**3. Results**

#### *3.1. General Overview of Articles*

In Figure 2, we can see, firstly, that most of the articles focus on European cities, and, secondly, that research on the assessment of the urban ES of di fferently spatial-scaled green infrastructure types is growing. Although the databases were screened for published articles from the year 2000, published articles corresponding to the described search criteria were only identified from 2008.

**Figure 2.** Numbers and geographical distribution of case studies in the articles published between 2008 and April 2019. The figures in brackets beside each year are the total number of articles published in that year; the total numbers of articles from each continent are also indicated. A polynomial trend line illustrates a predicted increase of the number of articles. (Note: One study is excluded due to its analysis of a fictional urban model).

There is a clear rise in the number of studies assessing the urban ES of green infrastructure types on di fferent spatial scales, especially over the last two years. The first paper to o ffer a smaller-scale assessment of explicitly urban ES was published in 2008. While the highest number of relevant publications was identified in the year 2018, by April 2019 more than 50% of the total number of articles from 2018 had already been published. The polynomial trend line also draws a predicted increase in the number of articles in this field (see Figure 2). Most of the reviewed publications focus on green infrastructures within European cities (55); a much smaller number deal with cities in Asia (8) or North (7) and South America (4). Only one publication dealt with the assessment of urban ES in an African city, specifically Cape Town (found in the gray literature document TEEB (The Economics of Ecosystems and Biodiversity) [29], referring to [30,31]). The scientific articles were sourced from 32 di fferent journals in diverse fields, such as ecosystem services, environmental science, ecology and urban planning.

#### *3.2. Assessed Es of Urban Green Infrastructure Types*

In total, we identified 40 different ES classes assessed in regard to different green infrastructure types (Table 2, for detailed ES class overview see Supplementary Materials (S3)). The ES section Regulation & Maintenance according to CICES V5.1 specifies 16 different ES classes. The ES section Provisioning examines 12 different ES classes. A total of nine different ES classes were studied within the Cultural ES section.

**Table 2.** Overview of assessed ecosystem services (ES) sections and classes (numbers in brackets) according to CICES V5.1 in regard to urban green infrastructures.


With a closer look at the analyzed ES classes (see Supplementary Materials (S3)) we can see the varying frequency of the examined individual ES classes. The most frequently assessed ES classes were "Filtration/sequestration/storage/accumulation by microorganisms, algae, plants, and animals" (2.1.1.2) and "Regulation of temperature and humidity, including ventilation and transpiration" (2.2.6.2). In general, most studies focused on an assessment of ES classes within the ES section Regulation & Maintenance (a total of 177 instances, see Table 2). The ES classes within the Cultural ES section were the second most frequently examined. In total, ES classes were assessed 115 times in this section (see Table 2). Specifically, the most frequently assessed classes were "Characteristics of living systems that enable activities promoting health, recuperation or enjoyment through active or immersive interactions" (3.1.1.1) and "Characteristics of living systems that enable activities promoting health, recuperation or enjoyment through passive or observational interactions" (3.1.1.2). ES were assessed 37 times in the Provisioning ES section (see Table 2). This represents the most rarely assessed ES for urban green infrastructure types.

#### *3.3. Investigated Green Infrastructure Types and Dimensions*

Figure 3 shows the number of investigated green infrastructure types at city- and site-dimensions in the reviewed publications. In general, the articles investigated a wide range of green infrastructure types. It can be seen that the majority of the studies analyzed ES at city-dimension (Figure 3, left side), especially in regard to "urban green spaces" and "structural types" with a much smaller number of studies analyzing single structures in cities (Figure 3, right side). The two dimensions show a similar number of assessments of "parks".

Regarding the assessment of ES for green infrastructure types (Table 3), our results show that most of the studies concentrated on assessing the Regulation ES classes "Filtration/sequestration/ storage/accumulation by microorganisms, algae, plants, and animals" (2.1.1.2) and "Regulation of temperature and humidity, including ventilation and transpiration" (2.2.6.2) in "urban green spaces" at city-dimension. This spatial structure type was also the focus of a large number of assessments of the cultural ES classes "Characteristics of living systems that enable activities promoting health, recuperation or enjoyment through active or immersive interactions" (3.1.1.1) and "Characteristics of living systems that enable activities promoting health, recuperation or enjoyment through passive or observational interactions" (3.1.1.2). Within the site-dimension, the ES class "Filtration/sequestration/storage/accumulation by microorganisms, algae, plants, and animals" (2.1.1.2) was most frequently assessed in "neighborhoods" and "parks".

**Table 3.** Assessed ES classes according to CICES V5.1 in relation to the considered green infrastructure types. The figures in brackets indicate how often an ES was assessed for the corresponding spatial structure. Since multiple ES were analyzed in some articles, the sum in brackets is higher and does not represent the total number of articles reviewed. (Note: The specification of the considered ES class is missing in some articles; these were assigned to the ES section "in general, specifications missing" in each ES section.)



**Table 3.** *Cont*.

#### *Land* **2020**, *9*, 150


**Table 3.** *Cont*.

**Figure 3.** Urban green infrastructure types investigated by the articles, subdivided into city- (**left**) and site-dimension (**right**).

#### *3.4. Methods Used to Assess the Urban Es of Green Infrastructure Types*

The majority of the reviewed publications applied "spatial proxy methods" followed by "surveys and questionnaires" (see Figure 4). "Social media-based methods" were the least commonly used method for ES assessment for different green infrastructure types in cities. Our analysis showed that over a quarter of the studies used more than one method to assess urban ES. "Surveys and questionnaires" and "model-based methods" were most frequently combined, followed by "spatial proxy methods" and "model-based methods".

**Figure 4.** Numbers (and % of total share) of methods used in the reviewed articles.

Table 4 categorizes the different methods used to assess urban green infrastructure types. At city-dimension, the reviewed studies most frequently applied "spatial proxy methods" and "surveys and questionnaires". The ES of "urban green spaces" were the most studied in this dimension by applying methods that belonged to the categories "spatial proxy methods", "surveys and questionnaires" and "model-based methods". "Structural types" and "trees" were also the focus of many assessments of urban ES. Here, the most common methods were "spatial proxy methods" and "model-based methods". At site-dimension, the most frequently used method was "surveys and questionnaires", largely to assess the urban ES of "parks".

**Table 4.** Numbers of methods identified in the reviewed articles to assess green infrastructure types. The figures in brackets show how often a method was applied to the corresponding structure type. Since multiple methods were used in some articles, the sum of methods is higher than the actual number of articles.

