Potential of urban roadside hedges for particulate matter removal – importance of species choice, and its economic impact

Urban hedges provide multiple ecosystem services including microclimate regulation, flood and pollution mitigation, and biodiversity provision. Their relatively compact nature may suit higher-density housing developments more readily than many other forms of green infrastructure. However, hedge plant species differ in their capacity to promote different services, so it is important that the decision to plant hedges is evidence-based. Our research focused on testing which hedge species (and which associated leaf and canopy properties) are required to maximise the capture and retention of airborne pollutant particles, which are harmful to human health. We collected leaf material from species with morphologically different leaves, located by major and minor roads in Reading (UK) after periods of dry summertime weather and analysed the quantities of deposited particulates. The species included hairy-leaved Cotoneaster franchetii Bois, waxy/smooth-leaved Acuba japonica Thunb. and Crataegus monogyna Jacq., along with coniferous Thuja plicata Donn ex. D. Don. Results show that hedges with hairy, rough, and oval leaves (e.g. Cotoneaster), best captured airborne particles. Results also suggest that a significant extent of coverage and hedge depth is required, particularly to reduce the concentration of fine particles and in species with smaller inherent capacity for particulates' capture. This is evidenced in similar concentrations of particulates we detected at the front and back of Crataegus and Thuja even when the hedge depth was in the 1.5-2 m range, suggesting that a greater hedge depth (and/or denser planting) is needed to reduce concentrations of fine particles. Furthermore, planting choice made a significant difference to the extent of capture only on major roads, where the pollutant concentrations are highest. Based on experimental measurements of particulates' capture we estimated the impact of planting 100 km of the best-performing species, Cotoneaster franchetii, in Reading over an appraisal period of 10 years. Results (central estimate) indicated that approximately £300,000 of health, environmental and economic damage costs to society could be avoided from road transport-associated PM10 and £15,000 of PM2.5 costs