## 2 Materials and Methods  
### Study Area and Dataset  
For this study, we selected the normal humid cold temperate climate in the cold temperate climate of the Koppen-Geiger climate zone [25]. We also selected 14 evergreen needleleaf forest (ENF) flux sites in this climate zone, in locations with continuous carbon flux measurements for at least ten years (Table 1). The specific locations of the selected forest sites are shown in Figure 1.  
The EC technique provides a direct and continuous assessment of the net carbon and water fluxes between the biosphere and atmosphere [26]. The carbon flux datasets we used are publicly available from FLUXNET ([[https://fluxnet.org/](https://fluxnet.org/)]([https://fluxnet.org/](https://fluxnet.org/)), accessed on 11 March 2023), the Integrated Carbon Observation System ([[https://www.icos-cp.eu/](https://www.icos-cp.eu/)]([https://www.icos-cp.eu/](https://www.icos-cp.eu/)), accessed on 11 March 2023), and Ameriflux ([[https://ameriflux.lbl.gov/](https://ameriflux.lbl.gov/)]([https://ameriflux.lbl.gov/](https://ameriflux.lbl.gov/)), accessed on 13 March 2023). We used data from 14 forest sites that had a minimum available data period of 10 years, including the annual and monthly data of four environmental factors: temperature, precipitation, VPD, and short-wave radiation (SW), as well as three carbon fluxes: NEP, GPP, and RE. GPP, NEP, and RE mentioned in this study are annual values. The selected flux dataset was  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Site Name** & **Site ID** & **Latitude** & **Longitude** & **Elevation** & **Forest Type** & **Years** & **Samples** \\ \hline Manitoba & CA-Man & 55.87962 & \(-\)98.48081 & 259 m & ENF & 1994–2008 & 15 \\ Saskatchewan & CA-Obs & 53.98717 & \(-\)105.11779 & 628.94 m & ENF & 1997–2010 & 14 \\ Turkey Point & CA-TP3 & 42.70681 & \(-\)80.34831 & 184 m & ENF & 2002–2017 & 16 \\ Bily Kriz & CZ-BK1 & 49.50208 & 18.53688 & 875 m & ENF & 2004–2020 & 17 \\ Oberbärenburg & DE-Obe & 50.78666 & 13.72129 & 734 m & ENF & 2008–2020 & 13 \\ Tharandt & DE-Tha & 50.96256 & 13.56515 & 385 m & ENF & 1996–2020 & 25 \\ Hytytiala & FI-Hyy & 61.84741 & 42.29477 & 181 m & ENF & 1996–2020 & 25 \\ Letttsuo & FI-Lett & 60.64183 & 23.95952 & 111 m & ENF & 2009–2020 & 12 \\ Sodankyla & FI-Sod & 67.36239 & 26.63859 & 180 m & ENF & 2001–2014 & 14 \\ Renon & IT-Ren & 46.58686 & 11.43369 & 1730 m & ENF & 1999–2020 & 22 \\ Fyodorovskoye & RU-Fyo & 56.46153 & 32.92208 & 265 m & ENF & 1998–2020 & 23 \\ GLEES & US-GLE & 41.36653 & \(-\)106.2399 & 3197 m & ENF & 2005–2020 & 16 \\ Howland Forest & US-Ho2 & 45.2091 & \(-\)68.7470 & 61 m & ENF & 1999–2020 & 22 \\ Niwot Ridge & US-NR1 & 40.0329 & \(-\)105.5464 & 3050 m & ENF & 1998–2016 & 19 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Location and details of the selected forest ecosystem flux data with at least ten years of data for evergreen needleleaf forests (ENF) in the normal humid cold temperate climate.  
Figure 1: The specific locations of the 14 selected forest sites. (**a**) A total of six sites on North American continent. (**b**) A total of eight sites on the European continent.  
preprocessed before analysis. Rows with a high number of missing values were removed, and missing values were filled using nearest neighbor interpolation. Data from the flux dataset was quality controlled, with data collated based on a quality control value (QC) greater than 0.7, and missing data were interpolated to eliminate spurious trends caused by sensor degradation and calibration changes. Consistent selection of variables and climate partitioning ensured the reliability of cross-site comparisons [27].