The best trees to improve the face (and air) of cities

Trees play an important role in reducing emissions and smog in big cities. There are species that are more suited for this purpose, here are a few.

From Chinese metropolises to European cities, most urban centres have been fighting the problem of pollution for decades. Some, like Paris, have put forward a draconian plan to reform mobility, proposing a future ban on the most polluting cars. Nevertheless, there’s another key element to solve this problem: increasing the space dedicated to urban forests or green areas. Relying on the role of trees in cities is vital not only for air quality at the local level but also in order to fight climate change, in an attempt to fulfil the objectives set by the Paris Agreement.

paris trees avenue
An avenue lined with trees in Paris © K.G.Hawes/Flickr

Green spaces are a source of wellness for all living beings, including humans. In conurbations they can have many functions, including the reduction of “heat islands” with consequent decreases in temperature and pollutants, and their ability to absorb atmospheric CO2. But there are some tree species that are particularly suited to these purposes due to their physical and physiological characteristics. It’s recommended to opt for them when expanding urban forestry in big cities.

Read more: Urban forests, cities’ answer to climate change (and much more)

Trees that reduce air pollution

Bologna‘s Institute of Biometeorology in Italy, led by Doctor Rita Baraldi, compiled a list of trees that can reduce air pollution. “This study, a cooperation between Bologna Municipality and the European Life+ project, set out to find out which plants are best suited to a city like Bologna,” Doctor Baraldi explains. Considering a life cycle of 30 years, researchers evaluated the ability of each plant to absorb CO2 and subsequently transform it into biomass, as well as its ability to fix chemicals such as benzene, nitrogen oxide, dioxin and others through its cuticles and plant hairs, parts that have a detoxifying function because they filter air pollutants.

Moreover, the study evaluates each plant’s potential in producing volatile organic compounds (so-called VOCs, odorous substances perceived by humans as well as animals) that can increase the concentration of ozone in the atmosphere. Therefore, these should be limited especially in densely populated areas.

“We categorised the plants according to their CO2 accumulation mechanism, at least 2 tonnes in 30 years, and in every category we included subcategories concerning the reduction of pollutants and particulate matter,” professor Baraldi explains. These assessments resulted in a list that public administrations as well as citizens can consult.

Mediterranean hackberry (Celtis australis), cleans the air

Mediterranean hackberry Celtis australis trees cities
A hackberry in Paris’ Jardin Des Plantes © Chris Waits/Flickr

A fast-growing plant that can grow up to 20-25 metres. It’s particularly suited to absorbing carbon dioxide (3,660 kilos in 20 years) and effectively capturing pollutants.

Field elm (Ulmus minor), absorbs CO2

Field elm Ulmus minor trees cities
Common elm © Manuel/Flickr

This columnar tree is also large and can grow up to 30 metres. It too can transform thousands of kilos of CO2 into biomass, with a medium-high potential of absorbing pollutants.

Common ash (Fraxinus excelsior), the grand

Common ash Fraxinus excelsior
A majestic specimen of common ash in the countryside © AJC/Flickr

A large deciduous tree that can exceed 30 metres in height. It is fast-growing especially in its early years and can store over 3 tonnes of CO2 in 30 years. This is also an excellent plant to reduce pollutants.

Littleleaf linden (Tilia cordata), typical of urban centres

Small-leaved lime Tilia cordata trees cities
An avenue lined with small-leaved lime, or lindens in autumn © Pablo Flores/Flickr

A tall variety, one of the ancestors of the common linden. Lindens are typically planted in cities and gardens. This variety is excellent in capturing CO2 and effectively reducing smog.

Norway maple (Acer platanoides), 5 tonnes of CO2 in 30 years

Norway maple Acer platanoides trees cities
A maple in a park in Vancouver © Wendy Cutler/Flickr

A tall, fast-growing tree, that can grow up to 25 metres. It has a high CO2 storage capacity (4,807 kilos in 30 years).

Turkey oak (Quercus cerris), the majestic

Turkey oak Quercus cerris trees cities
A beautiful specimen of Turkey oak in the southern UK © Rockman of Zymurgy/Flickr

A tall tree of the Fagacae family that can even reach 35 metres in height. It absorbs large amounts of carbon dioxide (4,000 tonnes) both if planted in a city or park.

Ginkgo (Ginkgo biloba), the “dinosaur” of trees

Ginkgo Ginkgo biloba trees cities
Gingkos in autumn become yellow, creating beautiful landscapes © Luca Boldrini/Flickr

The last specimen of an ancient group of plants now gone extinct, a living dinosaur. It grows slowly, has an excellent capacity of cleaning the air and absorbing CO2.

Largeleaf linden (Tilia platyphyllos), a faithful ally

Largeleaf linden Tilia platyphyllos trees cities
A majestic specimen of largeleaf linden

A fast-growing plant that is widely used to line the avenues of cities and parks. This is another species highly capable of reducing urban smog.

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