ReviewUrban forests and pollution mitigation: Analyzing ecosystem services and disservices
Introduction
Literature on urban forests, pollution, and sustainability promotes and advocates the positive contributions of trees in maintaining environmental quality (Beckett et al., 1998, Escobedo et al., 2008, McPherson et al., 1998, Nowak et al., 2006, Yang et al., 2005). In general these studies have led to a normative assertion by environmental managers that any increase in urban forests is desirable and will mitigate pollution problems. However, increasing rates of urbanization, lack of prioritization of tree preservation by city officials, inclusion of trees by decision makers in environmental policies, and a widespread neglect of the costs associated with urban forests call into question the assumption that the net effect of urban forests on environmental quality is always positive.
The purpose of this paper is to critically assess the role of trees in mitigating urban pollution and maintaining quality of life by integrating the concepts of ecosystem services and disservices into urban forest management. Specifically, we develop a connection between the ecosystem services of urban forests and their efficacy in mitigating urban environmental pollution problems. We develop this relationship by first reviewing the literature on urban forests and pollution mitigation. Second, we adapt existing ecosystem services definitions from the economics and ecological literature and apply them to urban forests. Third, using air quality improvement and carbon sequestration by urban forests as examples, we consider the role of context, scale, heterogeneity, and management intensity on the benefits – and costs – of urban forests, and highlight the need to define which ecosystem services and specific pollution problems are relevant given the particular economic, social, and environmental goals of urban sustainability. Finally, we present some methods for valuing the benefits provided by different urban forest management alternatives and considering the costs urban forests impose on communities and individuals living in cities.
Integrating ecosystem services of urban forests into environmental quality strategies makes it possible to identify the type and composition of trees that maximizes people’s overall quality of life in a specific city at least cost. This analysis can make explicit the tradeoffs associated with different management strategies, which is necessary for the sustainable management of urban forests and to address urban environmental pollution problems and enhance urban quality of life.
Section snippets
Urban forests and ecosystems
Urban forests – or the sum of all urban trees, shrubs, lawns, and pervious soils – are located in highly altered and extremely complex ecosystems where humans are the main drivers of their types, amounts, and distribution (Dobbs et al. 2011). Urban areas are characterized by high population densities and a network of non-natural, built-up infrastructure (Sanders, 1984, Williams et al., 2009). High per-capita energy and material consumption patterns and large resource inputs have contributed to
Ecosystem services and disservices of urban forests
Cities are characterized by multiple actors with different interests and varying perceptions towards trees and pollution (Agbenyega et al., 2009, Lyytimäki and Sipilä, 2009, Popoola and Ajewole, 2001). Nevertheless, the normative assumption in the literature is that in general all trees provide ecosystem services such as pollution removal and thus are desirable. A more nuanced assessment acknowledges that urban forests simultaneously produce multiple and wide ranging ecosystem outputs.
Context, scale, heterogeneity, and management intensity
Fig. 1 illustrates the relationship among ecosystem end-products (i.e., services and disservices) and ecosystem structure and function discussed in the previous sections. This figure delineates a process starting with ecosystem structure as a driver of ecosystem function and the ecosystem services and disservices end-products. The dashed arrows represent these biophysical flows. To differentiate between ecosystem services and disservices it bears reiterating that ecosystem services and
Assessing the efficacy of urban forests in mitigating urban pollution
The dynamics between urban pollution and forests has received attention in this and other journals (Beckett et al., 1998, Harris and Manning, 2010, Paoletti, 2009) and other studies such as McPherson et al. (1998) and Escobedo et al. (2008) have modeled the cost-effectiveness of using urban forests to improve air quality. Heal (2000) discusses that conserving and managing for the ecosystem services of the New York City Catskills watershed was shown to be more cost-effective than construction of
Valuing the ecosystem services provided by urban forests
Recent literature suggests that promoting ecosystem services and accounting for ecosystem disservices, as opposed to just quantifying ecosystem functions is more useful to decision makers (Boyd and Banzhaf, 2007, Kroeger and Casey, 2007). Managing for urban forest ecosystem services and disservices at the appropriate scale and context will also make the subsequent benefits and costs more relevant to communities and decision makers – not just researchers and environmental managers (Fig. 2). By
Using forests to mitigate urban pollution and enhance quality of life
Unfortunately, urban forest ecosystem services and disservices are reported principally from an academic perspective, but most urban citizens and policy makers do not see the relative value or direct benefits of urban forests in mitigating pollution, while they generally recognize the costs through personal experiences or losses (Agbenyega et al., 2009). People value ecosystem services based – among other things – on the information they have and the perceived importance of particular ecosystem
Acknowledgements
We thank Cynnamon Dobbs, Min Zhao, Matt Cohen, Dave Nowak, and participants of the 2010 Urban Environmental Pollution Conference for discussions and suggestion that helped us develop this paper. We are particularly grateful to Dr Min Zhao for help with Fig. 3 and the 4 anonymous reviewers for their helpful and insightful comments.
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