Some trees are making air pollution worse in cities.
- Eduardo García Rodríguez
- Sep 2, 2022
- 7 min read
Updated: Apr 3, 2023
You wouldn’t have thought it, after all the emphasis in adding some greenery to urban landscapes, but some city trees may be actually be worsening the conditions of the air we breathe.
A 2016 report from the National Institute for Health and Care Excellence (NICE) raised alarms about what kind of trees could combat air pollution and what type of vegetation cities should use to help improve air quality. The report showed that leafy, green avenues at street level, where vehicles emit the most pollutants, can have higher than poorer air quality due to restricted ventilation due to overhead canopies.
The 60-page report stated that “Street trees were unlikely to reduce air pollution in most street designs and could worsen in some cases” because “leaves and branches slow down air currents, causing pollutants to settle out”.
This wasn’t the first study to reach this conclusion. A 2012 study by Belgian researchers also measured air quality and pollution concentrations on roadsides with urban vegetation and saw that trees on city streets could be contributing to reduced ventilation.
Five years later, we revisit the case surrounding trees in urban areas and how cities should take care when making these decisions to guarantee to safeguard clean air in cities.
The link between trees and air pollution is nothing new
The NICE study came with some significant timing. With several European capitals trying to adopt measures to improve their air quality, amid an alarming rate of deaths related to pollution, there were calls for tougher regulation against polluters, who have benefited from government complacency regarding extras fees for diesel cars in urban areas.
The call is now to adopt more efficient measures for tracking and reporting pollutants and improving and encouraging more eco-friendly practices such as car swapping, public transport or cycling, amongst other factors that may harm the environment. The effect of trees on specific urban landscapes must also be analysed.
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Trees are still vital, but not all are equal.
This report should not underestimate the importance of trees and plants when designing infrastructure projects. Urban vegetation has been proven to help mental and physical health, reducing stress and increasing exercise habits.
However, whilst trees do effectively pull ozone and particulates into their leaves and out of the air that we breathe, care must be taken when dealing with green spaces. With councils being cash-strapped, cities and towns are losing their high-maintenance green spaces to make room for open but artificial spaces that facilitate residential construction.
Keeping green spaces requires thoughtful consideration and effort. Street design, tree placement and density and the effects of seasonal changes and wind levels must all be taken into account in each specific case.
Trees are vital, but not every tree has the same effect on its environment. Factors like the pungency and characteristics of some pines and cedars have been to come from organic compounds that can produce ozone when interacting in sunlight with the nitrogen oxide from car emissions that can be highly harmful to people.
Measures to ensure efficient green urban mapping.
Whilst it turns outs that it is not as simple as planting any tree to have adequate urban vegetation, the “hassle” is undoubtedly worth it. Initiatives like Transport of London’s proposal to invest more money in cleaning the city’s air pollution should incorporate projects to monitor the effects specific trees have on their surroundings. New York City, for example, took a step further with its TreesCount initiative, which maps all of its trees with unique identification codes and photographs to monitor each individual tree’s rainwater retention, reduction in air pollution and ecological benefits, which could bring interesting results as to which best trees adapt to which conditions. The City of London and other major cities like Paris could take valuable lessons from this as part of their tree planting initiatives in the city.
You wouldn’t have thought it, after all the emphasis on adding some greenery to urban landscapes, but some city trees may be worsening the conditions of the air we breathe.
Paris mayor Anne has unveiled plans to plant trees and gardens alongside four of its historic sites, including Hotel de Ville, to improve the air quality and address climate change.
Trees and Air Pollution in 2022. Choose the best trees to beat air pollution.
We have seen that not every tree counts when it comes to curbing the impact of climate change. So, what trees are the best for tackling air pollution?
As cities grow, they are increasingly taking green space. Clearing forests has been a common activity since prehistoric times. However, modern urban planners are increasingly appreciating the importance of greenery and incorporating them into landscapes and also bringing wildlife back to the city.
With cities planning on deploying green spaces in their city centres, some trees are more effective at filtering pollutants than others and improving air quality.
The trees chosen to protect air quality can do so directly or indirectly. For example, planners should also consider trees that can provide a lot of shade and reduce temperatures. Buildings that are surrounded by trees that provide shade are generally cooler and do not need to use that much air conditioning, subsequently reducing the emission of greenhouse gases and the risk of harmful pollutants that usually spike on hot days.
Cities may revert to Conifers, like pines and cypresses, to retain particulate matter (PM) in the air. These trees are evergreen but also efficient at trapping PM in their hairy leaves. Overall, trees with rough, rugged and hairy surfaces are the best filters for PM. But it is also these types of trees that can react with substances from vehicles and suddenly reduce air quality.
However, other trees are also effective. Silver birch, yew and elder trees and honey locust are also great at capturing particles. Nettles are not too efficient at capturing PM.
What trees should cities avoid?
When it comes to trees that can worsen air quality due to their reaction with chemical substances emitted from cars, some deciduous trees can provide some negative side effects. Poplars or black gum trees, for example, can emit high levels of volatile organic compounds (VOCs). Other deciduous trees like elms, horse chestnut trees and basswood emit fewer VOCs.
Interestingly, in some places like Chile, it is often non-native species that produce more VOCs than native species, while in others, non-native species thrive as the best solution for air pollution reduction.
In the end, it´s about finding the right balance and the right trees for each city. Each city has its own context. With this in mind, Prashant Kumar, the founding director of the Global Centre for Clean Air Research at the University of Surrey, presented a guide to help urban planners choose species that can help mitigate roadside pollution, along with recommendations for tree-planning schemes in cities. In his guide, Kumar lists 61 species and 12 traits suited for air-pollution reduction, as well as undesirable variables like pollen, VOCs and high-maintenance species. His conclusion: Pine species, conifers, evergreen oaks and common yew are the most effective trees.
Where do you plant the best trees to combat air pollution?
Choosing the most appropriate trees is only part of the journey. The next step is knowing where to plant them. Many cities like Beijing and Mexico City often have green areas far from city centres, which ultimately is not too beneficial. Trees need to be planted close to where people and sources of pollution are found while also keeping in mind factors like landscape structure and wind direction.
Narrow streets found in areas like Rome or downtown Manhattan can leverage airflow to trap pollutants that are close to the ground. But in these areas, tall trees with big canopies can worsen conditions by preventing the pollution from dispersing. In this scenario, hedges or green walls are preferable options to trees. Some examples of hedges that work well include red tip photinia, viburnum, privet and bay laurel.
Broad roads surrounded by low-rise buildings, as can often be found in American suburbs, allow air to flow more freely, so trees and hedges are viable options there.
A final factor to bear in mind is to consider planting trees that are relatively low maintenance and have long lifespans.
Biodiversity is crucial in urban tree planning.
Even if a specific tree stands out as a champion against pollution, urban planners should ensure that biodiversity is respected in green areas. No more than 5-10% of urban forests should consist of the same species or family.
There are so many variables to consider that it is easy to understand why planning for green areas and knowing which species to use where can be challenging. What´s more, monitoring and measuring the efficiency of these trees can be difficult, too, as reports and tools can differ between scientific teams.
Scientists, however, are increasingly using forest management tools intended to help urban designers identify and select the best species for each unique location.
4 Examples of Forestry Management Software that can help urban planners
As mentioned on their website, i-Tree Species is a tool that helps urban planners select the most appropriate tree species based on their potential benefits and their geographic area. The tool ranks species based on a set of variables, including carbon storage, air pollution removal, stormwater impacts, wind reduction, air temperature reduction, VOC emissions and more.
A turnkey solution for local governments and organisations, Forestree manages the location, management, maintenance and watering of public trees. Focused on the Australian and US markets, the tool can help planners reduce risks, manage planting and maintenance and access data to make data-driven decisions when planning their urban forest.
Companies are trying to make the most out of new technologies to improve urban forestry. PlanIT Geo is a global urban forestry consulting and software development firm that uses new technologies to plan urban forestry, parks and arboriculture. The tree plotting tool provides mapping and visualisation instruments to help cities and governments build Greenspace strategies and policies and carry out tree care operations.
ArborPro is an Urban Forestry Consulting Firm Tree Management Software that provides cities and institutions with tree inventories, reporting and diagnostics across the United States, though it has yet to expand to other countries.
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