Botanists have created a ‘plant selection guide’ of tree species and their traits that can help to combat air pollution.
The black pine, the common yew, the Spanish oak and Turner’s oak all combat air pollution caused by road traffic, including harmful nitrogen oxides.
UK researchers identified 12 ‘influential’ traits for 61 tree species that can affect their effectiveness as a barrier against pollution.
These include small leaf size, high foliage density, long in-leaf periods and even ‘leaf hairiness’.
Of the 61 species, the researchers say 12 in particular can help form an effective physical barrier between humans and areas of high pollution if grown in quantities.
There is evidence that ‘green infrastructure’ can divert and dilute clouds of pollution and reduce outdoor concentrations of pollutants.
Certain biophysical traits, such as leaves with ridges and grooves, can also help them trap micro particles in the air.
The team’s research paper could help urban planners, landscape architects and garden designers make better decisions on what vegetation to use in built up areas with high levels of pollutants.
‘The use of green infrastructure as physical barriers between ourselves and pollutants originating from our roads is one promising way we can protect ourselves from the devastating impact of air pollution,’ said Professor Prashant Kumar, founding director of the Global Centre for Clean Air Research at the University of Surrey.
‘We are all waking up to the fact that air pollution and its impact on human health and the health of our planet is the defining issue of our time.
‘We hope that our detailed guide to vegetation species selection and our contextual advice on how to plant and use green infrastructure is helpful to everyone looking to explore this option for combatting pollution.’
Transport emissions are a dominant source of urban air pollution, and come in the form of gas, such as carbon monoxide and nitrogen oxides, as well as particulate matter (PM), including black carbon.
Even non-exhaust-related emissions from traffic, such as wear from brakes against the rubber wheels, are considered significant sources of PM.
Professor Kumar said that air pollution is responsible for one in every nine deaths each year – a number that could intensify with projected population growth.
The University of Surrey team conduced a review of various research papers on the effects of trees and hedges for their guide, which has been published in NPJ Climate and Atmospheric Science.
The 12 beneficial influential traits they identified were five leaf characteristics – type, size, complexity, margin (the boundary area extending along the edge of the leaf) and surface, or ‘leaf hairiness’ (glossy or smooth).
The remaining seven traits were canopy density, size when mature, drought tolerance, salt tolerance, biogenic volatile organic compound (BVOC) emissions and pollen emissions, as well as successional size – the process of change in the species structure of an ecological community over time.
‘For example, some early successional species, such as Betula pendula (silver birch), generally thrive in open sites that may be characterised by conditions of high light and wind exposure and poor soil quality,’ Professor Kumar said.
‘Late successional species, such as Fagus sylvatica (European beech or common beech) tend to be shade tolerant and thrive in humid conditions with nutrient-rich soil.’
However, two of the 12 traits – BVOC and pollen emissions – are detrimental to air quality as far as human air intake is concerned.
While pollens can cause hay fever, BVOC in the presence of nitrogen oxides emitted from traffic and sunlight can produce a secondary pollutant – ozone – ‘to make things worse’, Professor Kumar said.
By including these traits, green planners using the guide can weigh up the disadvantages with any possible advantages, along with the environmental context in question.
The effectiveness of a plant in protection against pollution are determined by such a context – such as whether it will be used in an open road, a residential area or a city centre.
Previous research has highlighted the benefits of employing porous greenery as opposed to solid barriers, including walls and parked cars to mitigate air pollution.
The researchers conclude that the context and conditions of the planting site, and the target air pollutant type are taken into account, the selection of plants and their traits can act as a ‘roadside vegetation barrier’.
The team narrowed down the 61 species to 12 (evergreen oaks, pine species and one yew) that are potentially most effective.
These 12 were chosen as such because they are relatively tolerant of pollution and include beneficial leaf traits, as well as other positive traits.