**5. Conclusions**

While the importance of ecosystem services is today widely acknowledged, our results also show that there is still a gap in assessment methods on urban local scales [14,19,27]. The aim of our review was to raise awareness of this issue and provide a framework for further research. Previous reviews on the assessment of urban ES have mainly focused on larger spatial scales within cities. Our findings follow on from here, giving an update as well as checking whether any additional studies have examined even smaller spatial structures in the context of urban ES assessments (such as "green roofs", small garden patches, individual trees, etc.). Our results show that a research gap still exists at this point.

The significance of urban ES is readily acknowledged by scientists, and first steps have already been undertaken to adapt this concept for policymaking [17,62]. The high numbers of studies in the last years indicate that this topic is still evolving, especially in Europe. Several published reports have acknowledged that Europe, North America and northeast Asia are the main centers of research into the interrelationship between human well-being and green spaces [34,63,64].

In recent years, multiple ES classes have been investigated in cities as well as in their local spatial structures. Since the review by Haase et al. [14], more studies have been published on ES on the level of small urban structures; yet, the majority of work still focuses on larger spatial structures, mostly applying generalizing methods that provide results with a poor fit to reality [54]. Our results have shown a distinction between the numbers of ES assessments at city- and site-dimension. For a more rigorous implementation of the ES concept in urban planning as well as to develop adapted measures and design clearly additional research is needed, especially in evaluation methods on local urban scales. Improving our understanding of the ecosystem services provided by site-scaled green infrastructure types will constitute an important step towards setting policy objectives and creating suitable measures for sustainable urban development [9,19].

Approaches are needed that require data gathering [19], as these lead to credible and more realistic assessments of urban ES [54]. Furthermore, Beichler et al. [25] have stressed the importance of spatial scale in ES assessments, arguing that the exclusion of settlements and built-up areas from investigations can cause us to overlook ES provided by small ecosystems within such structures. In the urban context, results from spatially comprehensive (natural and built-up areas) approaches ultimately form more

convincing arguments for the ecological and sustainable design of future cities with their small green structures and, in this way, could usefully influence the decision-making processes.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-445X/9/5/150/s1, **S1.** Overview of the studies assessed in this review sorted according to reading order; **S2.** Short descriptions of method categories; **S3.** Overview of assessed ES sections and classes according to CICES V5.1 in regard to urban green infrastructures.

**Author Contributions:** P.B. designed the study and conceptualized the methodology. Both P.B. and A.S. conducted the literature search and reviewed the articles. P.B. performed the analysis, visualized the data and wrote the manuscript in consultation with A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was mainly funded by the Federal Ministry of Education and Research (BMBF) in the joint project "HeatResilientCity" (subproject gran<sup>t</sup> number: 01LR1724A). The promoter of this project is the DLR project managemen<sup>t</sup> agency (DLR-PT).

**Acknowledgments:** The authors would like to thank Derek Henderson for language editing and proofreading. We also thank the three anonymous reviewers for their very constructive comments and valuable hints.

**Conflicts of Interest:** The authors declare no conflict of interest.
