Why we need urban trees

Photo C. Bass.

The presence of trees in urban areas evokes a multiplicity of responses, ranging from deep concern and annoyance to feelings of joy and contentment. The concerns range from fear that a tree or branch might fall and cause damage, injury, or even death, or that their roots will cause damage to pavements or undermine foundations of houses, give us the annoying Autumn chore of sweeping up dead leaves, or drip sap on our parked cars, give off pollen and biogenic volatile organic compounds, and of course, trees provide encouragement for children to climb them and risk their life and limb. On the plus side, many scientific studies show there is a strong positive correlation between the density and diversity of urban trees and the well-being of the people who live there.

Beware the Urban Tree...?

The dangers of urban trees are easy to imagine, but how real is the danger of being killed or injured by a tree? The actual numbers are revealing. While some tree species, like crack willow (Salix fragilis), have a reputation as ‘widow makers’, reports of people getting killed by trees make the front page not because they are frequent, but because they are so unusual. The number of people killed by falling trees or branches every year in the UK averages about 6 per year, which equates to one chance in 10 million of getting killed by a tree, about the same odds as getting struck by lightning or winning the lottery. Only half of these incidents are actually in public spaces, where they are usually due to the combination of extreme weather (high winds, storms) and a concentration of people in open spaces like parks and picnic spots.

Before-and-After in Middlefield Road in Bessacarr, where Doncaster Council felled 64 healthy trees, because of 'increasing tree root damage' to the pavements. The residents protested at the felling, but in vain. Which would you prefer: a street view with mature trees and bumpy pavements, or the view of smooth hard surfaces and saplings?

Photos Sera Serfozo/Richard Needham

About 55 tree-related injuries are reported every year, mostly through falling branches or trees, a significant fraction of which occur in private gardens. (For comparison, in the UK there are 2.9 million admissions to A&E departments for injuries resulting from leisure activities). Indeed, tree-related injuries are tiny in number compared to injuries from football (262 000 injuries per year), children’s swings (10 900), and wheelie bins (2 200). Mike Ellison developed a quantified tree risk assessment tool (QTRA) for Cheshire Woodlands that is now an internationally-used index to guide rigorous management protocols for trees in public places.

So while collapsing trees do pose a non-zero risk to humans, the risks to adults are tiny and even tinier for children. Comparable statistics show that daily we brave with no qualms many more high-risk events, like driving a car, riding a bicycle, crossing the road, gardening, not to mention the hazards within our own homes - and exposing ourselves to the hidden dangers of wheelie bins.

Photo K Martin

So much for the negative consequences of gravity’s action on trees. It may seem absurd to pose the question, given that trees are sessile, but can urban trees actually save lives? The answer is an emphatic yes, and here again we can refer to statistics to support the conclusion. The benefits that urban trees have are referred to by the technical term, ‘ecosystem services’, which, simply put, are the direct and indirect benefits ecosystems give to humans. Trees provide a long list of services for urban populations including: aesthetic appeal, air-quality improvement, carbon capture and storage, education, foraging, flood-risk reduction, green energy, health and well-being, heat mitigation, leisure activities, pest control, play, pollination, privacy, shelter and shade, soil conservation, water-quality improvement, wildlife habitats, and windbreaks.

Air quality

Particulate matter (PM) are air-borne pollutants generated by the usual culprits - vehicles, gas appliances, industry and other human activities like smoking and burning wood and fossil fuels. Pollen adds to the mix and the results are harmful and even deadly. The Great Smog of 1952 in London claimed at least 8000 lives and even now the Royal College of Physicians reports that 500 people a week are dying from air pollution in the UK. Of course, preventing air pollution at source through measures like the imposition of Ultra Low Emission Zones (ULEZ) is a more effective means of reducing urban air pollution than dealing with pollution once it has been generated.

The adverse health risks of breathing air contaminated with PM are proportional to the particle size, so that the smaller the particle the higher the risk. Thus, breathing in the smaller particles called ‘PM_2.5’ (the particulate size is between 0.2 – 2.5 microns in diameter) is a major health risk. Tree leaves capture airborne particles and can reduce PM_2.5 by 7%-25% depending on the tree species, those with hairy or waxy-leaved trees being the most effective. This reduction may not sound like much, but in Oxford, it is estimated that 65 tonnes of PMs are captured per year and in a large city like London 852 - 2121 tonnes per year are captured by trees like London planes, oaks, yews and conifers.

Photo C Bass.

Scrubbing gaseous urban pollutants can be more problematic, not least because trees themselves produce biogenic volatile organic compounds (VOCs), like isoprenes and monoterpenes, which give the characteristic scent of forests and which contribute to the stress-reducing and immune system-enhancing effects of shinrin-yoku (forest bathing). In urban areas, however, VOCs react with nitrogen oxide from exhaust fumes to produce ozone, which is a respiratory irritant and toxic to plants. The secondary reactions of VOCs with nitrogen and sulphur dioxides produce ultrafine particles like PM_2.5, which are also a significant health risk. These considerations underline the importance of measures like ULEZ to reduce the primary human-generated pollutants from urban areas.

Floodwater management

How often have we sheltered under a tree during a cloudburst? The function of trees as urban umbrellas is welcome to us users, but where roads, pavements and rooftops provide impermeable surfaces, a tree’s umbrella intercepts the rain that would otherwise be heading straight for the stormwater drains, with all the negative consequences we know for the downstream wastewater treatment plants and our rivers and seas. What water does make it through the tree canopy to ground level is slowed and thus can be more effectively absorbed. The tree roots themselves produce a less compacted soil with better absorption and so reduce the runoff to drains and filter the water as it passes to the groundwater stores. Trees further reduce groundwater by their natural process of transpiration.

Thus in contrast to the hard infrastructure we build for managing water in urban environment, trees provide a soft, nature-based solution by buffering the runoff from rainfall and redistributing the rainwater away from stormwater drains. It is estimated that trees in Oxford City intercept 255 000 cubic meters of rainwater annually that would otherwise be heading for the stormwater drains.

Trees stripped bare of virtually all their ecosystem services, even shade.

Not tree 'surgery', more like amputations without anaesthetic,

leaving only stark monuments to ignorance of modern arboricultural practices.

Photo C Bass

Heat

Imagine a treeless urban landscape, with the only shade coming from the buildings themselves. The hard surfaces would absorb and reflect short and long wavelength radiation, with no mitigation from trees. By shading and absorbing some of the radiation, trees can reduce the day time air temperature in a street by 5-10 deg C and the temperature of the tarmac and buildings by much more. In sunlight the leaves of the tree absorb radiation and use it for photosynthesis and for evapotranspiration. As its name implies, evapotranspiration is the process whereby water drawn in by the roots is evaporated from the leaves, causing cooling. A mature tree will use 300-400 l per day for evapotranspiration and its foliage will absorb as much as 30%. of the radiation from sun and radiation reflected from hard surfaces. By providing shade, absorbing radiation, particularly harmful ultraviolet and infra-red wavelengths, and by evapotranspiration, the urban tree acts as a biological air-conditioner.

Health

The benefits of urban trees on our physical health have been mentioned – cleaner air, reducing heat, blocking and filtering UV, and reducing stress and boosting the immune system by the sight of trees and by inhaling their natural aromas. The Covid pandemic reminded us of the importance for our cognitive, psychological, and emotional well-being of spending time in nature.

Photo C Bass

In urban environments, a diversity of trees species, and particularly of of old and large trees, has been found to correlate positively with perceptions of well-being. A study of London boroughs in 2015 found that the prescription rates for antidepressants correlated negatively with the density of trees on the streets where you lived – more trees per kilometer = fewer prescriptions. Similarly, a 2020 study in Leipzig found that living within 100m of streets with high densities of trees was correlated with lowered the risk of antidepressant use, with individuals of low socio-economic status being benefitted most. A study of 2 million people in Wales reported that living within a few 100 m of high ‘ambient greeness’ was associated with 20% less anxiety and depression than people living in areas with low levels of greenery.Urban treescapes may also lead to more prosocial behaviours. One study in Portland USA found that an increase in tree cover in high crime neighborhoods.was linked to a reduction in violent crime.

Measuring the girth of the Peace Oak to determine its age, although we know that it was planted to commemorate the end of …

These strong links between mental health and urban greenery led Prof. C. Konijnendijk of the Nature-Based Solutions Institute of the University of British Columbia to propose the 3-30-300 Rule of-Thumb. Rule 1 is that everyone should be able to see at least 3 substantial trees from their residence. Rule 2, there should be 30% canopy cover over every neighbourhood, and Rule 3, there should be a high-quality green space within 300m (i.e. within a 5-10 min walk). Cities like Zurich, Malmö, Barcelona, and Seattle have adopted the 3-30-300 Rule as a basis for planning.

Photo C Bass.

Oxford’s i-Tree Eco Study of 2018 showed that the canopy cover was 22%, with Headington having the highest and areas like Lye Valley and Blackbird Leys having the lowest cover. So Oxford City still has some way to go to reach the 30% canopy cover threshold and substantially boost the tree equity score of the lower socio-economic areas.

Climate Change.

Many of the plusses of trees listed above have mitigating effects of climate change – particularly with the extremes of weather we now experience that lead to heat islands in cities and widespread flooding. Trees work hard to combat the prime cause of global heating by removing carbon dioxide from the atmosphere and storing the carbon. The Oxford I-Tree Eco Study estimated that Oxford’s trees store 76 000 tonnes of carbon and sequester 2500 tonnes annually. Nearby Wytham Wood is probably doing a lot of the heavy lifting (unfortunately now contributing a large return of carbon dioxide to the atmosphere as ash-dieback has taken hold). Climate change impacts negatively on survival of trees. Common species like ash, which are already threatened by disease, are further stressed by climate change.

Photo Sue Chapman.

Released: February 09th, 2026 03:58 PM

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