The differential impacts of the spatiotemporal vertical and horizontal expansion of megacity Dhaka on ecosystem services

The relationship between ecosystem services (ES) and urbanization is crucial for sustainable development. Rapid urbanization threatens the natural capital of Dhaka city, affecting the delivery of ES through changes in land use and land cover. We used vertical growth (VG), nighttime light data (NTL), and population density (PD) as standard urbanization indicators alongside technomass, a three-dimensional indicator, to evaluate the degree of urbanization as a continuous spatial process. We modeled the spatiotemporal relationships between urbanization degrees and ES using the ecosystem service value (ESV) dataset applied in Dhaka, with regional modified value coefficients. Results from the geographically weighted regression (GWR) model showed that technomass emerged as a more appropriate indicator to analyze urbanization for ESV analysis (r > 0.61), followed by NTL (r > 0.56), and PD (r > 0.54) across all zones from 2000 to 2021. We observed a 68.34% decrease in net ESV, equivalent to $245.88 million (in 2021 USD), from 2000 to 2021. This decline was driven by the conversion of water bodies (-70.93%), agricultural land (-60.08%), forest and vegetation (-70.18%) into urban built-up areas and other uses. In contrast, net technomass increased by 243.11% due to the city's vertical growth. The digital building height (DBH) model revealed that the built-up area had expanded by 94.94% over the study period, with an average annual growth rate of 4.52%. Significant correlations (p < 0.05) were observed between ES and urbanization. The rural-urban fringe area exhibited the most significant increase in urbanization (r > 0.90), along with a 440.47% growth in technomass. Our results provide insights into the impact of urbanization on ES, particularly at the regional scale, and have highlighted the importance of integrating VG and technomass for urbanization analysis. These findings could be useful for environmental management, policymaking, spatial planning, and coordinating future ES protection and urban development.