## 3 Results
### Spatial Patterns of NEP in the Three Northern Protection Forest Project Area  
In our investigation, delineated in Figure 4, we employed the Theil-Sen trend assessment and Mann-Kendall tests to conduct an exhaustive examination of the spatial dynamics and evolution of Net Ecosystem Productivity (NEP) within the Three Northern Protection Forest Project Area over the span of two decades from 2000 to 2020. The study revealed significant spatial variations in the interannual patterns of NEP per unit area,  
Figure 3: The distribution of NEP in the Three-North Shelter Forest Program Area from 2000 to 2020.  
with a predominant trend of increasing NEP across the region. Notably, the extent of areas exhibiting an upward trend in NEP significantly surpassed those with a declining trend, indicating a widespread pattern of ecosystem recovery and enhancement. As Figure 5 highlights, our findings demonstrate that a substantial 32.97% of the areas experienced a marked increase in NEP, with an additional 24.18% showing moderate increases. In contrast, only 18.87% of the regions exhibited significant decreases in NEP. This distribution suggests a positive trajectory in ecological restoration and the amplification of ecological benefits within the region, carrying crucial implications for environmental conservation and sustainable development efforts. As illustrated in Figure 4, this study identified significant increases in Net Ecosystem Productivity (NEP) primarily in Xing'an League and Hulunbuir City in Inner Mongolia, Baicheng City in the Northeast, Zhangjiakou and Chengde Cities in Hebei, Yinchuan, Baiyin, and Guyuan Cities in Ningxia, as well as Hotan Prefecture, Bayingolin Mongol Autonomous Prefecture, Western Tianshan, and Altay Prefecture in Xinjiang. These increases are closely linked to local vegetation-restoration efforts, effective environmental management policies, and favorable climatic conditions. Conversely, notable decreases in NEP were mainly observed in Karamay and parts of Kashgar City in Xinjiang, Alxa League in Inner Mongolia, Haixi Mongol and Tibetan Autonomous Prefecture in Qinghai, and specific areas of Jiuquan City in Gansu Province, likely due to human activities and environmental pressures such as overgrazing, land degradation, and water scarcity. Figure 4: Evaluation of NEP trends and their statistical significance. Panel **(a)** illustrates the trend of NEP as determined by the Theil-Sen estimator, whereas panel **(b)** displays the trend analysis results derived from the Mann–Kendall (MK) test. This comprehensive study not only illuminates the ecological health of ecosystems within the Three Northern Protection Forest Project Area but also underscores the critical need for ongoing monitoring and effective management of the region's ecological environment. The observed upward trend in NEP is a testament to the success of long-term conservation strategies and sustainable land-management practices, playing an integral role in promoting and maintaining the ecological balance of the region. Correlation between NEP and Environmental Variables in the Three Northern Protection Forest Project Area  
Between 2000 and 2020, the study conducted a thorough analysis of the relationship between Net Ecosystem Productivity (NEP) and annual mean temperature across the Three Northern Protection Forest Project Area, employing partial correlation analysis and significance testing. This comprehensive approach allowed us to delineate the intricate dynamics between NEP and climatic variables. We observed a broad range of partial correlation coefficients for temperature, spanning from \(-0.91\) to \(0.85\). According to Figure 5, a significant negative correlation between NEP and temperature was identified in \(35.60\%\) of the regions at the \(0.05\) significance level. These findings were predominantly in the Xinjiang Uyghur Autonomous Region (notably in its northwestern parts), Gansu Province, the Inner Mongolia Autonomous Region, and Hebei Province.