A framework for developing urban forest ecosystem services and goods indicators

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Abstract

The social and ecological processes impacting on urban forests have been studied at multiple temporal and spatial scales in order to help us quantify, monitor, and value the ecosystem services that benefit people. Few studies have comprehensively analyzed the full suite of ecosystem services, goods (ESG), and ecosystem disservices provided by an urban forest. Indicators, however, are one approach that could be used to better understand the structure of an urban forest, the suite of ESG provided by urban forests, and their influence on human well-being using a simple, innovative and repeatable metric. This study presents a framework for developing indicators using field data, an urban forest functional model, and the literature. Urban tree and soil indicators for groups of ecosystem functions were used to statistically analyze the effects of urban morphology and socioeconomics on urban forest ESG. Findings show that the most influential ESG indicators were tree cover, soil pH, and soil organic matter. Indicators were significantly influenced by land use and time since urbanization, while analyses of property values and household income did not yield any particularly significant results. The indicators presented in this paper present a first approach to non-monetary valuation of urban forest ESG and can be used to develop urban forest structure management goals and to monitor the effects of urban greening policies on human well-being.

Research highlights

▶ Urban forest ecosystem services and goods are dependent on tree cover. ▶ Indicators for ecosystem services and goods are influence by land use and time since urbanization. ▶ Indicators presented in this study are the first approach for a non-monetary valuation of urban forest ecosystem services and goods. ▶ Indicators can be used as a tool for evaluation of policies or management regimes.

Introduction

Relative to natural ecosystems, urban ecosystems have been cited as possessing unique climate, soils, vegetation, social dynamics, and flows of energy as a result of different ecological patterns, processes, and disturbances (Alberti, 2009, Pickett et al., 1997, Trepl, 1995). However, other studies such as those of Niemelä (1999), argue that the ecological processes and patterns between urban and other ecosystems are essentially the same, differing only in the importance and prevalence of certain disturbances. If indeed this is the case, urban ecosystems can be studied using common ecological principles (Niemelä, 1999) and other approaches such as the human ecosystem model (Pickett et al., 1997). As such, studying urban ecosystems can elucidate the interactions between social and ecological processes acting at multiple temporal and spatial scales and lead towards a better understanding of the influence of increased population, economic growth, and land use policies on urban forest function, community dynamics, and species distributions (Hostetler and Holling, 2000).

Understanding these functions can in turn improve urban planning, vegetation management, urban sustainability, allocation of financial resources, and most importantly human well-being in cities (Alberti, 2009, Pickett et al., 2009). This study developed indicators of ecosystem services, goods and disservices with the purpose of better understanding ecological processes in urban ecosystems. Most importantly these indicators could assess how the provision of urban forest ecosystem services is influenced by ecosystem structure, urban morphology and socioeconomics. Integrating these indicators into a framework could also be used to monitor the effects or urbanization and policies on urban forests and subsequent human well-being.

Ecosystem functions are the physical, chemical and biological processes occurring in ecosystems that are necessary for its self-maintenance (Turner and Chapin, 2005) and are the result of interactions between the biotic and abiotic components of an ecosystem (De Groot et al., 2002). Daily (1997) refers to these functions as ecosystem services and defines them as those conditions and processes through which natural ecosystems, and the species that inhabit them, sustain and fulfill human life. More specifically, ecosystem services are defined by their contribution to human well-being, since they are end products of various ecosystem functions such as climate amelioration and recreation because they are enjoyed, consumed or used by humans. Ecosystem goods, a subset of ecosystem services, can be defined as tangible material products such as wood, fuel, or food that results from ecosystem processes (De Groot et al., 2002).

Other types of ecosystem functions and structures might have negative consequences on human life are referred to as ecosystem disservices (Agbenyega et al., 2008, Lyytimäki and Sipilä, 2009, Zhang et al., 2007) and are exemplified by urban parks that are habitat for rats, mice, vectors and their pathogens (De Stefano and Deblinger, 2005) and human fears related to personal safety in green areas (Jorgensen and Anthopoulou, 2007, Lyytimäki and Sipilä, 2009). Therefore, ecosystem services, disservices and goods are defined by humans who determine their importance and value (De Groot et al., 2002). As a result, differentiating among ecosystem disservices, services or goods will depend on humans, their preferences, and socio-political as well as biophysical contexts (Lyytimäki and Sipilä, 2009, Zhang et al., 2007).

Urban and peri-urban forests as defined in this study are the tree and soil components of an urban ecosystem and are characterized by their structure, amount (e.g. volume), size (e.g. height and diameter), distribution (e.g. covers), and composition (e.g. number of species, soil types). Urban forest structure is a determinant of ecosystem function which has been documented as a means of mitigating environmental quality problems associated with the urban built environment (Nowak et al., 2006). The structure and subsequent function of the urban forest will therefore determine the provision of ecosystem services and goods (ESG; De Groot et al., 2010). Thus, by altering the structure of the urban forest, we can alter certain ecosystem functions that maximize human well-being in cities.

Urban forests, however, can also incur costs due to maintenance and management requirements, contribute to the perceived risk of crime, and emit pollutants (Lyytimäki and Sipila, 2009). Since this could have a negative effect on human well-being, these functions are referred to as disservices and are common to human influenced areas such as urban and ecosystems (Agbenyega et al., 2008, Lyytimäki and Sipilä, 2009, Zhang et al., 2007). For example, highly maintained trees and lawns in cities (e.g. structure) can produce pollen and reduce water infiltration – relative to natural areas – increasing runoff (e.g. functions) which could result in human allergies and flooding events (e.g. ecosystem disservices; Ogren, 2000, Paul and Meyer, 2001). Increased runoff can also decrease water quality by washing off nitrogen and phosphorus excess from fertilizers or increase the concentration of dust particles (Brezonik and Stadelmann, 2002).

Indicators are numerical values that describe the state of a phenomenon or environment and are used as tools to summarize information about the condition of an ecosystem (OECD, 2001, Segnestam, 2002). They reduce dimensionality of data, simplify interpretations, and facilitate communication between experts and non-experts (Segnestam, 2002). Therefore, indicators could be used as metrics for key information concerning ecosystem structure, function and services. Ecological indicators can combine measurable characteristics of structure, such as habitat or landscape patterns, with inherent ecosystem functions and services (Niemi and McDonald, 2004). Conversely, they can oversimplify interactions existing across temporal and spatial scales (Dale and Beyeler, 2001). Furthermore, an indicator does not provide information on the causality behind the value assigned to a particular ecosystem service (Segnestam, 2002). Environmental indicators do, however, condense information about conditions and may show trends and provide a better understanding of the viability of a system (UNEP, 2007). As stated by De Groot et al. (2010), two types of indicators are needed to quantify the capacity of landscapes to provide ESG: (1) State indicators describing which ecosystem function is providing a service and how much and (2) How much of that service can be used in a sustainable way. This information therefore, can provide decision-makers with an evaluation tool for establishing baselines and developing management and maintenance regimes aimed at conserving urban and peri-urban forests (De Groot et al., 2002).

This study developed indicators to assess the state of urban forest ESG and disservices and determine how their provision varied according to urban forest structure, urban morphology, and socioeconomic factors (De Groot et al., 2010, James et al., 2009). The specific hypotheses addressed in this study were to determine if: (1) greater amounts of urban forest cover resulted in increased indicator values of ESGs, (2) affluent areas exhibited higher ESG indicator values, (3) densely populated areas are characterized by lower ESG indicator values, and (4) recently urbanized areas are characterized by lower ESG indicators values. Results can be used as part of a framework that uses indicators to assess the effects of urbanization and policies on urban forest structure and subsequent provision of ESG and disservices.

Section snippets

Study area

Gainesville has a population of 113 942 inhabitants (U.S. Census Bureau, 2000), is located at approximately 29°39′N and 82°20′W in North Central Florida and covers an area of 127 km2. The climate is humid, subtropical with average monthly temperature of 19.4 °C in January and 33 °C in June and mean annual precipitation is 1228 mm (Metcalf, 2004). Soils are sandy siliceous, hyperthermic aeric hapludods and plinthic paleaquults (Chirenje et al., 2004) and natural vegetation is temperate evergreen

Regulation function indicator

Regulation function values are shown in Table 2. Maintenance of soil productivity was high since soil bulk density and organic matter were appropriate for plant growth and phosphorus and calcium were within recommended ranges for Florida (Gilliand, 1976) and the United States (Craul, 1999, Shacklette and Boerngen, 1984). However, high nutrient contents could lead to decrease water quality in lakes and water resources thus detrimentally affecting well-being (Carpenter et al., 1998).

Overall low

Discussion

The ESG indicators outlined in this framework present a typology that can be used in urban ecosystems to link urban forest characteristics and their functions to provision of ESG and disservices as well. The indicators quantify these relationships and several of the selected indicators could be used to monitor the sustainable provision of ecosystem services (De Groot et al., 2010).

Gainesville's tree cover was high and well distributed in both urbanized and naturally forested areas within the

Conclusion

This study developed a framework and indicators of ESG and disservices provided by urban forests using an existing typology, urban forest field measurements, modeling, and the literature. The framework accounted for ecosystem structure and functions and linked them to well-being. Correlations at the landscape, habitat, and species level as well as multi-scale analyses were also incorporated into the framework. Once urban forest structure data is obtained, the calculation of the indicator is

Acknowledgements

We would like to mention Florida Agricultural Experiment Station and USDA Forest Service Centers for Urban and Interface Forestry for funding. We would like to thanks Mark Hostetler for his careful reviews, Wendell Cropper and Jane Southworth for their valuable intellectual and conceptual input.

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