Elsevier

Science of The Total Environment

Volume 615, 15 February 2018, Pages 1364-1378
Science of The Total Environment

Assessment of urban green space structures and their quality from a multidimensional perspective

https://doi.org/10.1016/j.scitotenv.2017.09.167Get rights and content

Highlights

  • Compact green cities require multifunctional open spaces.

  • Development of a multidimensional indicator set to assess urban green spaces.

  • Empirical study on the perception of park structure by citizens.

  • Need to differentiate artificial and natural elements in urban green spaces.

  • Findings can be integrated into a higher-scale assessment and planning of cities.

Abstract

Facing the growing amount of people living in cities and, at the same time, the need for a compact and sustainable urban development to mitigate urban sprawl, it becomes increasingly important that green spaces in compact cities are designed to meet the various needs within an urban environment. Urban green spaces have a multitude of functions: Maintaining ecological processes and resulting services, e.g. providing habitat for animals and plants, providing a beneficial city microclimate as well as recreational space for citizens. Regarding these requirements, currently existing assessment procedures for green spaces have some major shortcomings, which are discussed in this paper.

It is argued why a more detailed spatial level as well as a distinction between natural and artificial varieties of structural elements is justified and needed and how the assessment of urban green spaces benefits from the multidimensional perspective that is applied. By analyzing a selection of structural elements from an ecological, microclimatic and social perspective, indicator values are derived and a new, holistic metrics1 is proposed. The results of the integrated analysis led to two major findings: first, that for some elements, the evaluation differs to a great extent between the different perspectives (disciplines) and second, that natural and artificial varieties are, in most cases, evaluated considerably different from each other. The differences between the perspectives call for an integrative planning policy which acknowledges the varying contribution of a structural element to different purposes (ecological, microclimatic, social) as well as a discussion about the prioritization of those purposes. The differences in the evaluation of natural vs. artificial elements verify the assumption that indicators which consider only generic elements fail to account for those refinements and are thus less suitable for planning and assessment purposes.

Implications, challenges and scenarios for the application of such a metrics are finally discussed.

Introduction

Rapid global development of urban systems causes need to mitigate urban sprawl by designing compact and multifunctional cities. Compact cities, in this sense, refer to urban landscapes which are densely populated and provide functions for their citizens in compact, dense areas to avoid urban sprawl. The term compact green cities highlights the importance of sustainable use of resources, enhancement of mobility and reduction of city-specific particulate emissions within those cities (Stevenson et al., 2016). The term green, however, comprises also the provision of open green spaces in a city, i.e. free spaces with a significant coverage of intended vegetation (such as public parks and recreation areas).

Green spaces play a key role in the development of compact green cities by providing a variety of ecosystem functions and services. This makes green spaces valuable from a variety of perspectives: from the microclimatic point of view, they contribute to a balanced city climate (Bowler et al., 2010) and affect the local concentrations and distribution of air pollutants such as particulate matter (Litschke and Kuttler, 2008, Janhäll, 2015), ozone and its major anthropogenic precursors, volatile organic compounds (VOCs) and oxides of nitrogen (e.g., Sillman, 2002, Calfapietra et al., 2013). From an ecological perspective, they can provide habitable space for plants and animals (Nielsen et al., 2014, Farinha-Marques et al., 2011) as a basis to maintain and preserve basic ecological properties and processes (Mace et al., 2012, Geslin et al., 2013). From a social point of view, they provide functions to city dwellers in the form of meeting points and places to relax and to recreate (Peters et al., 2010). Furthermore, it was shown that urban green spaces have a positive impact on health and wellbeing (Nielsen and Hansen, 2007, Richardson and Mitchell, 2010).

For dense cities, it is therefore of particular importance to efficiently use the few open spaces (Jim, 2004) by designing urban green spaces in terms of ecological, climatic, and social considerations at the same time (Gill et al., 2007). This will increase the multifunctionality of a site and promote sustainable urbanization which is a basic prerequisite for the concept of compact green cities (Næss, 1995).

For the design of urban green spaces according to ecological, climatic and social criteria, adequate indicators and assessment strategies are needed which reflect the multifaceted functions urban green spaces fulfill (Mörtberg et al., 2017). Up to now decision-making in practical management is mostly based on cost and aesthetic considerations (Beer et al., 2003) but less on ecological or climatic criteria.

The aims of this research are (1) to identify gaps and challenges in urban green space assessment and (2) to develop a truly quantitative methodology for multidimensional, indicator-based assessment based on structural elements in urban green spaces. Furthermore, we aim (3) to provide a sample application as a proof of concept for the developed assessment approach and (4) to discuss the implications, challenges and application contexts within future planning procedures.

With regard to these aims the resulting novelties are:

  • (1)

    A brief overview that acquires insights about current gaps and challenges in urban green space assessment. We will review the shortcomings and explain how our approach addresses these gaps.

  • (2)

    A combined methodology in which urban green spaces are assessed from three different, but very important perspectives at the same time using an integrative approach. The underlying idea of the joint approach is to treat all perspectives as equally important and to combine them within one frame of references in order to make different assessment results visible. An empirical study to assess citizens' perceptions and preferences, the multidimensional indicator set and its development are presented.

  • (3)

    A way to quantify the multidimensional benefits of urban green spaces within a novel, holistic metrics on a local small-scale level. Such an assessment has not been conducted quantitatively in the literature so far, although there might be an understanding from planners' perspective that multiple factors need to be taken into account.

  • (4)

    Knowledge about the applicability of the indicator set taking selected structural elements of urban green spaces as examples. Differences in the valuation of structural elements by various scientific perspectives are identified.

The outcome provides a common ground for specific discussion about trade-offs and synergies in planning procedures of such urban green space in the future.

To develop livable compact and green cities, indicators are required that assess the quality of urban green spaces and serve as planning tools in the future. Based on a review of current indicators and evaluation procedures, three gaps in research and/or management procedures were identified. The first gap refers to the spatial level of current assessment indicators, the second to the unidimensionality of the assessment and the third to the generic treatment of structural elements.

Green space assessment, decision making and maintenance in cities and municipalities is based more and more on ecosystem services assessment approaches (Niemela et al., 2010, Hansen et al., 2015). The concept of ecosystem functions and services can help to improve existing green spaces or create new ones within a city (Konijnendijk et al., 2013). However, assessments of ecosystem services in an urban environment are mostly conducted on a landscape-level rather than on a local site-scale (Dennis and James, 2016, Haase et al., 2012). Especially with regard to the overall accepted aim to mitigate urban sprawl for economical, climatological and ecological reasons (Stone et al., 2010, Sushinsky et al., 2013, Whitmee et al., 2015), it is of particular relevance to focus on the specific, small-scale design and characteristics of the few open spaces. Therefore, a local scale perspective with high spatial resolution needs to be adopted. Because often ecological properties and processes can only be recognized and assessed on a small-scale level, it is indispensable to focus on single structural vegetative units or habitats (Lovett et al., 2005).

Urban research has been conducted on small-scale habitat structures before (e.g., Voigt et al., 2014, Lehmann et al., 2014, Byrne, 2007), considering structural diversity with regard to single functions (e.g., recreational services, microclimate effects or urban soil ecology) of green spaces. However, there is still a lack of knowledge regarding a multidimensional evaluation on this site-scale level and a test of its applicability. Therefore, the presented assessment approach of this paper aims at a local scale level.

It has been acknowledged that urban green spaces provide a variety of services to urban areas and that research on those spaces therefore requires multidimensional efforts (James et al., 2009, Luederitz et al., 2015). This is important because elements with a low ecological value might be rated high from a microclimatic or social perspective. This way, elements which would be dismissed on the basis of current indicators can thus be appropriately valued. Although the need for a multidimensional perspective is widely acknowledged and accepted (Balram and Dragićević, 2005, Borgström et al., 2006, De Ridder et al., 2004), it is still not realized in many assessment procedures. Current indicators (e.g., the percentage of green areas, percentage of sealed soil, green volume) used in the assessment of urban green spaces often imply single functions or services, and thus evaluate urban green spaces in a unidimensional manner (Grunewald and Bastian, 2015). In practice, there is still the challenge of capturing, operationalizing, and measuring functions and services from different disciplinary perspectives in a joint approach (Luederitz et al., 2015).

O'Sullivan et al. (2017) exemplify such an approach by characterizing different tree species in terms of several ecosystem services (e.g., air quality and biodiversity value). However, an aesthetic function is not covered by their approach, although including social parameters in the set of indicators can further enhance efforts of planning of urban green spaces according to public needs and wishes (Wilker et al., 2016), in the way that citizens' preferences for single structural elements are systematically assessed.

On the other hand, studies on perception of urban green spaces by citizens have already been conducted (Todorova et al., 2004, Sanesi and Chiarello, 2006, Shackleton and Blair, 2013) showing that there is conflicting evidence regarding people's attitude towards natural or artificial design, but also that the specific design of urban spaces, artificial or natural, is a pressing question. These studies, however, did not incorporate ecological and microclimatic aspects of the studied sites.

Integrating social parameters in urban green space assessments provides a way of equitable citizen participation next to, e.g., ecological considerations in urban green management. The necessity of including the social perspective in terms of citizen participation is stressed by Balram and Dragićević (2005) and provides an opportunity to address the mismatch between experts' and laypeople's perspectives on ecosystems (Garritt, 2006, Lazo et al., 1999, Bonnes et al., 2007, Hofmann et al., 2012).

These opposing preferences present a difficulty for the sustainable implementation of nature-conservation policies (Harrison et al., 1998, Bonnes et al., 2007), as policies introduced by experts may lack support by urban dwellers which are mostly laypeople. An integration of the citizen perspective is thus all the more important to design urban green spaces with the citizens rather than for them in a top-down manner (Gross, 2007, Zaunbrecher and Ziefle, 2016). By this, not only livable and socially valued urban green spaces are provided, but also the awareness of laypeople for the importance of sustainable urban green spaces in general is increased.

There is a large strand of research which connects at least two of the three parameters (ecological, microclimatic and social) in the context of urban green spaces. However, the research undertaken had another focus and aim than the indicator set which is developed and argued later in this paper.

On the one hand, there were approaches in which a social factor in the context of urban green spaces (Nielsen and Hansen, 2007, Peters et al., 2010, Sanesi and Chiarello, 2006) was combined with either a planning (Baycan-Levent and Nijkamp, 2009, Gobster et al., 2007), an ecological (Meacham et al., 2016, Arnberger and Eder, 2012) or a microclimatic perspective (Cohen et al., 2012, Mahmoud, 2011). To the best of our knowledge, in none of the papers all three parameters - social, ecological and microclimatic - have been combined, in particular not on the level of single structural elements (cf. Gap 1). In addition, most of this research takes an explanatory approach of the factors examined, investigating either the influence of urban green spaces on social parameters (urban green spaces as explanans, e.g., Hegetschweiler et al., 2017, Van Herzele and Wiedemann, 2003, Mambretti et al., 2005) or on attitudes and perceptions of residents towards urban green areas (urban green spaces as explanandum, e.g., Bertram and Rehdanz, 2015, Balram and Dragićević, 2005). What is missing is a neutral metrics or methodology for planners, with which they are able to describe and evaluate a given area taking different perspectives into account, without the need of explaining the interconnection between the factors. This is the novel idea of this paper - to contribute to a metrics which allows describing urban green spaces out of the three major perspectives in an equitable way.

The current numerical assessment procedures of habitats and landscapes take different land cover types and biotopes into account in a generic manner (Biedermann et al., 2008, Burkhard et al., 2009). This means that no differentiation is made within the respective categories or elements (e.g. trees, hedges, lawns etc.) on a site-scale approach.

Recently, Mörtberg et al. (2017) conducted a study in which ecosystem services are analyzed in conjunction with accessibility and urban planning. While presenting an example for a valuable, integrated approach to ecosystem services, the specific green areas considered are not analyzed in more detail, i.e. with regard to their ecological quality.

Similarly, while in Voigt et al. (2014) a multidimensional approach to urban green spaces is taken on the level of single structural elements, these elements are not differentiated according to their quality (artificial or natural).

A classification which considers only generic elements (like “hedge” or “tree”), however, fails to account for diversity in ecosystem services provided by different instances of the same element, e.g. a near-natural and a trimmed hedge or different species of the same element (O'Sullivan et al., 2017). Furthermore, very generalized indicators provide only vague guidelines with regard to the design of urban green spaces, since they only indicate which element is referred to but not in which variety or quality this should be implemented. The distinction between natural and artificial instances is not only relevant from a planners', but also from laypeoples' perspectives (Hofmann et al., 2012, Jankovska et al., 2010, Jim and Chen, 2006, Southon et al., 2017). The innovative approach of the indicator set which is developed in this work is therefore the explicit distinction between natural and artificial instances of structural elements.

The review of indicators and assessments used in the context of urban green spaces has revealed three major gaps and challenges that will be addressed in this paper.

The challenge of analyses on a site-scale (see gap 1) will be addressed by a comprehensive analysis of structural elements in urban green spaces. This local site-scale approach allows evaluation of single components in a green space rather than whole landscapes, which is a prerequisite to assess and compare different green sites and their design in a bottom-up approach within a holistic model.

The underlying concept of the framework is adapted from numerical valuation of biotopes in national (or rather regional) evaluation schemata to quantify compensation measures, which is used in various nature conservation and landscape management procedures (Biedermann et al., 2008, Hetzel et al., 2014). The valuation of ecological functions on a biotope-level to assess and compare the quality of nature has a long tradition in landscape ecology. Already in 1991, Ludwig and Meinig developed a systematic, numerical method for the valuation of biotope-types. This approach rates biotopes according to the criteria naturalness (“Natürlichkeit”), rarity (“Gefährdung/Seltenheit”), possibility of spatial and temporal replacement (“Ersetzbarkeit/Wiederherstellbarkeit”) and completeness (“Vollkommenheit”), which are still used in current planning procedures, for example in the evaluation of habitat types for compensation measures in the federal state of North Rhine Westphalia (Biedermann et al., 2008) or Bavaria (Hetzel et al., 2014), Germany. From these regulating intervention rules, a first draft for a standardized national directive was derived (BMUB, 2013).

In our assessment approach these criteria will be updated for usability of the approach in an urban realm with the concept of ecosystem processes and functions to adequately reflect recent research lines (Hansen et al., 2015) and to generate a multidimensional perspective on urban green spaces. We marry the well-tried, longstanding landscape ecological principles and strategies with innovative concepts addressing functions and services of ecosystems, which is required in urban research (Breuste et al., 2008). Because the scale is on the level of single structural elements rather than urban green spaces as a whole, the assessment will primarily be focused on the evaluation of certain ecosystem functions, since not all ecosystem services are derivable at this small scale. This refers especially to the cultural ecosystem services (such as “recreation and mental and physical health” or “sense of place”, Buchel and Frantzeskaki, 2015), as these refer more to urban green spaces as a whole than to single elements of those spaces.

The second gap that was identified refers to the dimensions according to which structural elements are assessed. In our approach, we widen the assessment by integrating microclimatic and social criteria, relating the different perspectives to each other and discussing the implications.

The third gap addresses the quality of the structural elements, which is usually only taken into account in a generic manner. A more detailed categorization is needed to adequately reflect different instances of the same element. For a selection of elements the paper will exemplify that differentiation between natural and artificial instances of the same element is meaningful not only from an ecological but also from a microclimatic and social perspective.

Therefore, we extend the current site-scale research (gap 1) by adopting a multidimensional perspective (gap 2) and by integrating the variability of structural elements (quality of a site, gap 3).

Section snippets

Structure-type classification system

Structural elements of green spaces were classified exclusively based on structural parameters (three-dimensional arrangement of physical matter and degree of biotic coverage). For this procedure “habitat structure” by Byrne (2007) was adopted and slightly modified. The resulting classification represents a system of ecological elementary vegetation units in an urban realm with a specific combination of characteristics. The elements of the classification system subdivide a green space into

Results

The results section will first present the evaluations of structural elements from the ecological, microclimate and social perspective, followed by results of the question how and if the citizens differentiated between natural and artificial elements of urban green spaces.

Discussion

Designing urban green spaces to meet the demands of an integrative view on different scales is a challenging task. The efficient use of free space must balance ecological, social and microclimatic objectives in a trade-off acceptable to all parties involved. This inevitably leads to a multifunctional design of green spaces which is of particular relevance for the concept of compact green cities. In this study, an indicator-based assessment for structural elements of urban green spaces was

Conclusion

In this paper we introduced a novel approach to assess urban green spaces: a methodology in which urban green spaces are assessed quantitatively from three different perspectives using an integrative approach. Taking single elements of green spaces as examples, e.g. lawns or hedges, the metrics reflects an assessment from all perspectives. The underlying idea of the joint approach was to treat all perspectives as equally important and to combine them within one frame of references in order to

Acknowledgements

Authors thank Lisa Schwier for research support. This study is part of the interdisciplinary research project Urban Future Outline (UFO) at the RWTH Aachen University, Germany. The project is funded by the Excellence Initiative of Germany's federal and state governments.

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    The term metrics is derived from the Greek word μετρικόϛ and is used in this paper as the comprehensive compilation of a set of indicators (ecological, microclimatic, and social). Note that the three indicator values are not weighed or mathematically combined to a global indicator, but represent a space in which all parameters are used in their original way to be used for an interdisciplinary assessment of urban green spaces.

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