More Trees in the City – Cleaner Air? According to a Researcher, It’s Not That Simple

Recent trends show that cities are becoming greener. We are seeing more trees, green spaces and vegetation in urban areas, which not only enhance the cityscape but also perform important functions for cities and their residents. Trees are often seen as one of the nature-based solutions that can help tackle air pollution, urban heat and other challenges associated with urbanisation.

But does having more trees in a city really mean cleaner air? Is simply planting more trees enough to maximise their benefits? Or does it matter not only how many trees there are, but also which species are planted, where they grow and what functions they perform?

In our “What Does Science Say?” series, we explore these and other questions with Dr Valda Araminienė, Senior Researcher at the Lithuanian Research Centre for Agriculture and Forestry, whose research focuses on urban green spaces and the potential of trees to reduce air pollution.

Why does tree species matter in cities?

The first question is a simple one: does having more trees in a city really mean cleaner air?

According to Dr Araminienė, the number of trees alone is not enough. “Simply having more trees does not guarantee cleaner air,” the researcher emphasises.

The results depend on the tree species selected, their size, canopy structure, planting location and the specific sources of pollution. Urban green spaces should therefore be planned not simply with the aim of increasing the number of trees, but by selecting appropriate species and planting them in suitable locations.

This means that a tree planted in a city is more than just a green element of the urban landscape. Depending on its characteristics and location, it can perform different functions – from capturing air pollutants to providing shade and improving the urban microclimate.

Which trees are most effective at improving air quality?

Trees can help improve air quality in two main ways: by capturing particulate matter on their surfaces and by absorbing gaseous pollutants through their leaves and needles. However, different species perform these functions to varying degrees.

“Reducing air pollution is one of the most important and quantifiable ecosystem services provided by trees,” says Dr Araminienė. According to her, when planning new urban greenery or renewing existing green spaces, the first step should be to analyse the sources of air pollution in and around the area and assess pollutant emissions and their chemical composition.

There is no single tree species that is universally best suited to all urban conditions. However, some species have greater potential to reduce air pollution. Norway maple and linden trees, for example, perform well in reducing concentrations of some major urban air pollutants, including nitrogen dioxide, ozone and particulate matter.

When selecting trees, their ability to remove pollutants should not be the only consideration. Their resistance to drought, diseases and pests, as well as their longevity, should also be taken into account. Species that emit higher levels of volatile organic compounds or produce highly allergenic pollen should also be avoided.

There is no single “best” tree

One way of comparing different tree species is the species-specific Air Quality Index, or S-AQI. It assesses not only a plant’s ability to remove pollutants from the air, but also other important factors, such as resistance to drought, diseases and pests, pollen allergenicity and its potential contribution to ground-level ozone formation.

Different tree species receive different ratings under this index. Norway maple and hawthorn are among the higher-rated species, followed by linden, ash and pine. However, these ratings do not in themselves mean that one tree is “good” and another “bad”. The suitability of a tree depends on the function that needs to be enhanced and the specific conditions of the area.

A tree suitable for one type of pollution may not be the best choice for another

Preliminary recommendations developed by Dr Araminienė and her colleagues show that the choice of trees may also depend on the specific air pollutant involved.

For example, in areas where nitrogen dioxide is the predominant pollutant, maples and lindens are recommended. Where high concentrations of particulate matter are a concern, Scots pine can be selected or combined with deciduous species such as maples and lindens.

The situation is even more complex when it comes to ground-level ozone. Under certain conditions, some tree species can contribute to increased ozone formation in urban air. Here too, it is therefore important to select species appropriate to the specific conditions.

This once again demonstrates that planning urban green spaces is not as straightforward as it may initially appear.

It matters not only what we plant, but where

Even a carefully selected tree species may not provide the greatest benefit if planted in the wrong location.

According to Dr Araminienė, urban green spaces are increasingly viewed not merely as attractive places, but as solutions to a range of urban challenges. Their effectiveness, however, can be limited by fragmentation, suboptimal spatial distribution or poorly selected tree species.

One widely recognised principle for planning greener cities is the 3–30–300 rule. It proposes that at least three trees should be visible from every home, school or workplace, that tree canopy cover should reach at least 30% in residential areas, and that a high-quality green space should be accessible within 300 metres.

However, according to the researcher, such guidelines alone are not enough. Biodiversity, the ecological integrity of green spaces and where their benefits are most needed must also be considered.

Before deciding where and which new trees to plant, it is therefore important to assess which species already grow in the city, how much area existing greenery covers and what ecosystem services it provides. This can help identify where there are already enough trees, where more are needed and where changes in species composition would be beneficial.

Lithuanian cities need solutions tailored to their conditions

The researcher highlights another important point: solutions developed in other countries cannot simply be transferred to Lithuania.

In Southern European cities, for example, approaches have already been tested in which tree species are selected specifically for their ability to remove air pollutants. Lithuania, however, has different climatic and environmental conditions, meaning that tree selection should be adapted to the needs of Lithuanian cities.

Dr Araminienė and her colleagues applied this approach in 2021–2022 while carrying out a project under the Lithuanian–French “Gilibert” programme administered by the Research Council of Lithuania. The study assessed tree cover in Kaunas and Aix-en-Provence, analysed the predominant species and evaluated their potential to reduce air pollution.

The study showed that different tree species dominate in different cities, meaning that solutions suitable for one city may not necessarily be appropriate for another.

Trees matter for more than just air quality

The importance of urban trees extends well beyond reducing air pollution. One of the most immediate benefits people can experience is their effect on the urban microclimate. This is particularly important on hot summer days, when tree shade can fundamentally change how people experience the city.

Dr Araminienė points out that street greenery is sometimes overlooked when planning newly expanding residential areas. As a result, walking or spending time outdoors can become difficult on hot, sunny days. Well-planned trees can provide shade and help mitigate the effects of heat.

Drawing on her own experience, the researcher illustrates just how much tree shade can change the way an environment feels. During a visit to Southern Europe, she experienced temperatures of around 40°C, yet walking through a tree-covered urban park remained comfortable even in such conditions.

Green spaces are also important for people’s emotional and physical well-being. Dr Araminienė points to research suggesting that spending time in urban green spaces can be associated with greater physical activity, better emotional well-being and lower stress levels. It is therefore important that green spaces are not only attractive but also easily accessible – as close as possible to where people live.

So how many trees does a city actually need?

Science does not provide a definitive number of trees that would automatically make a city healthier or cleaner – probably because the number of trees alone is not the most important factor.

“Planting more trees in urban areas is beneficial in any case,” Dr Araminienė emphasises. However, to maximise their benefits, tree species, their characteristics, location and the specific needs of the area must all be taken into account.

The recommendations developed by the researcher and her colleagues are still preliminary and will need to be refined in the future by including more tree species, environmental factors and longer-term data.

The question of what kind of city we want in the future is therefore not simply about the number of trees. It is about how intelligently we plan our environment.

A greener city is not necessarily one that simply has more trees. It is a city where trees are planted where they are most needed, where species are selected to suit local conditions, and where green spaces are accessible to people.

Perhaps the next time we walk past a newly planted tree, we should ask not only, “Why was it planted here?” but also, “Why was this particular tree chosen for this particular place?”

Interview by Domantas Zubrickis, RCL
Photo courtesy of the interviewee

Scientific references:

1. Khan A., Araminienė V., Uogintė I., Varnagirytė-Kabašinskienė I., Černiauskas V., Gudynaitė-Franckevičienė V., Džiugys A., Davulienė L., Misiulis E., Davtalab M., Byčenkienė S. 2025. Evaluating the role of urban green infrastructure in combating traffic related microplastic pollution. Science of the Total Environment, 983 (179688), pp. 1–13.

2. Araminienė V., Sicard P., Černiauskas V., Coulibaly F., Varnagirytė-Kabašinskienė I. 2023. Estimation of air pollution removal capacity by urban vegetation from very high-resolution satellite images in Lithuania. Urban Climate, 51: 101594.

3. Sicard P., Coulibaly F., Lameiro M., Araminienė V., De Marco A., Sorrentino B., Anav A., Manzini J., Hoshika Y., Moura B. B., Paoletti E. 2023. Object-based classification of urban plant species from very high-resolution satellite imagery. Urban Forestry and Urban Greening, 81: 127866.

4. Araminienė V., Sicard P., Anav A., Agathokleous E., Stakėnas V., De Marco A., Varnagirytė-Kabašinskienė I., Paoletti E., Girgždienė R. 2019. Trends and inter-relationships of ground-level ozone metrics and forest health in Lithuania. Science of the Total Environment, 658: 1265–1277.

5. Sicard P., Agathokleous E., Araminienė V., Carrari E., Hoshika Y., De Marco A., Paoletti E. 2018. Should we see urban trees as effective solutions to reduce increasing ozone levels in cities? Environmental Pollution, 243: 163–176.

Last updated: 29-09-2026

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