## 4 Discussion  
### Response of Peak Vegetation Growth to Climate Data  
The research findings indicated that there were similar spatial patterns between NDVImax and GPPmax on the TP. In most regions, both NDVImax and GPPmax showed increasing trends. Approximately 65.28% of the pixels exhibited an increasing tendency for NDVImax, with 16.35% showing a significant increase (Figure 2c). Similarly, approximately 72.98% of the pixels showed an increasing tendency for GPPmax, with 16.69% displaying a significant increase (Figure 2f). These findings were in line with the one-time observations of widespread and sustained global greening documented in remote sensing studies [14; 15; 50]. In contrast,  
Figure 7: The distribution of mean ridge regression coefficients of NDVImax (**a**) and GPPmax (**c**) to VPD along gradients of annual mean temperature and annual total precipitation. The distribution of mean ridge regression coefficients of NDVImax (**b**) and GPPmax (**d**) to SM along gradients of annual mean temperature and annual total precipitation. SM and VPD exhibited distinct spatial patterns. From 2000 to 2022, SM decreased in approximately 70.86% of the regions, with approximately 25.82% of the regions displaying a significant decrease (Figure 2i). On the other hand, VPD increased in approximately 74.75% of the regions, with approximately 31.86% of the regions showing a significant increase (Figure 2l). These findings were consistent with previous research indicating that SM and VPD exhibit opposite trends [51]. The evolution of drought involves a complex transition of water resource shortages. It begins with meteorological anomalies such as high atmospheric temperatures, low precipitation, and high evapotranspiration, and continues for a certain period, resulting in abnormal energy transfer within the hydrosphere due to water vapor interactions. Here, it can be seen that the continuous decrease in SM may exacerbate soil drought, and the increase in VPD represents an increase in the atmospheric evaporation demand, which may intensify meteorological drought. However, the formation of drought is influenced by multiple factors and requires more in-depth research. By studying how SM and VPD affect vegetation peaks, we can observe that NDVI\({}_{\text{max}}\) and GPP\({}_{\text{max}}\) exhibit similar trends. NDVI\({}_{\text{max}}\) is positively correlated with SM and negatively correlated with VPD, and the correlation between them and GPP\({}_{\text{max}}\) is consistent with NDVI\({}_{\text{max}}\). However, overall, the inverse correlation between vegetation peaks and VPD is particularly significant in the northeastern area of the TP. Moreover, the significant positive correlation between vegetation peaks and SM predominates; although the regions vary, they remain largely consistent. This indicates that VPD mainly adversely affects vegetation peaks, while SM helps to promote the growth of vegetation peaks. On the one hand, SM directly determines the amount of water that plant roots can extract, akin to a reservoir for plants [31]. Thus, SM can directly influence the growth status of the vegetation, subsequently affecting the vegetation peak values. On the other hand, VPD has the potential to prompt plants to close their stomata to decrease water loss [52; 53; 54; 55], which could limit the photosynthesis within the ecosystem [31; 35]. VPD has a significant impact on the physiological activities of plants. When VPD is high, plants will close the stomata on their leaves to prevent rapid water evaporation. While this behavior helps to reduce water loss, it also hinders the entry of carbon dioxide (CO\({}_{2}\)), thereby affecting the efficiency of photosynthesis. Specifically, when VPD exceeds a certain critical value, it will limit the photosynthesis and growth of the plants. In the face of high-VPD conditions, the plants will adjust the opening and closing of stomata to seek a balance between CO\({}_{2}\) absorption and water loss, maintaining a higher photosynthetic rate and thus improving the water use efficiency.