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Editor’s Choice: Urban ecosystem services

September 2011 (Issue 48:5)

Davies, Z.G., Edmondson, J.L., Heinemeyer, A., Leake, J.R. & Gaston, K.J. 2011) Mapping an urban ecosystem service: quantifying above-ground carbon storage at a city-wide scale. Journal of Applied Ecology, 48, 1125-1134.

To fulfil international obligations to produce national inventories of greenhouse gas emissions by sources and removal by sinks, as well as meeting reporting requirements under the Kyoto Protocol, biological carbon emissions and sequestration arising from different land uses, land use change and forestry must be taken into account. This includes recording carbon loss and capture due to the conversion of land through the process of urbanisation. However, in the UK, once land is considered to be urban, biological carbon density at equilibrium is assumed to be zero. This is likely to be a legacy of the perception that urban ecosystems have limited ecological value because they are heavily modified by humans and relatively small in size. Yet, during the past century, the global urban population has grown ten-fold and at present more than half of the world's population lives in cities. As a result of this rapid expansion, urban areas continue to grow at a faster rate than any other land-use type. However, although urbanisation is a primary driver of land-use change globally, there have been surprisingly few attempts to quantify ecosystem service provision at a city-wide scale. One service that is an increasingly important aspect of climate change mitigation policies is carbon storage.

In this Editor’s Choice researchers, led by Zoe Davis from the University of Kent in Canterbury, UK, demonstrate that there is a pressing need to produce detailed maps of carbon stocks across cities, at a scale and resolution pertinent for urban landscape management and policy-making. They examine the quantities and spatial patterns of above-ground carbon stored in a typical British city, Leicester, and consider how carbon density differs in domestic gardens, indicative of bottom-up management of private green spaces by householders, and public land, representing top-down landscape policies by local authorities. They estimated over 230 000 tonnes of carbon is stored within the above-ground vegetation of Leicester, equating to 3.16 kg C m−2 of urban area, with 97.3% of this carbon pool being associated with trees rather than herbaceous or other woody vegetation. In contrast, domestic gardens store just 0.76 kg C m−2, which is not significantly different from herbaceous vegetation (0.14 kg C m−2). The greatest above-ground carbon density is 28.86 kg C m−2, which is associated with areas of tree cover on publicly owned/managed sites. By comparing a national ecosystem service map with the estimated quantity and distribution of above-ground carbon within Leicester they found that existing national carbon storage maps underestimate urban carbon pools by two orders of magnitude. In addition, they show that high resolution distribution data of carbon stocks across cities are essential for informing policy and management decisions at a scale that allows for the diverse patterns of land ownership to be accounted for.

The findings are not only of significant in theory in assessing ecosystem carbon pools in urban area, but also of importance in management practice for enhancing carbon sequestration in urban ecosystem. The UK government has recently set a target of an 80% reduction in greenhouse gas emissions, from 1990 levels, by 2050. Local authorities are therefore central to national efforts to cut carbon emissions, although reductions required at city-wide scales are yet to be set. This has led to a need for reliable data to help establish and underpin realistic carbon emission targets and reduction trajectories, along with acceptable and robust policies for meeting these goals. Zoe and colleagues illustrate the potential benefits of accounting for, mapping and appropriately managing above-ground vegetation carbon stores, even within a typical densely urbanised European city. For example, most of the publicly owned land across Leicester is grassland and if just 10% of this was planted with trees, the existing carbon pool across the city could be increased by 12%.

In addition, the carbon densities they report can be used to generate better quality estimates of carbon storage for other UK and northern European cities, where suitable landcover datasets are available. In the UK, explicit urban tree surveys, such as the National Forest Inventory and Trees in Towns, will have a valuable role to play in refining these estimates of above-ground carbon by monitoring temporal patterns and trends in the provision of this ecosystem service in the future. The study will be of clear importance to policy-makers at all levels of government, urban land managers, academics and those interested in sustainability science more broadly.

However, while such information is critical if the ecological performance and service provision of urban ecosystems is to be maximised, the quantities of carbon stored within the above-ground vegetation is not a permanent sink. The carbon captured as a plant grows will ultimately be released back into the environment when it dies or is destroyed, and replacement is therefore necessary to counterbalance the carbon emitted from removed vegetation. Moreover, to maximise net carbon storage provided by above-ground vegetation, fossil fuel consumption related to management activities (e.g. through the use of lawn mowers, chainsaws, vehicles, chipping machines) must be minimised, decomposition of waste material should be limited via long-term carbon storage solutions (e.g. land fill, making wood products) and the biomass used where possible as an alternative renewable fuel source. The study is part of £2.5m multi-institutional project, funded by the Engineering and Physical Sciences Research Council (EPSRC), that aims to assess the size of the urban carbon footprint.

Philip E. Hulme
japplecol@hotmail.com

Reference

Davies, Z.G., Edmondson, J.L., Heinemeyer, A., Leake, J.R. & Gaston, K.J. (2011) Mapping an urban ecosystem service: quantifying above-ground carbon storage at a city-wide scale. Journal of Applied Ecology, 48, 1125-1134.