## 4 Discussion
### Difficulties in Calculating NPP as a Part of GPP Using Different Approaches  
The use of allometric relationships between stem diameter (DBH) and tree biomass can lead to systematic errors in aboveground NPP estimates [14]. It has also been suggested that the use of inadequate allometric equations and uncertainty in the estimates of fine-grained litter production potentially introduces significant errors in biometric estimates of NPP [2; 61; 62; 63]. Several reports have showed that discrepancies between biometric and EC measurements on an annual basis may be caused by the time-lag between carbon assimilated in photosynthesis and its allocation represented by stem growth [64]. Nevertheless, relative annual changes of NEP were found to be comparable between the two estimation methods, even when there were differences in its absolute values [65; 66; 67]. In our research, lower productivity in spring and summer (GPP) was associated with less severe drought events in 2020, although Reco increased in the summer and autumn of this year and was thus higher than in 2019. In general, the highest respiration fluxes were found to be in the summer, apparently caused not only by high temperatures but also higher overall plant activity (leaves and fine-roots are present and active, growth is occurring, etc.) [31]. The total EC-derived sum of NEP from March to October was higher in 2019 by 0.5 t C ha\({}^{-1}\) than in 2020 (Table 3). Higher NPP values were estimated for the same period in 2020 with biometric methods. Thus, the results obtained in our study suggest that higher NPP values estimated with biometric equations are associated with lower GPP and higher CUE. These may occur together with a lower NEP derived from EC measurements in the same season (Table 3). Notably, it is assumed that if the share of GPP used for respiration changes depending on soil conditions, NPP should also vary according to the ratio of NPP to GPP. [PERSON] and [PERSON] attempted to answer the question posed by [PERSON] et al. [52] 20 years earlier, and investigated if NPP of forests is a constant fraction of GPP. Their results provided evidence to reject the hypothesis of a universal or constant ratio of NPP to GPP, which in fact changes under disturbances, ageing and changes in biomass accumulation with changing climate, soil fertility and management practices. These have all been indicated to influence the ratio of NPP to GPP, likely in a non-mutually exclusive manner [68]. However, this approach was already implemented as a simplified concept in a large number of ecosystem models. In our research, such variation was also true: very similar GPP values estimated for different years under varying conditions did not change proportionally to NPP totals in the studied period. On the contrary, they indicated greater wood growth in a moderately wet year with higher soil water content during spring and summer. It is often considered that low CUE results do not reflect a significant underestimation of NPP, but are more likely related to the overestimation of GPP, since CO\({}_{2}\) fluxes measured by the EC method are prone to uncertainties. Different conclusions have been reached when using the open path (OP) analyzers as compared to closed path (CP) analizers. For instance, [PERSON] et al. compared random CO\({}_{2}\) fluxes using concurrent CP and OP measurements above soybean and maize crop [69]. The authors found differences in random flux uncertainty to be negligible during the day, while at night the random uncertainty of the OP system exceeded that of the CP systems by up to 20%. On the contrary, another study suggested very small differences in the random flux uncertainty between CP and OP systems [70]. Some of the latest results from Bog Lake Peatland flux tower (US-MBP on AMERIFLUX) suggest that during the growing season, the OP system measured larger net daytime CO\({}_{2}\) uptake than the CP sensor did, while the OP and CP CO\({}_{2}\) fluxes tended to converge at night [71].