New work by researchers at University College London shows that pockets of this urban jungle store as much carbon per hectare as tropical rainforests. Indeed there are actually more trees in London than people and is almost a forest.
More than half of the world’s population lives in cities, and urban trees are critical to human health and well-being.
Trees provide shade, mitigate floods, absorb carbon dioxide (CO₂), filter air pollution and provide habitats for birds, mammals and other plants. The ecosystem services provided by London’s trees–that is, the benefits residents gain from the environment’s natural processes–were recently valued at £130m a year.
This may equate to less than £20 a year per tree, but the real value may be much higher, given how hard it is to quantify the wider benefits of trees and how long they live. The cost of replacing a large, mature tree is many tens of thousands of pounds, and replacing it with one or more small saplings means you won’t see the equivalent net benefit for many decades after.
Trees absorb CO₂ during photosynthesis, which is then metabolised and turned into organic matter that makes up nearly half of their overall mass. Urban trees are particularly effective at absorbing CO₂, because they are located so close to sources such as fossil fuel-burning transport and industrial activity.
The UCL team use a combination of cutting-edge ground-based and airborne laser scanning techniques, to measure the biomass of urban trees much more accurately. Lidar (light detection and ranging) sends out hundreds of thousands of pulses of laser light every second and measures the time taken for reflected energy to return from objects up to hundreds of meters away.
Space is at a premium in urban areas. Building new parks, especially in European cities, is difficult. Yet there are acres of space, if we cast our eyes upwards, that could be utilised for our health and well-being. For high above the pavement and asphalt, there are plenty of roofs that could offer alternative green spaces. Places where ‘the air is fresh and sweet’ as the Drifters were telling us in 1962.
While there is a degree of evidence that green roofs can remove particulates associated with air pollution, it’s the space they provide for people that is perhaps more important. Especially with cities becoming denser. The denser the city, the less room there is for green space at ground level. Buildings make up the majority of a city’s footprint. These buildings should be the perfect platform for parks, gardens and unofficial nature reserves in the sky. In doing so, planners and developers can ensure that people – and wildlife – have access to greenery. And where there is greenery the air is likely to be fresher.
Although most new developments over the past ten years in London have green roofs, these are only a drop in the ocean. The existing cityscape and its old roofs hold huge potential for greening. Nearly 32% of central London could be greened tomorrow (see the report we were commissioned to write for the Mayor in 2008 here). And that is just at roof level! Whilst the proportion of these spaces may not be physically accessible, many will be visually accessible from window from other buildings. We must not underestimate the visual impact of greenery at roof level on our health and well-being
So cities have a huge potential to transform themselves into dynamic green centres. Healthy roofs mean healthy citizens mean healthier buildings which also means better property portfolios for developers. Green roofs have the potential to grow value both financially and environmentally. It is important, then, that any issues surrounding initial costs need to be put firmly into perspective in the light of the long-term benefits of healthier cities on their citizens.