## 3 Results
### Historical Trend Analysis of NDVI
Through the decomposition of monthly NDVI data for China using the STL method, the seasonal characteristics and long-term growth trend of NDVI have been clearly identified. Further, according to China's climate classification, the country can be divided into several regions characterized by distinct climatic features, including Temperate Monsoon Climate (TMC), Temperate Continental Climate (TCC), Plateau Mountain Climate (PMC), Subtropical Monsoon Climate (SMC), and Tropical Monsoon Climate (TRMC). The spatial  
Figure 2: NDVI changes in China from 2001 to 2023. Analysis of seasonal fluctuations, long-term trends, and residual components using the STL method: panels (**a–d**) represent the original data, seasonal component, trend component, and residual component, respectively. long-term dynamic changes in NDVI across different climatic zones in China were analyzed using STL decomposition, with results presented in Figures S1-S5. Research has found that although the NDVI in different climate zones has shown different trends and fluctuations over the past 20 years, the trend component exhibits a common characteristic: a general decline followed by an increase. The seasonal component shows clear annual cyclical fluctuations, with each cycle lasting 12 months, demonstrating a distinct seasonal pattern. In the early stage, from 2001 to 2003, the NDVI trend values in different climate zones showed some changes. Specifically, the trend value of TMC slightly decreased from 0.430 to 0.411; the trend value of TCC decreased slightly from 0.145 to 0.142. At the same time, the trend value of SMC dropped from 0.593 to 0.576. Notably, PMC experienced a more significant decline, with its trend value decreasing from 0.197 to 0.180. In contrast, the trend value of TRMC only saw a slight decrease, from 0.659 to 0.640, indicating a relatively stable characteristic during this period. During the period from 2004 to 2008, the trend values in all climate zones generally showed significant improvement. The value of TMC increased to 0.471, and TCC rose to 0.161. Meanwhile, the value of SMC saw a substantial jump to 0.630. PMC also showed signs of recovery, with its value rising to 0.196. In addition, the value of TRMC increased to 0.690. During this phase, the trend values in all climate zones experienced a growth of over 0.05 units, indicating a positive change in vegetation cover. Since 2009, the system has entered a phase of fluctuating adjustments, with significant changes observed in the temperate climate zones. The trend value of the TMC zone fluctuated and decreased from 0.471 to 0.460, while the trend value of the TCC zone dropped from 0.161 to 0.154. Meanwhile, the monsoon climate zones showed a divergent trend: the trend value of the SMC zone decreased from 0.630 to 0.580, and the value of the TRMC zone also dropped from 0.690 to 0.626. In contrast, the PMC zone remained relatively stable between 2012 and 2016, with its value staying around 0.186. In recent years, most climate zones have shown a trend of stability and recovery. Specifically, the NDVI trend value in the TMC zone has rebounded from 0.460 to 0.467, while the TCC zone has increased from approximately 0.146 to 0.152. At the same time, the trend value in the SMC zone has seen a slight increase, rising from about 0.580 to 0.590. The PMC zone experienced a small fluctuation, rising from around 0.183 to 0.189. Additionally, the trend value in the TRMC zone has also improved, increasing from 0.626 to 0.635. To thoroughly examine the long-term dynamics of NDVI in China, this study categorized vegetation types into forest land, cropland, shrulband, and grassland, analyzing each type's NDVI from 2001 to 2023, as shown in Figure 3. The results indicate that the vegetation index for cropland and forest land exhibited certain fluctuations but was generally stable or slightly improved over time; in contrast, grassland and shrubland experienced fluctuations or slight declines, particularly grassland, likely due to natural factors or changes in land use.