## 3 Results  
### Spatiotemporal Variations of GPP  
Figure 3 illustrates the spatiotemporal changes in GPP across ecological restoration areas in Northern China from 2001 to 2020. During this period, the annual average GPP exhibited a significant upward trend, increasing at a rate of 6.7 gC m\({}^{-2}\) per year, with a coefficient of determination (R\({}^{2}\)) of 0.82. GPP values rose from 316.7 gC m\({}^{-2}\) in 2001 to 483.3 gC m\({}^{-2}\) in 2020, representing a substantial 52.6% increase over the two decades. We conducted a change point detection for GPP from 2001 to 2020 and found a turning point in 2011. Before 2011, the GPP increased by 4.9 gC m\({}^{-2}\) per year, with an R\({}^{2}\) of 0.39. After 2011, the increase was 6.5 gC m\({}^{-2}\) per year, with an R\({}^{2}\) of 0.61. Figure 4 and Table 2 present the spatial distribution and statistical analysis of GPP across Northern China's ecological engineering areas from 2001 to 2020. The annual average GPP within the region exhibited substantial variability, ranging from 0.005 gC m\({}^{-2}\) to 1438.9 gC m\({}^{-2}\). The multi-year average GPP was significantly higher in the APTM region compared to SPMRYR, with SPLR and TNSP displaying progressively lower values. High GPP values were predominantly observed in Qinling, Taihang, and the Yanshan Mountains, with a general decreasing trend from east to west and from south to north across the study area. By 2020, GPP had increased across all four main regions compared to 2001, with increments of 127.0 gC m\({}^{-2}\) (TNSP), 306.3 gC m\({}^{-2}\) (APTM), 145.5 gC m\({}^{-2}\) (SPLR), and 328.4 gC m\({}^{-2}\) (SPMRYR), corresponding to 46.4%, 66.6%, 33.7%, and 94.1% of the 2001 GPP values, respectively. These increases represent significant growth, particularly in the APTM and SPMRYR regions, highlighting substantial improvements in vegetation productivity over the past two decades. Figure 5 illustrates the long-term trends in GPP from 2001 to 2020, with corresponding significance levels as detailed in Table 2 (\(p<0.05\) and \(p<0.01\)). The GPP trends ranged between \(-41\) gC m\({}^{-2}\)yr\({}^{-1}\) and \(48\) gC m\({}^{-2}\)yr\({}^{-1}\). A substantial portion of the areas under ecological restoration programs exhibited increases in GPP. Specifically, \(60.9\%\) of the regions demonstrated highly significant enhancements in vegetation greenness, \(14.0\%\) showed significant improvements, and \(22.2\%\) experienced non-significant improvements. Only \(2.9\%\) of the areas saw vegetation degradation over the last two decades. Significant GPP increases were predominantly observed in the northeastern Three-North Shelterbelt, Taihang Mountains, Loess Plateau, and other regions. In contrast, some areas in Qinghai, Xinjiang, and northwestern Gansu exhibited non-significant or slight decreases in GPP. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **TNSP** & **APTM** & **SPLR** & **SPMRYR** &
\begin{tabular}{c} **The Entire** \\ **Areas** \\ \end{tabular} \\ \hline Mean & 355.1 & 642.3 & 527.5 & 552.3 & 422.0 \\
2001 & 273.7 & 459.7 & 432.3 & 349.1 & 316.7 \\
2020 & 400.7 & 766.0 & 577.8 & 677.5 & 483.3 \\
2020–2001 & 127.0 & 306.3 & 145.5 & 328.4 & 166.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Distribution of GPP (gC m\({}^{-2}\)) in the TNSP, APTM, SPLR, and SPMRYR regions for the years 2001, 2020, and multi-year average from 2001 to 2020 (“mean”). “2020–2001” represents the GPP differences between 2020 and 2001. Abbreviations: TNSP, Three-North Shelterbelt Program; APTM, Afforestation Program for Taihang Mountain; SPLR, Shelterbelt Program for Liaohe River; SPMRYR, Shelterbelt Program for Middle Reaches of Yellow River. Figure 3: Interannual variations of annual average GPP (unit: gC m\({}^{-2}\)). The trends of GPP dynamics from 2001 to 2020 (green solid line), 2001 to 2011 (blue dotted line), and 2011 to 2020 (red dotted line) were plotted. Figure 4: Spatial distribution patterns of GPP in the ecological restoration programs in 2001 and 2020.