London’s urban ‘forests’ can store almost as much carbon as tropical rainforests

Trees in Hyde Park, London. Image: Getty.

Most people would never think of London as a forest – yet there are actually more trees in London than people. And now, new work by researchers at University College London shows that pockets of this urban jungle store as much carbon per hectare as tropical rainforests.

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.

The trouble with measuring trees

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.


This carbon storage potential is an extremely important aspect of their value, but is very hard to quantify. A 120-year-old London plane tree can be 30 metres tall and weigh 40 tonnes or more, and some of the carbon in its tissues will have originated from Victorian coal fires.

Measuring the height of a tall tree is difficult, because it’s rarely clear exactly where the topmost point is; estimating its mass is even harder. Typically, tree mass is estimated by comparing the diameter of the trunk or the height of the tree to the mass of similar trees (ideally the same species), which have been cut down and weighed in the past. This process relies on the assumption that trees of a certain species have a clear size-to-mass ratio.

But a fascinating property of trees is how variable they can be, depending on their environment. So inferring the mass of urban trees from their non-urban counterparts introduces large uncertainties.

Lidar over London

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, which stands for 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 metres away.

When mounted on a tripod on a city street, lidar builds up a millimetre accurate 3D picture of everything it “sees”, including trees. The team are using lidar methods, which they pioneered to measure some of the world’s largest trees, and applying them to trees in the university’s local London Borough of Camden.

The UCL team used publicly available airborne lidar data collected by the UK Environment Agency, in conjunction with their ground measurements, to estimate biomass of all the 85,000 trees across Camden. These lidar measurements help to quantify the differences between urban and non-urban trees, allowing scientists to come up with a formula predicting the difference in size-to-mass ratio, and thus measuring the mass of urban trees more accurately.

The findings show that Camden has a median carbon density of around 50 tonnes of carbon per hectare (t/ha), rising to 380 t/ha in spots such as Hampstead Heath and Highgate Cemetery – that’s equivalent to values seen in temperate and tropical rainforests. Camden also has a high carbon density, compared to other cities in Europe and elsewhere. For example, Barcelona and Berlin have mean carbon densities of 7.3 and 11.2 t/ha respectively; major cities in the US have values of 7.7 t/ha and in China the equivalent figure is 21.3 t/ha.

A story to tell

Trees matter, to all of us. Recent protests in Sheffield, Cardiff, London and elsewhere, over policies of tree management and removal show how strongly people feel about the trees in their neighbourhood. Finding ways to value trees more effectively is critical to building more sustainable and liveable cities.

Measuring trees in new ways also helps us to see them from a new perspective. Some of these trees have incredible stories to tell. Just one example is an ash, tucked away in the grounds of St. Pancras Old Church, one of London’s – and indeed Britain’s – oldest Christian churches. 

The tree has an extraordinary arrangement of gravestones around its roots, placed there when the railway was built from St Pancras in the mid-19th century. The job of rehousing the headstones was apparently given to a young Thomas Hardy, working as a railway clerk before going on to achieve literary fame. The UCL team’s 3D lidar data are helping monitor the state of this “Hardy Ash” tree in its dotage. This is just one of the ways new science is helping tell the stories of old trees.

Mathias Disney, Reader in Remote Sensing, UCL.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

Segregated playgrounds are just the start: inequality is built into the fabric of our cities

Yet more luxury flats. Image: Getty.

Developers in London have come under scrutiny for segregating people who live in social or affordable housing from residents who pay market rates. Prominent cases have included children from social housing being blocked from using a playground in a new development, and “poor doors” providing separate entrances for social housing residents.

Of course, segregation has long been a reality in cities around the world. For example, gated communities have been documented in the US cities since the 1970s, while racially segregated urban areas existed in South Africa under apartheid. Research by myself and other academics has shown that urban spaces which divide and exclude society’s poorer or more vulnerable citizens are still expanding rapidly, even replacing public provision of facilities and services – such as parks and playgrounds – in cities around the world.

Gated developments in Gurgaon, India, have created a patchwork of privatised services; elite developments in Hanoi, Vietnam, offer rich residents cleaner air; and luxury condos in Toronto, Canada, displace local residents in favour of foreign investors. An extreme example is the Eko Atlantic project in Nigeria – a private city being built in Lagos, where the majority of other residents face extreme levels of deprivation and poverty.

A commodity, or a right?

Although these developments come with their own unique context and characteristics, they all have one thing in common: they effectively segregate city dwellers. By providing the sorts of facilities and services which would normally be run by public authorities, but reserving them exclusively for certain residents, such developments threaten the wider public’s access to green spaces, decent housing, playgrounds and even safe sewage systems.

Access to basic services, which was once considered to be the right of all citizens, is at risk of becoming a commodity. Privatisation may start with minor services such as the landscaping or upkeep of neighbourhoods: for example, the maintenance of some new-build estates in the UK are being left to developers in return for a service charge. This might seem insignificant, but it introduces an unregulated cost for the residents.

Privatising the provision of municipal services may be seen by some as a way for wealthier residents to enjoy a better standard of living – as in Hanoi. But in the worst cases, it puts in a paywall in front of fundamental services such as sewage disposal – as happened in Gurgaon. In other words, privatisation may start with insignificant services and expand to more fundamental ones, creating greater segregation and inequality in cities.


A divided city

My own research on branded housing projects in Turkey has highlighted the drastic consequences of the gradual expansion of exclusive services and facilities through segregated developments. These private housing developments – known for their extensive use of branding – have sprung up in Istanbul and other Turkish cities over the past two decades, since the government began to favour a more neoliberal approach.

By 2014, there were more than 800 branded housing projects in Istanbul alone. They vary in scale from a single high-rise building to developments aiming to accommodate more than 20,000 residents. Today, this development type can be seen in every city in Turkey, from small towns to the largest metropolitan areas.

The branded housing projects are segregated by design, often featuring a single tower or an enclosing cluster of buildings, as well as walls and fences. They provide an extensive array of services and facilities exclusively for their residents, including parks, playgrounds, sports pitches, health clinics and landscaping.

Making the same services and facilities available within each project effectively prevents interaction between residents and people living outside of their development. What’s more, these projects often exist in neighbourhoods which lack publicly accessible open spaces such as parks and playgrounds.

This is a city-wide problem in Istanbul since the amount of publicly accessible green spaces in Istanbul is as low as 2.2 per cent of the total urban area. In London, 33 per cent of the city’s area is made up of parks and gardens open to the public – which shows the severity of the problem in Istanbul.

These branded housing projects do not feature any affordable units or social housing, so there are no opportunities for less privileged city-dwellers to enjoy vital facilities such as green spaces. This has knock-on effects on excluded residents’ mental and physical health, contributing to greater inequality in these respects, too.

Emerging alternatives

To prevent increasing inequality, exclusion and segregation in cities, fundamental urban services must be maintained or improved and kept in public ownership and made accessible for every city-dweller. There are emerging alternatives that show ways to do this and challenge privatisation policies.

For example, in some cities, local governments have “remunicipalised” key services, bringing them back into public ownership. A report by Dutch think-tank the Transnational Institute identified 235 cases where water supplies were remunicipalised across 37 countries between 2000 and 2015. The water remunicipalisation tracker keeps track of successful examples of remunicipalisation cases around the world, as well as ongoing campaigns.

It is vitally important to keep urban services public and reverse subtle forms or privatisation by focusing on delivering a decent standard of living for all residents. Local authorities need to be committed to this goal – but they must also receive adequate funds from local taxes and central governments. Only then, will quality services be available to all people living in cities.

The Conversation

Bilge Serin, Research Associate, University of Glasgow.

This article is republished from The Conversation under a Creative Commons license. Read the original article.