###### Abstract  
The accurate estimation of an increase in forest stand biomass has remained a challenge. Traditionally, in situ measurements are done by inventorying a number of trees and their biometric parameters such as diameter at the breast height (DBH) and height; sometimes these are complemented by carbon (C) content studies. Here we present the estimation of net primary productivity (NPP) over a two years period (2019-2020) at a 25-year-old Scots pine stand. Research was based on allometric equations made by direct biomass analysis (tree extraction) and carbon content estimations in individual components of sampled trees, combined with a series of stem diameter increments recorded by a network of band dendrometers. Site-specific allometric equations were obtained using two different approaches: using the whole tree biomass vs DBH (M1), and total dry biomass-derived as a sum of the results from individual tree components' biomass vs DBH (M2). Moreover, equations for similar forest stands from the literature were used for comparison. Gross primary productivity (GPP) estimated from the eddy-covariance measurements allowed the calculation of carbon use efficiency (CUE = NPP/GPP). The two investigated years differed in terms of the sum and patterns of precipitation distribution, with a moderately dry year of 2019 that followed the extremely dry 2018, and the relatively average year of 2020. As expected, a higher increase in biomass was recorded in 2020 compared to 2019, as determined by both allometric equations based on in situ and literature data. For the former approach, annual NPP estimates reached ca. 2.0-2.1 t C ha-1 in 2019 and 2.6-2.7 t C ha-1 in 2020 depending on the \"in situ equations\" (M1-M2) used, while literature-derived equations for the same site resulted in NPP values ca. 20-30% lower. CUE was higher in 2020, which resulted from a higher NPP total than in 2019, with lower summer and spring GPP in 2020. However, the CUE values were lower than those reported in the literature for comparable temperate forest stands. A thorough analysis of the low CUE value would require a full interpretation of interrelated physiological responses to extreme conditions. carbon sequestration; net primary productivity; carbon use efficiency; water deficit +
Footnote †: journal: Physics Letters B  
1 Meteorology Laboratory, Department of Construction and Geogeningering, Faculty of Environmental Engineering and Mechanical Engineering, Poznan University of Life Sciences, Piatkowska 94, 60-649 Poznan, Poland; klaudiazem@upp. Poznan1 (); zjunsz.olejnik@upp. Poznan.pl (); marek. ally exchanged between forest ecosystems and the atmosphere. In principle, undisturbed forest ecosystems and aforested stands on non or marginal agricultural land are a significant sink of atmospheric CO2[2; 3]. It was estimated that forests account for approximately half of the annual global net primary production (NPP) of all terrestrial ecosystems [4]. Forests contribute to the terrestrial carbon balance in two ways: as principal pools of carbon stored in plant biomass due to photosynthesis, and through the release of CO2 via ecosystem respiration [5]. These carbon pools are composed of living and dead biomass both above and below ground (AGB and BGB, respectively), such as dead wood, litter, and soil organic matter [6]. The productivity of any forest ecosystem corresponds to its carbon sequestration potential [7] and depends mainly on the age and species of trees, as well as the habitat and meteorological conditions. It has been assumed that the recent increase in the tropospheric concentration of ozone (O3) has reduced terrestrial carbon uptake compared to the preindustrial era [8]. This is mainly caused by O3 molecules penetrating the leaves through the stomata, hence damaging the photosynthetic apparatus. In addition to O3 damage, the productivity of forest ecosystems can be severely limited by water scarcity [9]. Carbon sequestration (stock) estimation is essential to understand the overall storage efficiency of a forest ecosystem [6].