Trees and woodlands are a key part of green infrastructure, delivering a wide range of benefits to those living nearby and beyond. Trees have a widely recognised role in protecting and enhancing the local environment but they also have a key role to play in fighting climate change with their ability to store carbon as well as making urban areas more resistant to the effects of climate change including flooding, heat, pollution and soil nutrient depletion.
Trees are therefore an important consideration for planners and woodland preservation, management and creation is a key part of planning for new developments. In this article we outline some planning considerations and benefits of trees in built areas and discuss how they can add value to new developments.
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Tree roots and underground pipes
There are many benefits to trees on housing developments, but planners will also need to consider their maintenance and the impact that they could have on landscaping and below ground services. Forest Research have a useful guide on practical considerations for trees around buildings. Instances of pipes being broken by the growth of roots are rare, but blockage of damaged pipes is not uncommon as roots tend to extend to exploit water and nutrients in the soil.
The cooling effect of trees
The urban heat island effect happens when built up areas have a much warmer temperature than surrounding rural areas due to the high levels of concrete and pavement. They absorb and retain heat during the day creating localised warming. As climate change causes more extreme weather events and warmer summers, this effect will make living conditions uncomfortable and sometimes dangerous for the many people living in urban areas. This year the UK experienced record-breaking temperatures of over 40oC this year, a stark reminder that we need to find ways of keeping cities cooler and trees could be an important part of the solution.
Tree-covered areas in cities have a much lower land surface temperature compared with surrounding areas. This is not surprising considering the combination of the shade that is provided by their canopies and the effects of transpiration. Transpiration uses energy from the surrounding environment for evaporation of water from within the tree. What is surprising though is that in comparison, the cooling provided by treeless green spaces is negligible.
The cooling effect of trees is not just important generally in an urban environment but can also have a direct impact on individual buildings. Studies have shown that trees properly planted around buildings can reduce air conditioning needs by 20-50%.
Trees can reduce floods
Flooding is a key impact of climate change and planners will need to find ways to make our cities more flood resistant. Trees, hedgerows and woods can reduce floods as they are a key component of natural flood management. Not only can they directly intercept rain, but they can also promote higher soil infiltration rates and ‘hydraulic roughness’ which describes the increased resistance caused by trees when water flows across land. The water use by trees removes water from the catchment area reducing pressure on drainage systems.
Health benefits of trees
The World Health Organisation states that air pollution is a growing issue with 9 out of 10 people now breathing polluted air and polluted air killing 7 million people globally each year with one third of deaths from stroke, lung cancer and heart disease being due to air pollution. The UK Centre for Ecology and Hydrology has created a calculator to show how trees can improve air quality and cut health costs. This tool builds on previous research carried out by CEH and effect (a consultancy on economic for the environment) for the Office of National Statistics that estimates that plants in the UK remove 1.4 million tonnes of air pollution saving 1 billion in avoided health costs each year. Plants remove air pollution by providing a large surface area for particulate matter to settle on and by active uptake of gases into the leaves or chemical reactions with the leaf surface. Collectively these processes are called ‘dry deposition’.
Further research published in the Lancet makes the link between the presence of a tree canopy and the prevention of heart disease, hypertension and diabetes. Therefore the inclusion or preservation of trees in any new development is likely to be a welcome addition, however the choice of species is important. The use of pollutant -tolerant species such as alder, birch, sycamore and poplar needs to be balanced with the concerns over emissions of pollen and VOC (volatile organic compounds) from trees.
Trees don’t just impact physical health though. Trees and green space are strongly linked to reduced negative thoughts, reduced symptoms of depression, better reported moods and increased life satisfaction. Their mental health benefits have been extensively researched with countless studies showing evidence including reduced hospital recovery times, reduced stress and encouragement to exercise. Another way that trees can help increase well being is by reducing the noise from traffic. Forest Research (FR), the research agency of the Forestry Commission (FC) states that planting “noise buffers” composed of trees and shrubs can reduce noise by five to ten decibels for every 30m width of woodland, especially sharp tones, and this reduces noise to the human ear by approximately 50% but to achieve this affect the species and planting design must be chosen carefully.
Trees as a community resource
Trees can make for a more interesting landscape that people want to spend time in bringing the benefits of exercise and being outdoors. Trees can provide shelter from sun, rain and wind and for children can create places to climb, hide near and view wildlife in. Desirable community spaces can also help to build local communities. The presence of trees has also been found to reduce violence and aggression in households and limit criminal activity in neighbourhoods.
For children, trees have been shown to be important in improving classroom engagement and cognitive development. One study found that children that have views of trees and green spaces from their classrooms scored substantially higher on tests measuring attention and had a faster recovery from a stressful event.
Biodiversity
Developers are required to where possible protect existing habitats and ecosystems when building a new development. The Environment Act 2021 gained Royal ascent in Autumn 2021 with secondary legislation to be released making it mandatory for developers to consider biodiversity net gain for any upcoming projects in England. Any loss of habitat should be offset by an increase in biodiversity post development. Biodiversity Metric 3.0, which will become the new industry standard, differs from Biodiversity Metric 2.0 (2018 version) as it includes changes in how woodland is assessed in terms of distinctiveness to recognise the role of urban trees.
Urban trees provide a large range of habitat opportunities including lichens, bryophytes, invertebrates and birds. They also increase connectivity and can contribute to biodiversity critical mass between already established patches or sites. One mature Oak can be home to as many as 2300 different species, 326 of which are entirely dependent on oak for their survival.
Economic benefits
Many of the benefits of trees to communities can also translate into economic benefits which can include:• Reduction of air conditioning costs • Reduction in risk of flooding• Reduction of healthcare costs due to health benefits• Increased house value• Value of amenity
However, for many of these economic benefits it can be difficult calculate an exact monetary value. But without this monetary value urban trees will be viewed as just a cost and turned into a liability rather than an asset. A study by Woodland Trust on trees outside woods (TOWS) reviewed a range of valuation methods which combined factors including air quality amelioration, carbon sequestration, energy saving, amenity value and property prices and used median values from studies in 8 UK urban areas to give a total annual benefit per tree per year of £81.45 this compared to international studies where the value ranged from £45.68 to £167.95 per tree per year.
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