## 4 Discussion
### Divergent Responses of Vegetation Change in the Three Agglomerations  
Vegetation provides numerous vital ecosystem services for human well-being; thus, understanding its influencing mechanism is crucial for sustainable urban development. The three urban agglomerations varied in vegetation change and their responses to land use change and climatic factors. Our study found that in unchanged forest lands, vegetation productivity growth exceeded the growth in greenness in BTH. Conversely, in the YRD and PRD, there was a tendency for vegetation greenness to grow faster than productivity. One potential explanation for these differences lies in the varying vegetation types across these regions [26]. The YRD and PRD are primarily characterized by evergreen broadleaf forests, which exhibit rapid growth in leaf area during the vegetation growth process. This rapid leaf area expansion contributes to the faster growth of greenness observed in these regions. On the other hand, the BTH region is dominated by deciduous broadleaf forests and coniferous forests. These forest types exhibit a slower change in leaf area compared with productivity throughout the vegetation growth process. Consequently, the increase in vegetation productivity surpassed the increase in greenness in this region.  
Spatial heterogeneity in vegetation dynamics also existed in the regions with land use change. The transition of various land types, such as croplands, grasslands, and forest lands, into construction lands typically resulted in a decline in greenness and vegetation productivity. However, the extent and nature of these changes varied significantly across the YRD, PRD, and BTH. In the YRD and PRD, this transition led to a decrease in both greenness and productivity. In contrast, in BTH, while greenness decreased, vegetation productivity actually increased. This occurrence can be ascribed to several contributing factors. Firstly, the proportion of grasslands and croplands converted into construction lands in BTH was relatively small (the proportion accounted for 0.23% and 2.39%, respectively), resulting in less significant vegetation loss compared with the YRD and PRD. Secondly, this increase may be linked to urban greening efforts and the establishment of green infrastructure, as in BTH, greenness and productivity increased by 15.69% and 36.18%, respectively, in unchanged construction lands (the growth rate was the largest in all agglomerations). Furthermore, this study highlights that the negative impact of converting vegetation into bare lands is generally greater than that of converting vegetation into construction lands. This observation suggests the indirect positive impacts associated with construction lands, such as green infrastructure construction and urban microclimate [54; 55].  
The effects of climatic factors on vegetation greenness and productivity varied across the three agglomerations. They were negatively correlated with rainfall and positively correlated with Radia in the YRD and PRD. Conversely, in BTH, they were positively correlated with rainfall and negatively correlated with Radia. These variations can be explained by the disparities in vegetation types and climate backgrounds among the three regions. In the YRD and PRD, relatively abundant rainfall ensures that the needs of local vegetation growth are met. However, excessive rainfall can increase cloud cover, which in turn reduces the amount of necessary Radia required for optimal vegetation growth. Therefore, when rainfall is sufficient in these regions, it can actually inhibit vegetation growth, while Radia acts as a promoting factor [26; 56; 57]. On the other hand, in BTH, there is relatively less rainfall and more Radia. This climate condition satisfies the needs of vegetation for Radia. As a result, excessive Radia does not further promote vegetation growth, while Rainfall acts as a positive factor for local vegetation growth in BTH.