h}\) and between 6.7-9.5 Mg C ha\({}^{-1}\) a\({}^{-1}\) for \(R_{a}\) with higher values in evergreen forest compared to deciduous forest [9]. This might be a result of different species and growth strategies.