Ecosystem services
Trees for ecosystem services
The urban forest provides a vast range of benefits contributing to food production, air purification, amenity value and flood management.
There is now a global understanding on the links of human well-being and nature via the Intergovernmental Platform on Biodiversity and Ecosystem Services. The UK NEA analysed the natural environment including the urban forest in terms of the benefits it provides for society and economic prosperity. The UK NEA found that health, wellbeing and economic productivity of the country depended on the range of services provided by ecosystems and their constituent parts, such as water, soil, nutrients and organisms.
But for the urban forest to perform these tasks effectively the i-Tree Eco project `Valuing London’s Urban Forest’62; revealed that ‘there needs to be trees of all shapes and sizes and the right proportions to ensure that benefits can be continued to be delivered for future Londoners’. i-Tree Eco can pick out if there is sufficient succession, a requirement for more tree planting or if there is an over-reliance of over mature trees.
Trees and storm water management
In recent years flooding has become prevalent in urban areas and as a society we need to be more resilient as the effects of climate change become more apparent in our everyday lives.
Flooding in urban areas is estimated to cost a minimum of £270 million per year in England and Wales, with two thirds of the homes affected in the floods of 2007 due to surface water63, 64.
Surface water flooding happens when rainfall runs off land and buildings at such a rate that it is unable to drain away in streams, rivers, drains or sewers. Urban trees can play a pivotal role in counteracting this. Our streets and urban spaces generally have a high coverage of impermeable surfaces which prevents surface water from soaking into the ground, increasing the risk of flooding and pollution from heavy rainfall65. If the urban forest is designed as part of and to compliment permeable paving, with swales, rain gardens and green roofs within a Sustainable Drainage System (SuDS) to mimic natural drainage, rainfall can be intercepted by trees, their root systems promote infiltration and water storage in the soil and prevent “grey” drainage systems becoming overwhelmed during storm events66. All developments in Coventry must apply SuDS and should ensure that surface water runoff is managed as close to its source as possible. Natural England has also highlighted the use of urban forestry in wetlands and floodplains to act as buffers to protect urban areas from flooding and pollution67.
Trees and water quality
Trees are nature’s water filter.
Improving water quality is crucial to healthy life. Urban forestry can help reduce the high speed of runoff, collect pollutants and detritus from urban surfaces, and reduce infiltration of precipitation, ensuring the quality of water is as good as it can be flowing through an urban catchment.
The EU’s Water Framework Directive establishes targets for ensure water quality in our environment. In many urban areas throughout the UK these targets are being missed. The incorporation of natural SuDS with existing and planned developments is one effective ;and environmental friendly way of improving water quality. Increasing woodland cover also has a benefit. For example in north Nottinghamshire the establishment of a new Community Forest over 24 years increased tree cover threefold and reduced annual recharge and runoff by 11%68.
Trees and noise pollution
Trees help to mask noise.
In Coventry, sources of noise from the airport, motorway network and the industrial areas are all sources of environmental pollution that can be reduced by effective planting of trees. The proliferation of prolonged exposure to high levels of noise can cause anxiety, stress and hearing loss. The reduction of noise pollution (sometimes called abatement) can be achieved by well planned and designed tree planting. Evidence from Forest Research suggests 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%. To achieve this effect, the species and the planting design must be chosen carefully.
Trees, carbon storage and sequestration
Trees lock up carbon from the atmosphere and help reduce the effects of global warming. The urban forest can help mitigate climate change by sequestering, or hiding away, atmospheric carbon as part of the carbon cycle. Tree stems, branches and roots can store carbon for decades or even centuries, equating to several tons of atmospheric carbon dioxide being absorbed over the lifetime of a single tree.
One large tree can absorb 150kg of carbon dioxide per year, as well as filtering airborne pollutants.
In London an estimated 2,367,000 tonnes (approximately 15t/ha) of carbon is stored in London’s trees with an estimated value of £147 million59. The number of trees present, their species and mass can affect carbon sequestration and Oak as a species stores the most carbon in the urban forest, as larger trees store more carbon in their tissues
Trees and food production
Trees play a huge part in the drive for urban gardening, Forest gardening69 and local food production, whether this is on allotments, community gardens or orchards. The ‘Feeding Coventry Steering Group’ seeks to increase access to healthy food, provide food-related educational and business opportunities, combat food poverty and reconnect communities with their local food suppliers. Joining local communities with these urban agriculture and local food assets within the urban forest, via footpaths and cycleways can encourage reconnection to the natural world further.
Trees and soil protection
Trees are vital for soil health. Trees and their roots aerate the soil and prevent erosion and compaction.
Wind and rain are two of the main forces that erode bare soil. Trees reduce the effect of erosive forces using their root systems and foliage. Tree roots create a network of flexible tendrils that help stabilise the soil around the tree and hold it in place. The leaves and branches of trees create a flexible screen that reduces the force of wind and rain in the surrounding area. Tree foliage intercepts falling rainwater and reduces the force it exerts when it hits the ground. Rain water caught in a tree’s foliage is channelled over the stems and down the trunk until it soaks into the soil. Groups of trees planted together can act as wind breaks and prevent soil from being carried away in the wind.
Trees and air pollution alleviation
Air pollution from vehicles and industrial processes has an impact on our health and air temperatures.
Urban air pollution predominantly comes from traffic emissions. Urban trees can alleviate air pollution directly by trapping and removing fine particulate matter70 and indirectly by reducing air temperatures. The strength of the effect71 of filtrating pollutants depends on many factors e.g. weather, the pollution concentration, extent of tree cover, leaf area, species and quality of vegetation72. The structure of large trees and their rough surfaces cause interception of particulate matter (of less than 10 microns diameter) by disrupting wind flow. Therefore the uptake of SO2, NOx and ozone is higher in broadleaved species than conifers, but conifers capture larger amounts of PM10 than broadleaved trees due to the larger total surface area of needles, giving conifers larger filtering capacity than broadleaved trees73. Trees also provide a surface area for capture between 2 to 12 times the area of land they cover.
As a consequence, urban planning needs to consider a combination of parklands, buildings, street trees, and gardens to create a rough surface of differing heights, to create essential turbulence, increasing mixing, and pollutant dispersion74. In the West Midlands, a study has suggested that doubling tree cover across the region would reduce the concentration of fine PM10 by 25% and could prevent 140 air pollution related deaths in the region each year75, 76, supporting proposals for planting new urban woodlands.
But can urban trees make pollution worse at a street level? In some circumstances this can be the case, but always the best way to improve air quality is to remove the emission sources – road traffic - rather than the tree. Natural chemicals produced by the tree called volatile organic compounds can on very hot days with strong sunlight mix with pollution to form ozone, which at street level, is a pollutant with negative health impacts77. For a significant health impact this would require millions of trees and take several hours. This effect is large-scale and the ozone formation occurs hundreds of miles away from the original source. Dense avenues of street trees with large interconnected canopies can trap air at street level if the pollution source is located within this zone78, but most importantly if the source is located outside, the tree canopies will create locally cleaner air. Therefore green corridors need to be master planned across cities to reduce pedestrian exposure to pollution by providing alternative routes79 and acting as a green barrier, increasing the pathway between pollution source and receptor, and speeding up the mixing and remediation of pollutant concentration78.
Trees and the urban heat island
Trees are nature’s air conditioners.
Well planned and designed urban places and spaces with trees are crucial for reducing the long term effects of climate change.
Urban areas in Coventry experience elevated temperatures compared with rural areas, because the urban fabric, e.g. tarmac and concrete, absorb and retain heat80. Climate change projections suggest a trend towards elevated temperatures, but urban forestry has an important role to play in cooling air temperatures through the evaporation of water81, 82, shading83, and the conversion of solar radiation to latent heat. Through modelling it is possible to determine the cooling effect of the urban forest and associated green space e.g. in Birmingham (BUCCANEER project84). Trees can cool cities by between 2°C and 8°C and when planted near buildings, can cut air conditioning use down by 30%, and reduce heat energy consumption by 20-50% (UN Urban Forestry Office). ;
Trees and traffic calming
Traffic and trees can work together to make our streets safer and more distinctive.
Well designed streets and urban areas with carefully positioned trees can have a positive effect on slowing traffic and making spaces more pleasant for pedestrians and motorists. Carefully positioned trees can frame and segregate pedestrian areas and subconsciously inform vehicle drivers. Improving sightlines and helping to slow down cars in urban settings can be used as an alternative to bollards and speed bumps or to reinforce their presence and enhance the role of a central reservation.
Trees and distinctive design
Coventry’s “leafy character” is synonymous with its rich and mature treescape and creates a distinctive environment.
Successful urban forestry embraced by the local community, which relates to the landscape character and heritage of the locality, can contribute to the local sense of place.
Trees shade buildings, shield from winter winds and regulate temperatures through evapotranspiration, influencing the energy consumption to heat and cool the building. In the summer, trees reduce building energy consumption, but in the winter months can either increase or decrease building energy use, depending on the location of trees around the building.
Street trees present aesthetic qualities to our urban spaces; provide distinctive landmarks and can evoke memories, which are particularly important for the sensory development of young children and recognition for seniors suffering from dementia. Streetscapes can be injected with vibrancy, beauty and light when trees have been planted, making them distinctive places, and as a consequence can be a catalyst for regeneration and enhance house prices (when compared with similar streets without trees and investment). The visual appearance and attractiveness of towns and cities has been found to be strongly influenced by the provision of green space85. Distinctive trees can potentially result in a boom in tourism, stimulating job opportunities as a result
