City landscapes: Pay attention to detail

In gardening terminology there is a much-used phrase: “Right plant, right place”. The more we learn about the multi-functional capacity of plants to deliver benefits for urban environments, the more appropriate this term becomes to city landscapes.

At the recent AIPH International Green City conference this message came across clearly. Beijing, host city to the conference, has ambitious plans to expand green space by more than 60,000ha to address air pollution and deliver ecological benefits to the city. Research at Beijing Forestry University, presented by Prof Yu Xinxiao, explores the science behind how plants capture the particulate matter component of air pollution, and how this research informs expansion of the city’s green space. Different vegetation components (shrubs, grass and trees) have different capacities to capture particulate matter, and different species provide further variation. It is also essential to consider placement of plants when applying these results to practice.

DEFRA report

The recently published DEFRA report (UK’s Department for Environment, Food and Rural Affairs) on the Impacts of Vegetation on Urban Air Pollution reinforces the importance of placement of plants as well as informed species selection. Configuration and density of trees has a major impact on airflow and turbulence, both of which affect dispersion of pollutants. Prof Paul Monks, Chair of the DEFRA Air Quality Expert Group, told the conference that he is confident that trees in cities are tremendously important, and encourages authorities and residents to continue with their efforts to increase tree planting. While trees may moderate air pollution on a local scale, however, he cautioned that they should not be seen as the answer to city-wide air pollution. Their benefits are numerous and species selection should consider priorities and multiple benefits. Ranking tree species according to their capacity to deliver various benefits aids selection (Forest Research, UK). For example, Oak spp sequester more carbon that London Plane, yet have a lower capacity to remove air pollution.

Traffic patterns

A different study on air pollution showed that traffic patterns influence exposure of pedestrians to pollution. It is obvious that in urban street canyons more traffic and idling traffic will increase local pollution. Less obvious is that free flowing traffic creates air turbulence that both disperses air pollution, and brings it into greater contact with tree canopies thereby increasing their potential capacity to capture air pollution. Integrated urban planning and policy development need to consider traffic flow patterns alongside urban greening strategies. Commonly, turbulence induced dispersal pushes air pollution upwards, which opens the question of the importance of Green Walls and Green Roofs in relation to air pollution.

Green walls and roofs

Green walls and green roofs are becoming increasingly important in cities as potential greenspace, largely because this is a vast space available for greening in highly built urban areas. Melbourne has more than 300ha of space for green roofs; London has 150ha of suitable roofs available, and Chicago already has well in excess of 50ha of green roofs installed. Impressive results are becoming available to enable us to choose particular species, and even varieties by their known capacity to deliver on ecosystem services

To round off the conference theme of “Urban Greening for Clean. Healthy Cities”, the audience heard about examples of exciting projects and plans around the world. Presentation and videos are available on the AIPH website.

AIPH International Green City Speakers (left to right) Mr Yu Bo, Ms Judith van der Poel, Prof. Paul Monks, Dr Audrey Gerber, Mr Li Zheng, Mr Gao Dawei, Mr Bill Hardy (AIPH Green City Chair and representative of the Canadian Nursery and Landscape Association).

Different vegetation components (shrubs, grass and trees) have different capacities to capture particulate matter, and different species provide further variation. This photo was taken at Expo 2019 Beijing.