Urban expansion drives both loss and compensation in city vegetation productivity

Abstract

Urbanization alters vegetation productivity by both direct (ω_d) and indirect (ω_i) effects. The direct effect is from the change of vegetated area indicated by impervious surface intensity (ISI), while indirect effects arise from changes in urban environmental factors, such as near-surface air temperatures, precipitation, urban heat island (UHI) intensity, and population density (POP). The respective contributions of ω_d and ω_i to vegetation net primary productivity (NPP) under various phases of urbanization are not well quantified. Using multisource remote-sensing data from 1990 to 2020, we analyzed the spatiotemporal variation in urban expansion and the effect that ω_d and ω_i had on NPP in the megalopolis of Beijing, China, over 5-year intervals. During this period, Beijing underwent significant planar expansion rates of about 58.9 km²/yr. Annual mean loss of NPP by ω_d was estimated to be about 77.1 g C/(m²·yr) during the 1990-2020 period, while annual mean improvement to NPP by ω_i amounted to an increase of 28.9 g C/(m²·yr). The NPP losses were partially offset by NPP improvements in the order of 18.6 %-69.3 %. The impact of forcing variables on NPP varied spatially. Air temperature, precipitation, UHI, POP, and ISI explained about 13.8 %, 23.2 %, 23.7 %, 14.7 %, and 24.6 % of the spatial variation in NPP. The impact of air temperature on NPP was related to available moisture, negatively affecting NPP in regions with water deficits. Our findings demonstrate the dual impact of urbanization on vegetation and underscore the necessity for spatially adaptive ecological management strategies in regions experiencing rapid urban growth.