Early advantage for carbon sequestration of monocultures and greater long-term carbon sink potential of broadleaf mixed forests: 20-year evidence from the Shanghai Green Belt

In the context of carbon neutrality and urbanization, there is a notable research gap regarding the carbon sequestration benefits of urban forests and the identification of suitable tree species for achieving sustainable carbon sequestration. In this study, we examined the dynamics and drivers of urban forest carbon sequestration in Shanghai, one of the most urbanized cities in China. First, we compiled a comprehensive field inventory data set spanning 20 years for the Shanghai Green Belt. Second, we quantified and compared the total carbon accumulation and growth rates of six monoculture forests and two mixed forest types consisting of six common tree species. Third, we conducted drivers analysis of the effects of stand-related and anthropogenic factors on carbon accumulation and growth rates. Overall, our results revealed that each forest type exhibited a unique growth pattern of tree biomass carbon (TBC), with the Chapman–Richards growth function providing an accurate representation of carbon dynamics. During the early stages of development (within 20 years), the carbon accumulation rate of Populus L. reached 4.13 Mg ha−1 yr−1, which was approximately 127 % higher than that of broadleaf mixed forests. In late stages, the potential tree biomass carbon accumulation of broadleaf mixed forests reached 172.8 Mg ha−1, which was 34 % higher than that of Populus L. over the entire growth cycle. Forest types, stand age, canopy density class, ownership, closure management, strongly influence the accumulation rates of TBC. Overall, these findings contribute to the understanding of the advantages of mixed and monoculture plantations for carbon sequestration, and these results can be used to predict future patterns of carbon storage in urban forests in Shanghai.