Background

Cities and urban areas are mainly a place of carbon emission (through transport, heating, etc.). Urban greenspace is the main urban land use that contributes to carbon storage.

Urban forests can contribute to reducing the atmospheric concentration of CO₂ in three ways:

• Trees directly sequester CO₂ as woody and foliar biomass as they grow; soils equally have the potential to sequester carbon
• Trees near buildings can reduce the demand for heating and air conditioning, reducing emissions associated with electric power production
• Paths and cycle lanes through greenspace are usually very attractive and can lead to a significant shift from cars to more sustainable means of transport. Green spaces also offer recreational activities that people currently practise outside towns.

Opportunities

Urban forests and greenspace have various functions. They contribute to the quality of life, and offer recreational activities to the inhabitants. They have a beneficial impact on the urban climate.

An additional function can be their contribution to limiting the city’s carbon footprint. The vegetation and soil of a greenspace can sequester carbon, contributing directly to a reduction in atmospheric CO₂ concentration. They can also affect the carbon balance indirectly, through their effects on the urban energy balance and thus on CO₂ emissions related to energy use (for example by reducing urban air temperatures and therefore building energy use). A related issue is the study of urbanisation, and the impact, in terms of carbon fluxes and carbon pools, of the conversion between natural space and the built environment in urban areas.

Practical considerations

Depending on their location in relation to buildings, urban trees can reduce incident radiation through shading effects, altered wind patterns, and increased evaporative cooling (Oke, 1989). Most studies have estimated a net decrease in energy use and subsequent CO₂ emissions from electricity use as a result of placing trees near buildings (Akbari, 2002; Akbari and Konopacki, 2005).

Field measurements support reduced summertime temperatures near urban forest canopies (Mueller and Day, 2005), as trees and shrubs provide protection from both heat and UV radiation by direct shading. Potcher et al. (2006) have shown that open spaces with a higher number or larger area of trees have been found to have lower temperatures than those with fewer trees.

Parks of at least 3 ha have been shown to be cooler than that of the surrounding urban areas. However, the temperature in parks of less than 3 ha is more variable, and the quantity of paved surfaces in a park also causes variation in park temperatures (Chang et al., 2007).

Estimates by Nowak and Crane (2002) suggested that if fossil fuels are used to maintain urban vegetation, net effects will eventually become negative (net emitters of carbon) at varying rates, depending on species, disposal techniques and maintenance intensity, unless trees are planted in energy-conserving locations (i.e. around buildings to reduce their energy use) to offset maintenance emissions.

McPherson et al. (2005c) estimated the carbon balance of several municipal urban forests (street and park trees) including direct carbon sequestration, indirect effects of energy savings, and fossil fuel emissions from forest maintenance and management. Carbon costs and benefits varied regionally, but in most cities energy savings constituted around 40% of reductions in net CO₂ emissions. In all cases, the presence of urban trees resulted in a net reduction in CO₂ emissions. Notably, these calculations pertain to growth and indirect emissions associated with the vegetative component of urban forests only; the impact of soil C pools in urban forests is more uncertain. These studies need to be extended to include a more extensive range of greenspace (including, for example, private gardens).

It is unlikely that carbon sequestration alone will justify the creation of greenspace, but it is clearly an associated benefit. The methodology to quantify the carbon sequestration of greenspace is being put in place, but still needs to be improved. Carbon sequestration has been included in the ecological indicators considered by Whitford et al. (2001) for a study in England.

Case studies

Carbon balance of urban forest in US cities

In an American study, McPherson et al. (2005a-c) estimated the carbon balance of urban forest (streets and park trees) in seven US cities. This study, which focused on trees on public land, was based on operational data gathered by the city department in charge of greenspace, some additional field measurements, and a model called STRATUM. It aimed to quantify the costs and benefits of urban forest.

Data from McPherson et al. (2005a-c) compiled by Pataki et al. (2006).^a Avoided emissions: emissions reductions from energy savings associated with tree cover. ^b Sequestered: carbon stored in the trees (the soil component is not considered in this study). ^c Maintenance: emissions associated with urban forest management activities (e.g. pruning, mowing etc.). ^d Decomposition: amounts of carbon returning to the atmosphere after decomposition of leaves.

The presence of urban trees in all cities resulted in a net reduction in CO₂ emissions. Notably, these calculations relate to the indirect emissions associated with the vegetative component of urban forests only; the impact of soil carbon pools in urban forests is more uncertain.

The structure and climate of European cities are different from those of American cities, and these results cannot be directly extrapolated to our case – but they do give some broad results, and an indication of the methodology that can be followed in further studies.

Services

Forest Research is improving the knowledge of the carbon balance of British forests and the products derived from them by:

• Producing a National inventory of woodland and trees – this Britain-wide map includes individual trees and linear features that constitute greenspace
• Monitoring short-term carbon fluxes associated with woodland and preparing detailed carbon budgets
• Developing models and methodologies for carbon accounting
• Improving our knowledge of the greenhouse gas balance of different types of UK forest.

Further information

Akbari, H. (2002). Shade trees reduce building energy use and CO₂ emissions from power plants. Environmental Pollution 116: S119–S126.

Akbari, H. and Konopacki, S. (2005). Calculating energy-saving potentials of heat-island reduction strategies. Energy Policy 33: 721–756.

Chang, C. R., Li, M. H. and Chang, C. (2007). A preliminary study on the local cool-island intensity of Taipei city parks. Landscape and Urban Planning 80 (4), 386–395.

Grimmond, C.S.B., Souch, C. and Hubble, M.D. (1996). Influence of tree cover on summertime surface energy balance fluxes, San Gabriel Valley, Los Angeles. Climate Research 6: 45–57.

McPherson, E.G. and Simpson, J.R. (1999). Carbon Dioxide Reduction through Urban Forestry: Guidelines for Professional and Volunteer Tree Planters. General Technical Report PSW-GTR-171. Albany, CA, USA: USDA Forest Service, Pacific Southwest Research Station.

McPherson, E.G., Simpson, J.R. and Peper, P.F. (2005a). City of Boulder, Colorado Municipal Tree Resource Analysis. Davis, CA, USA: USDA Forest Service, Pacific Southwest Research Station, Center for Urban Forest Research.

McPherson, E.G., Simpson, J.R. and Peper, P.F. (2005b). City of Minneapolis, Minnesota Municipal Tree Resource Analysis. Davis, CA, USA: USDA Forest Service, Pacific Southwest Research Station, Center for Urban Forest Research.

McPherson, E.G., Simpson, J.R. and Peper, P.F. (2005c). Municipal forest benefits and costs in five US cities. Journal of Forestry 103: 411–416.

Mueller, E.C. and Day, T.A. (2005). The effect of urban ground cover on microclimate, growth and leaf gas exchange of oleander in Phoenix, Arizona. International Journal of Biometeorology 49: 244–255.

Nowak, D.J. and Crane, D.E. (2002). Carbon storage and sequestration by urban trees in the USA. Environmental Pollution 116: 381–389.

Oke, T.R. (1989). The micrometeorology of the urban forest. Philosophical Transactions of the Royal Society of London B 324: 335–349.

Pataki, D.E., Alig, R.J., Fung, A.S., Golubiewski, N.E., Kennedy, C.A., McPherson, E.G., Nowak, D.J., Pouyat, R.V. and Lankao, P.R. (2006). Urban ecosystems and the North American carbon cycle. Global Change Biology 12: 2092–2102.

Potchter, O., Cohen, P. and Britan, A. (2006). Climatic behavior of various urban parks during hot and humid summer in the Mediterranean city of Tel Aviv, Israel. International Journal of Climatology 26 (12), 1695–1711.

Whitford, V., Ennos, A.R. and Handley J.F. (2001). City form and natural process – indicators for the ecological performance of urban areas and their application to Merseyside, UK. Landscape and Urban Planning 57: 91–103.

Glossary

Albedo

The albedo of an object is the extent to which it diffusely reflects light from the Sun.

Sequestration

A compound or metabolite is locked away so as not to be readily available. In the context of climate change, the term denotes the permanent storage of carbon dioxide or other compounds so they will not be released to the atmosphere where they would contribute to the greenhouse gas effect.