Tree shade protection factors by Jacaranda mimosifolia: a summertime comparison of UV dosimetry and whole sky image analysis for residential urban environments

This study aims to characterize Jacaranda mimosifolia in Queensland as a shade protection species for residential urban environments during peak summertime ultraviolet exposure periods, assessed near solar noon and taking into account changes in tree aspect measured relative to the main trunk and with variation in seasonal foliage. Ultraviolet polysulphone dosimeters and whole sky image (WSI) analysis was employed to derive site protection factors (PF) measured on the north, south, east and western sides of trees planted alongside urban street locations in southern Queensland, Australia for the summer months of December, January and February. Dosimetry and WSI analysis of the site PF measured in 5-minute intervals one hour before and following noon confirmed there is a seasonal variation in protection from ultraviolet radiation due to the presence of Jacaranda mimosifolia . Site PF varied from 2 to 12 with protection peaking at the end of summer when canopy foliage is greatest. Protection factors were shown to follow a monthly cycle confirming observed patterns in critical daytime PF either side of noon for the same species derived previously in research from southern Australia. This study contributes new knowledge as the first direct comparison of seasonal and daytime protection factors (PF) for Jacaranda mimosfolia in Queensland, a location of elevated ambient solar ultraviolet. The study also demonstrates that polysulphone film remains a viable tool for the assessment of ultraviolet PF, capable of showing seasonal variation and trends in canopy protection with site aspect. Our correlation of measurements performed using traditional dosimetry techniques with whole sky image (WSI) assessment utilising modelled UV index (UVI), erythemally effective ambient irradiance (E ambient ) and erythemally effective surface irradiance (E site ) demonstrates the potential for WSI analysis to be utilised for more widespread assessments of ultraviolet protection by tree shade in urban environments. • UV comparisons of street tree protection factor (PF) are defined for J. mimosifolia . • Image processing techniques and UV dosimetry measures of PF are compared. • Dosimeters and digital imaging confirm the same median summertime PF of 5.0. • Outcomes define canopy PF for residents living in an elevated UV climate.