Urban forests and pollution mitigation: Analysing ecosystem services and disservices

Urban forests: could they be doing us a disservice?

There is a growing body of scientific research into the health benefits of urban forests, such as improving air quality and providing recreational space. However, new research challenges the assumption that their overall impact on quality of life is always positive and land planners need to take into account ecosystem disservices as well as services, say the researchers.

‘Ecosystem services’ describes the components of a natural ecosystem that are directly enjoyed, consumed or used to produce measurable benefits to humans. Trees, shrubs and areas of grass or soil in cities, when combined, known as urban forests, offer a number of ecosystem services, including improving air quality, flood protection, micro-climate regulation, shade and recreational facilities. However, not all urban forests provide these ecosystem services equally and factors that could turn a perceived benefit into a net cost are often overlooked.

According to the new study, the main issue currently missing from environmental assessments of urban forests is that the benefits need to be weighed up against any factors that negatively affect human well-being, the ‘ecosystem disservices’ or the costs entailed to economy and society. These might include allergens, pests and insects, leaf litter and debris, falling branches, wild animal droppings and bites, obscured views and enhanced fear of crime. These disservices are mainly classified as social nuisances. However, the removal of fallen debris and pest-control, for example, also incur financial costs. Some activities, such as pruning and maintenance, may even be a source of noise and air pollution through the fossil fuel-burning equipment used.

The researchers argue that city planners need to look at three specific factors to determine how important different ecosystem services (and disservices) are for a given community. These factors are ‘context’, ‘scale’ and ‘management-intensity’. To demonstrate how this approach works in practice, the researchers use the specific example of the ability of urban forests in different countries to improve air quality by filtering out airborne pollutants and sequestering carbon.

‘Context’ refers to the fact that urban forests are used by many different people in different ways and certain ecosystem services will not be equally important to all urban communities. For example, improving air quality might be more highly valued in inland cities than coastal cities or carbon sequestration might be more relevant in tropical than in arid cities.

‘Scale’ is important as the value of a particular ecosystem service or disservice will increase with the size of the forested area. Scale also means looking at a broad range of economic, social and temporal factors to fully assess the impact or relevance of a service or disservice on the population.

Different urban forests will also be characterised by different species, each with their own requirements for human inputs, such as water, fertiliser and maintenance. For example, highly maintained parks and streets are likely to be more ‘management intense’ than a naturally-occurring forest. This knowledge could be used to strategically maximise the desired ecosystem services and minimise any identified disservices.

The researchers point out that air pollution is just one of several important ecosystem services and that planners need to weigh up all of these services against the disservices for several alternative management approaches in order to maximise human well-being.

Source: Escobedo, F.J., Kroeger, T. & Wagner, J.E. (2011). Urban forests and pollution mitigation: Analysing ecosystem services and disservices. Environmental Pollution. 159; 2078-2087.
Contact: fescobedo@ufl.edu
Theme(s):Air pollution, Forests, Urban environments

Fuente: Science for Environment Policy DG Environment News Alert Service