A robust case for trees' environmental benefits has helped a London borough gain political and financial support for an ambitious planting programme.
In what is thought to be the largest programme of its kind in London, Barnet Council has pledged to plant 4,500 trees over the next five years to boost the borough’s air quality, reduce the risk of flooding, replace ageing trees and improve parks.
The council’s planting will target "urban heat islands" — areas with little shade that are prone to higher temperatures during hot spells, including at night when heat built up during the day is released.
New trees will also help reduce air pollution around arterial roads in the borough, while hundreds more will be planted along other streets as well as near schools and in parks.
The programme is guided by the London Borough of Barnet Tree Policy, published last October and guided by national legislation as well targets set by the Greater London Authority. The borough’s tree officer Andy Tipping tells Horticulture Week: "Our tree policy sits under the broader Parks & Open Spaces Strategy, which highlighted these ecosystem services and determined the figures the council wanted to plant. There are real issues with air quality in London and fortunately we have senior managers and councillors here who are supportive of efforts to mitigate this."
A subsequent report, approved by full council, ensured funding for planting, which has already benefited from schemes such as the Government's Big Tree grant, the mayor of London's grants for air quality, local improvement plans and Outer London Fund, and from the Forestry Commission's Re:Leaf programme.
Tipping, who is vice-chair of the London Tree Officers Association, adds that the policy "prioritises larger-growing, shade-providing trees following urban heat island targets, scaling down to smaller ornamental trees where larger trees are not suitable". The policy also calls for "a diversity of tree species to mitigate against pests and diseases that can threaten entire species". Tipping adds: "We also have a biosecurity clause here so we are not importing pests and diseases into the country."
The borough’s initiative has been given the thumbs-up by national tree advocates. Forestry Commission adviser Jim Smith says: "Barnet is leading the way and demonstrating how councils across the country should be improving and replacing their urban forest. Investment in their future is essential to ensure residents, visitors and businesses alike benefit and thrive from the presence of trees."
Woodland Trust external affairs officer for London Richard Barnes adds: "Cleaner air, cooler temperatures, reduced flooding — these are just a few of the benefits our urban trees provide, but ones we’re more in need of than ever. Barnet is recognising a need to invest at a time when all local authorities are under pressure."
The north London borough is London’s largest by population and fourth largest by area, extending from rural Hertfordshire to Hampstead Heath. Barnet Council manages around 30,000 street trees and 850ha of green space, including more than 160ha of woodland.
To maintain its existing tree stock in line with its environmental aims, its highways team is installing a rubber crumb surface around tree pits that allows rainwater to drain into the root zone. "Where tree roots are causing damage to the footway, all engineering options will be considered before tree removal is considered," says Tipping. "We simply don’t get requests for removal any more."
Building the case
A new study by the University of Reading and Forest Research provides further costed evidence of the environmental and economic benefits of urban trees. The research, published in Urban Forestry & Urban Greening, found that air-conditioned buildings save energy when in close proximity to trees, which cool and humidify incoming air due to evapotranspiration.
Attempting to quantify the effect in an area of inner London, the researchers found that energy consumption of air-conditioning units near trees is between 1.3% and 13% less, corresponding to an estimated annual saving of between £2.1m and £22m for the area.
University of Reading lecturer Dr Stefan Smith says: "We were surprised to see the extent to which the trees affected energy consumption in air-conditioning units. Large trees were particularly significant in their cooling effects, even without consideration for the cooling effect that shading provides."
But even modest-sized trees "offer the potential to create evaporative cooling in locations without sufficient space for a large tree canopy", he adds.
Co-author Dr Kieron Doick, head of the Urban Forest Research Group at Forest Research, says: "This work adds to the suite of urban tree benefits that we can already value.
"Quantification and valuation of the ecosystem services provided by urban trees is important as it can help make the case for protecting tree planting and maintenance budgets and to integrate building climate change resilience into cities."