## 4 Discussion  
### Spatial and Temporal Distribution Characteristics of EVI and GPP  
Analyzing the spatial and temporal dynamics of EVI (Enhanced Vegetation Index) and GPP (Gross Primary Productivity) is crucial for a comprehensive understanding of regional disparities in ecosystem productivity and vegetation health, as well as their responses to environmental changes. Both EVI and GPP exhibit positive growth trends across the nation and in various climatic zones, indicating significant improvements in vegetation growth conditions and steady increases in ecosystem productivity. Specifically, in humid regions, the substantial growth of EVI and GPP is likely closely associated with positive correlations to temperature and precipitation. Vegetation in these regions is highly sensitive to changes in temperature and precipitation. Warmer climates and increased precipitation provide favorable conditions for vegetation growth and enhanced photosynthesis, thereby promoting better vegetation health and increased ecosystem productivity [23; 24; 25]. Conversely, in arid regions, the growth of EVI and GPP is relatively slow, likely due to limited water resources. Water stress is more pronounced in arid regions, restricting vegetation growth and productivity improvements. Moreover, vegetation responses to climate change vary significantly across different climatic regions. Vegetation in humid regions, with abundant water resources, is more sensitive to temperature changes, whereas vegetation in arid regions is more constrained by water availability [26; 27]. During the study period, all land cover types exhibited a positive inter-annual change trend. This trend reflects the dynamism of land cover types in the context of global change and their response to environmental and socioeconomic factors. The positive change trend suggests an improvement in land cover quality, which may be closely related to the global greening trend, urbanization processes, and agricultural expansion [28; 29]. However, significant differences in change trends were observed among different land cover types, which may be closely related to their respective ecological functions, geographical distribution, intensity of human activities, and policy orientation [30]. These differences indicate that land cover change is a complex process influenced by multiple factors. It is necessary to comprehensively consider the mechanisms of natural and human factors and their interactions [31; 32]. Moreover, land cover change has far-reaching impacts on ecosystem services, biodiversity conservation, and global climate change. Therefore, an in-depth understanding of these change trends and their driving forces is crucial for formulating effective land management policies and sustainable development strategies. The spatial aggregation trends and clustering characteristics of the Enhanced Vegetation Index (EVI) and Gross Primary Productivity (GPP) can reflect the vegetation growth conditions and ecosystem productivity in different regions of China. High EVI values are predominantly concentrated in the southern regions, likely due to favorable climatic conditions (e.g., higher temperatures and precipitation) and the prevalence of vegetation types such as evergreen broadleaf forests. These conditions support robust vegetation growth and photosynthesis. Conversely, the low-value aggregation of GPP in the Yangtze River Delta region may be attributed to urbanization and human activities, which can alter land use patterns and degrade the ecological environment [33; 34]. Additionally, the clustering analysis of EVI and GPP reveals significant differences in vegetation growth conditions between the northern and southern regions, which may be related to the distinct climatic zones and vegetation types. The low-low (L-L) distribution in the northern regions is likely associated with arid and semi-arid climates, while the high-high (H-H) distribution in the southern regions corresponds to humid and subtropical climates.