Assessment and Valuation of the Ecosystem Services Provided by Urban Forests

Urban forests, composed of trees and other vegetation, are integral parts of urban ecosystems. Whether planted intentionally or left by default, urban forests appeared even in the earliest settlements. In urban areas, the constituent greenery provides a broad range of benefits, including opportunities for residents to have daily contact with nature, and to enjoy attractive landscapes and recreational activities (Grey and Deneke, 1986; Rowntree, 1986; Ulrich, 1986; Dwyer et al., 1992; Miller, 1997; Bolund and Hunhammar, 1999; Tyrvainen and Miettinen, 2000). In addition, vegetation in cities moderates microclimate extremes and reduces regional pollution (Botkin and Beveridge, 1997; Whitford et al., 2001). They contribute to an improved quality of urban life in many ways, even though these functions are often taken for granted by the public and some city authorities. The environmental benefits and natural functions provided by urban forests can be interpreted as ecosystem services, which are defined as benefits that the human population can derive, directly or indirectly, from ecosystem functions (Costanza et al., 1997). The urban population must rely mainly on services derived from external ecosystems, such as food and energy. However, the diversified benefits generated by urban forests, which are limited in comparison with the amount of imported ecosystem services, could be more instrumental in solving local environmental problems. They could significantly improve the quality of urban life, and play a paramount role in stabilizing and sustaining urban ecosystems (Daily, 1997; Bolund and Hunhammar, 1999; Jensen et al., 2000). Such ecosystem services, however, are not very tangible and generally not well understood or appreciated. Recent studies have generated a wealth of scientific information on the magnitude of their benefits. A more direct interpretation of these benefits for laypersons could promote their preservation and enhancement. A useful approach is to quantify these natural services, and then follow with a valuation of these nonmarket and noncommodity goods. The results could be translated into the universal language of monetary units, and be compared with alternatives to facilitate decision making related to natural resources or the environment. Valuation is inseparable from the choices and decisions humans have to make about ecosystems (Bingham et al., 1995; Costanza et al., 1997; Barbier et al., 1998; Costanza, 2000). Some studies have attempted to quantify the ecosystem services generated by urban