Building shading affects the ecosystem service of urban green spaces: Carbon capture in street canyons

The urban building environment has dramatically transited into a high-rise style. Consequently, street canyons have changed the daytime solar radiation distribution and indirectly affected the photosynthesis of the surrounding ecosystems. Because of the interdisciplinary nature of the topic, quantitative studies are rare. In this study, a new approach combining radiation simulation and in situ observations was proposed, aiming to provide support towards sustainable urban spatial planning and management. Taking the Central Business District (CBD) of Beijing as the research area, the range and intensity of the shading effect were assessed based on a Digital Surface Model (DSM) and Solar Radiation Analysis (SRA). The findings reveal the following: 1) The daytime carbon capture of green space was 33.14 ± 12.43 gCO2 m-2 d-1, which included 85.6%, 11.9%, and 2.5% from trees, shrubs, and grassland, respectively. 2) The Carbon Capture Index (CCI) of the arbor zone was approximately two and five times that of shrubs and grassland, respectively. The sensitivity of carbon capture to the building height increased in the order shrubs < grassland < trees. To enhance current carbon capture, trees and shrubs should play a dominant role in urban greening planning. 3) When constructing enclosed buildings and planting inside, as well as to the north of high buildings in northern hemisphere cities, a setback line is strongly suggested according to the building height. 4) When the building height increased by 10% or by one floor, the carbon capture declined by 0.29 tCO2•km-2 d-1 or 0.40 tCO2•km-2 d-1, respectively. These results suggest that the modern urbanization process reduces the utilization efficiency of daytime solar energy and potentially impairs the ecological service of the urban green space.