## 3 Results
### The Suppressive Effect of Afforestation Projects on NPP
#### 3.3.2 Decoupling of NDVI-NPP  
Although afforestation projects significantly enhance NDVI, they exert a suppressive effect on NPP (Figure 7). Under drought stress scenarios (S1-S3), afforestation-induced \(\Delta\)NPP\({}_{2}\) exhibited consistent negative values across four representative drought events. The most pronounced declines occurred in the TNSP region, spanning the Hetao Plain, Ningxia Plain, western Loess Plateau margin, and Hehuang Valley, where \(\Delta\)NPP\({}_{2}\) reached significantly negative levels. During DEe, afforestation-induced \(\Delta\)NPP\({}_{2}\) consistently remained below \(-80\) gC\(\cdot\)m\({}^{-2}\). This severe carbon loss resulted from intensified water competition in high-density plantations, which simultaneously reduced both soil moisture content and light use efficiency. These coupled constraints ultimately elevated autotrophic respiration, driving significant NPP reduction. However, in the Loess Plateau, the SPMRYR project core zone, the mixed forest structure resulted in \(\Delta\)NPP\({}_{2}\) > 0, forming a local carbon-sink gain. Ideal climate scenarios (S2-S4) further confirm the decoupled climate independence. The TNSP project area maintained negative \(\Delta\)NPP3 values, whereas the SPMRYR area sustained positive \(\Delta\)NPP3 with greater variability than \(\Delta\)NPP\({}_{2}\), demonstrating how vegetation configuration modulates ecosystem functionality (Figure 8). Figure 7: Spatial distribution of \(\Delta\)NPP2 during four typical drought events in the Yellow River Basin (**left**), average NPP over all grids during the drought events folded plot of the distribution (**center**), and fiddle plot of the NPP distribution over all grids during the drought events (**right**). During most drought events, the absolute value of \(\Delta\)NPP\({}_{2}\) exhibited a certain degree of attenuation characteristics. During the DEe, the first month's \(\Delta\)NPP\({}_{2}\) reached \(-\)40.21 gC\(\cdot\)m\({}^{-2}\), while it attenuated to 1.40 gC\(\cdot\)m\({}^{-2}\) at the end. This nonlinear attenuation process suggested that there may be some physiological regulatory mechanisms within the ecosystem to mitigate the negative effects of resource competition. Spatially, the amplification effect showed significant heterogeneity. Across the Hehuang Valley, Ningxia Plain, and Hetao Plain, \(\Delta\)NPP\({}_{2}\)-\(\Delta\)NPP\({}_{3}\) consistently fell below \(-80\) gC\(\cdot\)m\({}^{-2}\). This severe carbon loss primarily resulted from soil moisture content dropping below critical thresholds combined with a persistent imbalance between evapotranspiration demand and precipitation supply. However, in the Guanzhong Plain irrigation area, \(\Delta\)NPP\({}_{2}\)-\(\Delta\)NPP\({}_{3}\) ranging from 0 to 20 gC\(\cdot\)m\({}^{-2}\) due to artificial irrigation, which verified the buffering effect of water management on the amplification effect. Event DEf represented the sole anomaly with positive \(\Delta\)NPP\({}_{2}\)-\(\Delta\)NPP\({}_{3}\) values (+16.94 gC\(\cdot\)m\({}^{-2}\)). This deviation likely reflects its (2008) temporal proximity to the 2000-2004 baseline period, when newly established engineered vegetation lacked stable community structure and consequently exhibited reduced drought sensitivity. The suppressive effect exhibited a nonlinear attenuation with the duration of drought. During the event DEe, \(\Delta\)NPP\({}_{2}\)-\(\Delta\)NPP\({}_{3}\) reached \(-29.6\) gC\(\cdot\)m\({}^{-2}\) in the first month, but it recovered to +1.7 gC\(\cdot\)m\({}^{-2}\) in the fifth month. Vegetation gradually adapted to stress through stomatal regulation and redistribution of photosynthetic products. However, for most events, \(\Delta\)NPP\({}_{2}\) failed to recover to baseline levels within the drought cycle due to  
Figure 9: Spatial distribution and time-series line graphs of \(\Delta\)NPP\(2-\Delta\)NPP3 of four typical drought events in the Yellow River Basin.