## 4 Discussion
### NPP Biometric Estimates
[17] reported NPP values twice as high as those found at the ME site--roughly 4 t C ha\({}^{-1}\) year\({}^{-1}\) for forests of loblolly pine older than 20 years. These authors also report the highest NPP for trees younger than 20 years (assuming NPP as the sum of wood, foliar, and fine roots production), concluding that for this pine species, NPP generally decreases with age after reaching this maximum [17]. There has been broad discussion in the literature addressing the question of the decrease in NPP with forest age. This revealed, inter alia, that the decline in GPP--rather than an increase in Ra--was the cause of NPP decrease as pine trees aged [17]. Sapwood respiration, which is an important component of Ra, also decreases with age. The decrease in pine GPP was driven by the age-related decrease in canopy conductance, which is in line with the hydraulic limitation hypothesis of [PERSON] et al. [17; 49]. Hence, it is expected that in the future, stand biomass growth that was currently estimated for the Mezyk site will most probably decrease. Further studies are needed to show at what age exactly NPP reaches its maximum for this temperate Scots pine stand, and what meteorological conditions will be conducive to this. Differences between Meteorological Conditions in 2019 and 2020 and Their Impact on NPP and GPP, and CUE Values  
It has been suggested that carbon use efficiency decreases with forest age [19]. In our study, CUE increased in 2020 compared to the previous year, which comes from only slightly higher annual total of GPP with generally higher NPP. This can be explained by more favorable environmental conditions--higher average soil water content and higher precipitation totals, both annually and in the summer--which led not only to a higher NPP/GPP ratio, but also to higher total ET fluxes. Stomata minimize the amount of transpired water per given gross primary productivity (GPP) rate under scarce water conditions, which has a theoretical basis in photosynthesis and air humidity control on stomatal conductance [50]. Therefore, by reducing water loss during drought conditions while maintaining photosynthesis at a fairly stable level for as long as possible, drought conditions have a greater impact on the annual variation in ET totals than GPP. ET sums were lower both in spring and summer of 2019 (Table 3), when dry conditions were more severe. The differences in meteorological conditions between 2019 and 2020 resulted in only slight changes in photosynthesis and transpiration fluxes. Considering their influence on NPP, the impact was significant--almost 1 t C ha\({}^{-1}\) more in woody biomass in 2020 compared to dry 2019. The drought that occurred in 2019, especially in June and August, has particularly important consequences. Since the highest GPP values were observed in summer months (in 2020 constituting 52.8% of the total vegetation period GPP, and 44.3% of its annual total), their contribution was crucial on the annual basis. Thus, significant reduction due to drought can lead to the decrease in summer contribution to both vegetation period and annual GPP totals. Nevertheless, in 2019, especially with a very dry August, the share of GPP in relation to the entire year and the vegetation period was not less than in 2020 (52.8% and 45%, respectively). The average annual CUE from across a range of forest ecosystems has been estimated as approximately 0.5, which means that, on average, half of the carbon absorbed in the photosynthesis process is allocated to the growth of wood tissue [51; 52; 53]. Nevertheless, reported CUE ranges from 0.2 to 0.8 depending on species, environmental, age and methodology [19; 51; 53; 54]. According to the study of different-aged pine stands (6-, 19-, 34-, and 69-years-old) in southern Ontario, Canada, CUE varies greatly with age. Annual mean CUE during 2005-2008 amounted to 0.61, 0.33, 0.50, 0.43, respectively [2].