## 4 Discussion
### Site-Specific versus Multi-Site (Species-Specific) Parametrization  
CO\({}_{2}\) gas exchange rates as well as parameter values are correlated with site properties (see Figure 3). This reflects the fact that parameters restrict model applications to a certain range of environmental conditions. The fact that site-specific parameters perform better indicates that (a) the underlying process representation in the model misses some sensitivity to environmental drivers and (b) the model is limited in its ability to reflect plant adaptations to changes in site conditions. However, parameters derived at specific sites are often inferior to multi-site parametrizations at sites other than those where they have been obtained (Figure 4). Hence, site-specific calibration is particularly suitable to address responses to current climate conditions but should be used with caution if acclimation processes to local biotic and abiotic conditions are expected [20; 21; 26; 116; 117]. For example, the inter-site differences for model parameters involved for photosynthetic activity _i.e._, AMAXB, PSNTOPT, between boreal and temperate pine forests are large. Site-specific calibration for \"nitrogen dependency of photosynthesis\" (AMAXB) results in a higher value (in average 50%) in boreal compared to temperate pine stands which is in accordance with experimental findings [118]. Acclimation to local environmental conditions has been reported for pine [22], spruce [23] beech [24] and other tree species [21; 25]. [PERSON] and [PERSON] [116] as well as [PERSON] _et al._[21] point out that the variability of photosynthetic activity (in our model AMAXB) can be very large across geographical regions. Therefore, sometimes latitude information is used to describe a shifting response to environmental gradients of radiation, temperature, nutrient and water availability [119]. However, this is only an empirical work-around for missing process sensitivity that fails to describe an increase in C uptake efficiency originating from growing season length, radiation or nitrogen and water availability along latitudinal gradients [116]. Similarly, the \"optimum temperature for photosynthesis\" (PSNTOPT) is found to be much lower for boreal (FI-Hyy, FI-Sod, in average 8.74 \({}^{\circ}\)C) than for temperate forest (NL-Loo, BE-Bra). This is in agreement with field experiments where the link between photosynthetic response potentials and prevailing growing season temperatures had been demonstrated [120]. The differentiation of site-calibrated parameters in this study indicates the degree of acclimation of trees to specific environmental conditions. That the adaptation of only a few parameters such as AMAXB can improve the representation of NEE, GPP _etc._ across different climates has been demonstrated before [121; 122]. However, according to our knowledge, a comparison of site-specific paramemters and multi-site parameters has not been done in this context before. On the other hand multi-site parametrizations can reasonably perform over a wider range of environmental conditions: GPP and NEE (and thus total ecosystem respiration) during spring and summer, where 90% of the CO\({}_{2}\) exchange fluxes occurs, are well-represented (Figure 5). Also, an overall representation of \(r^{2}\) values between 0.66 and 0.87 indicates a good representation of GPP and NEE throughout the year (Table 5 and Figure 1). This ability of the model suggests that ecosystem responses to seasonal and inter-annual variations in environmental conditions are covered due to the representation of general principles of eco-physiology and biogeochemistry. It also indicates that the general parametrization can be used with some caution at a wider regional scale, such as for Central Europe. Figure 4: Cumulative daily mean values of the period 2001–2010 of Net Ecosystem Exchange (NEE) obtained with multi-site, boreal and temperate parameters calculated with the specifications of the pine forest of Loobos-Netherlands.