What are the environmental benefits of trees?

In praise of the urban forest.

That tree in front of your house is not just an object of beauty – there is much more to it than you might think. Sure, trees help create a sense of place and bring nature into our everyday life. But they also help us to be more sustainable, more comfortable and healthier, both mentally and physically.

A plant physiologist at the Institute of Bioeconomy of the Italian National Research Council, Rita Baraldi, explains that plants are often seen as the ’lungs’ of an ecosystem. But they can also act as an ecosystem’s ‘liver’, as they filter pollutants through their leaves. Not all species have the same efficiency in absorbing or filtering pollutants. Some species are more effective than others: bigger canopies or trees with larger leaves usually trap more particles. As Baraldi explains, leaves with rough, rugged and hairy surfaces are the ones that act as the ‘best filters’. Evergreen trees, such as conifers, are also particularly effective because they are always green and help reduce air pollution throughout the year.

There is more to it, though, and expertise is required for optimum benefits. For example, some trees emit high levels of volatile organic compounds, according to David Nowak, a senior scientist at the US Forest Service. Trees can trap pollution, but they can also affect the way pollution moves, and so they should be planted accordingly. As Nowak explains, in narrow streets surrounded by tall buildings, air flows can trap pollutants close to the ground. ‘Planting tall trees with big canopies can make matters worse in this situation by preventing the pollution from dispersing’ – in which case hedges and green walls might be more adequate. Kieron Doick, head of the Urban Forest Research group at Forest Research in Surrey, offers some additional advice by explaining that mixing trees of varying heights creates more air turbulence, which in turn helps to disperse air pollutants.

The urban forest has a potentially important role in reducing the ‘heat island’ effect. With temperatures increasing, moderate climates, such as that of the UK, should consider the beneficial effect of cooling from trees. Cooling is particularly relevant for larger cities. Heat islands are the result of several processes usually found in urban areas, where temperatures are higher than in their immediate surroundings due to the shrunk natural environment and the predominance of buildings, pavements, and other surfaces that absorb and retain heat. A perfect example is Los Angeles. National Geographic (July 2021 issue), reveals that a lack of tree-cover in LA’s low-income areas has left poorer residents vulnerable to the rising heat. High population densities correlate with higher air temperatures and can lead to an increase in health issues, unless external and internal temperatures are mitigated to ensure that our homes, offices and also our cities are comfortable and enjoyable.

A research note produced by the UK’s Forestry Commission and published in January 2019 assesses the role of urban trees and green spaces in lowering urban air temperatures. There is more to it than just the shading effect. A process called evapotranspiration ensures that part of the energy absorbed by plants evaporates water within the leaves, lowering their temperature. Plants also reflect more solar radiation than other artificial surfaces. This leads to a reduction in local air temperatures.

A Forest Research investigation (Doick, Peace and Hutchings, 2014) found that a large park in London had night-time air temperatures up to 4°C lower (1.1°C on average) than in surrounding built-up areas. The study also found that the cooling extended up to 440m from the greenspace. Large green spaces are more effective than small ones. The cooling effect also depends on the type of pavement surrounding the trees or on the soil type and condition. Other factors which influence the local temperature are the vapour pressure and the crown exposure to radiation. Urban trees should have a good quality of urban soil. Poor-quality soil, combined with a widespread use of impermeable pavements, can affect a tree’s health and therefore diminish its efficacy in lowering temperatures.

In winter, deciduous trees in front of a living room will allow direct sun to enter the space, thus naturally heating the room and reducing the need for a heating system. The increased daylight levels will also require less electric light, making homes cheaper to run and more sustainable, as there are fewer carbon emissions emitted from electric and mechanical systems. Trees can help decrease flood risk: Drops landing on the leaves evaporate, reducing the volume of water reaching the ground, and the roots slow down the water flows and help keep topsoil in place.

The positive impact on the environment and urban climate only partially explains the importance of trees. Social, economic and health benefits are also increasingly reported in research studies that quantify the regenerative properties of trees and show how they can make us happier and safer, as well as helping to build a more diverse and equal society.

The impact of street trees on our mental health is reported in a study from January 2021 conducted by researchers at the Helmholtz Centre for Environmental Research, the German Centre for Integrative Biodiversity Research, Leipzig University, and the Friedrich Schiller University Jena. The study confirms an association between antidepressant prescriptions and the number of streets at different distances from people’s homes. Trees were shown to reduce stress and anxiety - among the biggest reasons for absence from work.

Studies confirming the decrease in cortisol – our stress hormone – are published in the journal ‘Environmental Health and Preservative Medicine’. A project conducted at Maudsley Hospital in south London in 2016 aimed to put this research into practice by giving people the opportunity to spend time in green spaces by creating a sensory therapeutic garden for two wards. Creating a calming space helped patients alleviate feelings of anxiety and, more generally, promoted their wellbeing.

All that is fine, but we need to find time and resources to care for the urban forest. Care is required to match species, specimens appropriately to location and design of public realm. Ensuring that an appropriate management system is in place is equally important. Generally, larger trees provide more benefits than small ones. Urban trees are usually smaller than their rural brethren and they don’t live as long either, but their beneficial impact is nonetheless valuable so it’s vital that they are well maintained. Newly planted trees, in particular, should be well looked after until maturity so that they can grow to their full potential. Trees need sufficient water but also appropriate nutrients to remain healthy and to survive. Air pollution, impermeable paving and physical damage caused by humans, adjacent construction or storms all impact their health and longevity.

An extensive and healthy tree canopy is an essential element of urban infrastructure. Cities blessed with a rich green urban forest enjoy their living contribution to urbanity and civic pride, with avenues and parks that help shape beautiful, iconic, memorable public realm. But they don’t just add value in those terms. We must nurture our relationship with them as a vital element in the biosphere and for their generous contribution to the ecosystems that exist in their canopies and amongst their roots. But as urbanisation continues, densities increase, global warming persists, we will come to rely on them for our health and wellbeing, indeed our very survival.

By Elisabetta Li Destri Nicosia