Assessing the Spatiotemporal Characteristics of NPP and the Response to Urban Expansion in the Chang–Zhu–Tan Urban Agglomeration, China

Abstract Background In recent years, accelerated urbanization, accompanied by rapid economic growth, has substantially influenced the ecological environment and carbon storage. To examine the relationship between urbanization and carbon storage, the vegetation net primary productivity (NPP) of the Changsha–Zhuzhou–Xiangtan urban agglomeration (CZT) over the past two decades was estimated using the Carnegie–Ames–Stanford approach model. The multiyear spatiotemporal evolutionary characteristics of NPP changes and their responses to urban expansion were explored using various trend analysis methods. Results From 2000 to 2020, a fluctuating and moderate declining trend in the yearly average NPP was observed. In particular, after 2008, there was a noticeable acceleration in the rate of NPP decline, which was attributed to the implementation of the urban integration policy in the CZT. Furthermore, an increasing trend in NPP was noted in the core of the built-up region, which was attributed to the government's efforts toward urban greening. However, with the continuous outward expansion of the Changsha–Zhuzhou–Xiangtan region, a considerable reduction in NPP was observed in the suburbs. Future variations in NPP for most parts of the CZT will be contrary to those observed in the past. The primary factors influencing NPP changes in the CZT have gradually shifted from natural factors to human activities. Although a weak negative correlation was observed between the built-up area density and NPP in 63.04% of the CZT and urban expansion predominantly negatively affected vegetation NPP, the positive contribution of human activities to NPP still outweighed the negative contributions. Conclusion Measures to protect the ecological environment have effectively mitigated the reduction in vegetation carbon storage within urban agglomerations. This study provides a rigorous theoretical basis for the integrated development of urban expansion and environmental protection, as well as for carbon management.