The effects of urban forms on photosynthetically active radiation and urban greenery in a compact city

Urban greenery is increasingly recognized as an important component of urban ecosystems as it provides a range of ecosystem services for environmental and human well-being. However, the extent to which the performance of urban greenery is affected by availability of light in high-density urban environment is poorly understood. We examined the effect of shade on the levels of photosynthetically active radiation (PAR) in different urban forms, focusing on urban street canyon, vehicular flyovers, sky terraces, at-grade community gardens and rooftop community gardens surrounded by high-rise residential buildings. Within these urban forms, we examined the effects of PAR levels on the growth of shrubs, trees and palms. PAR was assessed though simulation using the building information modeling tool Ecotect, and direct measurements with quantum sensors. PAR was most reduced in sky terraces, with daily PAR restricted to about 15 % of unshaded conditions in Singapore. Daily PAR in rooftop community garden was about 60 % of unshaded conditions, whereas urban street canyon, planting area around and under vehicular flyovers and at-grade community garden had daily PAR of 41 to 52 % of unshaded conditions. Within these shaded environments, almost half of all shrubs and almost all flowering shrubs showed differences in vegetative and reproductive growth at lower PAR levels, respectively. Slenderness of trees was also significantly higher compared to trees grown in more unshaded conditions. The spatial distribution of PAR levels was highly non-uniform, with PAR differing between two and nine-fold within a site, and differed significantly to that of direct sunshine hours. These findings provide insights on how a better understanding of effects of shade on PAR helps to improve urban green space design and plant selection for better plant performance.