## 4 Materials and Methods
### Dataset
#### 4.2.2 Field Survey Data  
The forest plot productivity survey was conducted between 5 and 23 August 2013, and between 9 and 24 August 2016 over Daxing'anling. As the northernmost ecological barrier and important forest conservation area in China, Daxing'anling contains the most representative boreal tree species. Therefore, forty-six plots over Daxing'anling were selected, including 17 deciduous broadleaf forests (_Betula platyphylla Suk_) plots, 14 deciduous needleleaf forests (_Larix gmelinii_) plots, 7 evergreen needleleaf forests (_Pinus sylvestris Linn_. var. _mongolica Litv_.) plots, and 8 mixed forest plots (Figure 5).  
The plots were circular areas of land with a radius of 10 or 15 m. The surveyed vegetation types included major species in Daxing'anling, namely, _Larix gmelinii_, _Pinus sylvestris Linn_. var. _mongolica Litv_., and _Betula platyphylla Suk_. Two aspects were investigated for each plot, namely, (1) the tree growth status in the plot and (2) the productivity of the forest. For every tree in the plot with a diameter at breast height (DBH) of more than 5 cm, a single tree measurement was performed to acquire the relevant data to establish a height-DBH model. By inputting the DBH obtained from the tree ring data into the growth models of different species, the height of a standard sample tree was derived. Tree ring data were acquired in the plot at six diameter levels (5-10 cm, 10-15 cm, 15-20 cm, 20-25 cm, 25-30 cm, and \(>\)30 cm). For each level, three standard tree samples were selected, of which, the tree cores were acquired by drilling at breast height (1.3 m) in both the east-west and the south-north directions to reduce measurement error without compromising the quality of the core [36]. A total of 596 tree cores was used for the tree ring analysis.  
The results from the complete analysis of the tree cores are shown in Figure 6. Once a tree core was collected in the field and its length was measured, it was placed in a paper-sealed tube marked with the plot number and the DBH value and returned to the laboratory for processing. The tree cores were first air-dried and placed in a dry place for several weeks to stabilize. Subsequently, they were fixed on the wooden notch and polished with sandpaper until the tree rings became clearly visible. Lastly, the processed cores were scanned and measured to obtain the tree ring width and to calculate the annualgrowth. The tree ring width was measured using a special tree ring analysis instrument (WinDENDRO), with a measurement precision of 0.001 mm [37]. Due to its symmetry, the tree ring width could be measured in only one direction and, to eliminate interpretation error caused by a false ring during measurement, the tree rings in two directions of the tree were separately identified and plotted to obtain the framework diagrams (Figure 6). The tree ring widths were subsequently measured in both directions, the average of which was taken as the result. That is, tree ages and DBH increments were acquired through tree cores. The results were used to derive the annual growth of biomass using the forest growth model [38]. This model was adopted to derive the regional annual NPP, utilizing the conversion factors between carbon and biomass increments, and it was employed to verify the simulation results from the BEPS model.