Tree planting and urban greenery
- Conservative Environment Network
- 18 minutes ago
- 4 min read
Air pollution is the leading environmental threat to human health. Dirty air is detrimental to our quality of life. It causes low birth weight, premature birth and stunted cognitive development, as well as reducing lung function. It also increases the likelihood of developing asthma and heart disease, and the risk of having a stroke. Air pollution contributes to around 43,000 premature deaths per year in the UK.
The UK has legally binding air pollution targets. England has two legally binding targets to reduce the level of and exposure to fine particulate matter (PM2.5) by 2040. Additionally, where an area breaches the statutory nitrogen oxide (NOx) limit, the local authority must create a plan to bring levels within the legal limit as quickly as possible.
The impact of urban trees, hedges, and shrubs on air pollution is varied depending on location. Urban vegetation can be beneficial for air quality. However, it is not the sole solution to urban air quality problems. Only tackling the sources of the air pollution will lead to a meaningful and lasting improvement in air quality.
Benefits of tree planting for air quality
Urban vegetation can cause pollutants to disperse more widely. Strategically placed urban greenery helps disperse pollutants by improving air circulation through diluting their concentration and reducing our exposure to them.
Vegetation can absorb some pollutants, but it is more effective at removing certain pollutants over others. Air pollutants can be deposited on vegetation by sticking to plant leaves. Existing vegetation reduces the surface concentration of ammonia by 24% and sulphur dioxide by 30%, improving air quality. Greenery also removes 6% of PM10 and 10% of PM2.5, while making no marked change in NOx concentrations. However, the amount of pollution removed depends on the species, age, number, and condition of the vegetation.
Urban trees and greenery can act as a barrier to pollution. Along our roadsides, hedges and greenery can act as a barrier to pollution between traffic and pedestrians. The vegetation creates a sheltered area behind the ‘barrier’, meaning pedestrians walk in a protected space. One study found that a green wall between a busy road and a school playground reduced PM2.5 pollution by 80%.
Coniferous trees capture pollutants all year round. Plants with rough, hairy, or waxy leaves are the most effective at capturing pollutants. Conifers, such as pines, spruces, and firs, have needle-like leaves all year round, which can trap pollutants. Their rough surfaces and dense canopies help filter fine particles from the air throughout the year. However, these are not commonly planted in urban areas across the UK, equalling around 2.3% of all London’s Public Realm Trees.
Cooler ground-level temperatures generally lead to lower levels of ozone pollution. Ozone is a gas found at ground level and in the stratosphere. Ground-level ozone is formed when NOx and volatile organic compounds from sources like vehicles and industry react with sunlight. Ozone irritates our airways, worsens asthma, and causes lung disease. Urban trees can lower street temperatures by up to 12 degrees, slowing the chemical reaction that creates ozone.
Urban greenery can help encourage safer driving. Roadside greenery is associated with greater driver attention and fewer severe crashes. It is estimated to lower road speeds by an average of three miles per hour, helping residents to feel safer walking and cycling.
There are significant wider environmental benefits from trees. The UK is one of the most nature-depleted countries, and trees serve as a nature-based solution to both climate change and biodiversity loss. Planting the right tree in the right place can help create essential habitats for wildlife, while absorbing carbon dioxide and mitigating the worst effects of climate change, such as flooding.
Challenges of urban tree planting for better air quality
Trees and greenery have to mature over many years to start absorbing pollutants. Large canopies capture the most pollution, but it can take trees 15 years to reach this stage. With UK tree coverage historically low at 13.5% and our planting efforts off track, it is unlikely that planting more trees will make a meaningful difference to our air quality in time to meet our legally binding targets.
Small scale urban vegetation is largely ineffective at absorbing air pollution, as there is limited greenery to deposit pollutants onto. Local micro interventions, such as planters or living walls, may offer benefits very close to the absorbing surface in calm conditions. However, greenery needs to be planted on a large scale for the mitigation of exposure to PM10 or PM2.5 to be more than a few percent, but the practicalities of this are limited in an urban setting. Urban greenery is more effective at controlling the flow of pollutants by dispersing them away from a source and diluting them with surrounding air.
Vegetation does not absorb much NOx pollution. The little NOx pollution that is absorbed generally occurs in the daytime and in warmer months. Soil is also a source of NO meaning absorbed pollutants are generally offset. Therefore, urban planting must be accompanied by other policies to reduce air pollution, such as expanding access to EV charging points and tightening regulations on domestic burning.
Dense tree canopies on narrow roads can trap and worsen pollution. Dense vegetation on narrow roads with tall buildings restricts airflow, trapping pollutants in street canyons and preventing dispersion. Therefore, urban vegetation must be properly planned and well located to ensure it has a positive impact on air quality.
Some trees emit high levels of biogenic volatile organic compounds (BVOCs), worsening pollution. Species like oaks, willows, and poplars emit high levels of BVOCs, which react with NOx in sunlight to form ozone, a harmful air pollutant. Birch, hornbeam, yew, and elder trees are preferable for street planting as they emit lower levels of BVOCs.
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