Carbon-sink value of urban trees greatly underestimated

New research from the UK could prove useful to China's urbanisation drive and push for low-carbon economic development.

Published in the Journal of Applied Ecology, the research shows that plants, but in particular trees, make a significant contribution in capturing carbon emissions in their local environment. This is believed to be the world's first study to quantify how much carbon is stored within urban vegetation.

A combination of field-survey and satellite data was used to assess domestic gardens, public spaces, road verges and derelict industrial land within Leicester; a typical British city with a population of about 300,000.

Researchers discovered that an average of over 3kg of carbon was locked into vegetation per square metre of the city. Comparing this figure with existing estimates of carbon capture within the United Kingdom's ecosystem, the service provided by Leicester's above ground vegetation turns out to have been underestimated by an order of magnitude.

Lead researcher Dr Zoe Davies of the Department of Animal and Plant Sciences at University of Sheffield said current assessments hold that once land in the UK is classified as urban, its biological carbon density is assumed to be zero. "Our study illustrates that this is not the case," she explained.

The research team points out that as urbanisation increases opportunities to offset carbon emissions become ever-more important. China is projecting that urbanites will be 52 percent of its population by 2015 and 65 percent by 2030, according to the government's 12th Five-Year-Plan (2011-2015).

On the back of those facts Qian Zhimin, deputy director with the National Energy Administration, told a low-carbon forum sponsored by the Asia-Pacific Economic Co-operation in Tianjin last month, that by 2015 China will establish 100 model cities, 200 model counties, 1,000 model districts, and 10,000 model towns of green and new energy.

Little has been outlined in these or other plans for green space in these developments, or indeed on the 'greening' of existing towns and cities. However the new research provides food for thought.

Leicester's vegetation locks away over 231,000 tonnes of carbon, equivalent to 3.16kg per square metre of the city. In practical terms, the team found that an astonishing 97·3 percent of the above-ground carbon pool was associated with trees rather than herbaceous and woody vegetation.

While these figures may appear unimpressive in the great scheme of things (in particular the large amounts carbon cities emit), most countries are like the UK in not viewing cities as having any positive effects on the carbon cycle.

Britain's vegetation carbon stock is estimated to be 113.8 megatonnes, meaning that Leicester accounts for 0.2 percent of the nation's above ground carbon store, while only making up 0.03 percent of its area.

As China pushes for low-carbon urbanisation, leveraging this research provides another reason why urban trees and green spaces should be valued.

The study is part of £2.5 million (USD4 million) project, funded by the Engineering and Physical Sciences Research Council (EPSRC), that is investigating the size of the urban carbon footprint.