## 5 Conclusions  
Based on the data of gross primary productivity in the BTH from 2000 to 2020, this paper analyzed the spatial distribution and temporal changes of GPP in the past 21 years, the growth trend, and the correlation between the change trend and surface temperature, precipitation, downward surface shortwave radiation, etc. In addition, geodetectors were used to quantitatively detect the driving mechanism of each influencing factor on the spatiotemporal variability of GPP in the BTH by using the five selected factors, which include land surface temperature, precipitation, short-wave radiation, nighttime light data, and land use type. The main conclusions are as follows:  
1. As far as spatial distribution is concerned, from northwest to southeast, the value of GPP gradually increases from a low value and then gradually decreases, which showed a low-high-low trend. The GPP of grassland and cultivated land in the Zhangjiakou region was in the range of 300 to 500 g C/m\({}^{2}\), while that of the Yanshan and Tai-hang Mountains was in the range of 600 to 850 g C/m\({}^{2}\), and that of cultivated land was in the range of 300-500 g C/m\({}^{2}\). The growth trend and spatial distribution showed a similar trend, and the Yanshan and Tai-hang Mountains showed a strongergrowth trend, with a growth rate of 20-50 units. The growth trend was relatively slow in the northwest, while the growth area decreased in the central and southern plains, and no significant increase was observed in most regions. However, 68.73% of the area in the whole BTH showed an increasing trend.
2. In terms of time-series, GPP in the BTH presents a significant growth trend and has obvious seasonality, with a faster growth trend in summer and slower growth in spring and autumn. It also showed a relatively fast trend throughout the growing season.
3. The driving factors of GPP spatial differentiation in the whole BTH are land surface temperature, land use type, and nighttime light data, while precipitation and short-wave radiation contribute less. The increase of night light data indicates that human activities have an increasing influence on the spatial distribution of GPP.
4. The interaction among the five factors showed significant enhancement effects. In the future, proper attention should be paid to the effects of human factors and synergistic effects of multiple factors on the spatial differentiation of GPP, and the driving mechanism of spatial distribution of GPP should be further studied.  
Conceptualization, H.G. and C.C.; data curation, H.G. and X.Y.; formal analysis, H.G.; funding acquisition, C.C. and M.X.; investigation, H.G. and M.X.; methodology, H.G. and K.W.; project administration, C.C., M.X. and X.Y.; resources, C.C.; software, H.G. and Y.C.; supervision, C.C.; validation, H.G. and J.L.; visualization, H.G. and X.G.; writing--original draft, H.G., M.X. and X.Y.; writing--review & editing, H.G. and R.S.D. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the project of the National Key R&D Program of China grant number 2021 YFB3901104 and the National Natural Science Foundation of China grant number 41971394 and 2020 Report on Forestry Technological Developments and Monitoring and Assessment of Terrestrial Ecosystem Research grant number 2020132108.  
Not applicable.  
The authors declare no conflict of interest.