Pocket parks-a systematic literature review

The rapid process of urbanization and climate change have resulted in socio-environmental issues that necessitate viable intervention strategies to create green, sustainable, healthy, and livable built environments. One effective method of achieving this is through the development of pocket parks. Although several studies have explored the benefits and design principles of pocket parks, there is a lack of systematic and quantitative understanding, which could influence the decision-making process for the proper adoption of pocket parks. In view of this knowledge gap, this study employs bibliometric methods and critical review to quantitatively analyze and systematically review 276 papers published in the Web of Science and Scopus databases between 1977 and 2023. Firstly, a bibliometric analysis is conducted, providing a comprehensive overview of research on pocket parks. Secondly, a detailed critical review is conducted from three major perspectives: influencing factors, design methods, and benefits of pocket parks. Finally, the research trends and future directions are discussed. The results indicate that (a) pocket park development takes various forms, with a growing number of articles published each year. However, research on this topic is primarily concentrated in the United States and China, with significant knowledge gaps remaining globally. (b) The utilization of pocket parks is affected by various factors, including internal, external physical, and socio-economic factors. The mechanisms of these factors and their interactions demand deeper understanding. (c) There is a need to conduct an in-depth exploration of planning methods for pocket parks, and planning and design methods in different country contexts can be expanded and compared. (d) Further research is needed to compare the benefits of pocket parks in different climatic zones in terms of depth and width. The outcome of this study can contribute to the body of knowledge on pocket parks, foster their wider acceptance, and help urban planners, practitioners, policy makers, and government managers apply them more effectively in resilient and livable cities of the future.


Introduction
Urbanization and climate change are global concerns. Currently, more than 50% of the world's population resides in urban areas. It is projected that by 2050, 70% of the world's population will be living in urban areas as the global population continues to grow and urban land expands [1,2]. The rapid process of urbanization has led to significant environmental problems, including frequent rainstorms and flooding disasters, serious air pollution, and significant heat island effects [3,4]. Urbanization has also impacted people's physical and mental health and social life, with psychological stress and physical health problems due to a lack of contact with nature being some of the main issues [5].
Green infrastructure is a vital component of urban areas, providing essential ecosystem services such as mitigating the urban heat island effect [6][7][8], reducing surface runoff [9], improving air quality [10,11], and supporting biodiversity [12,13]. Additionally, natural environments offer a versatile setting for outdoor recreation, physical activities, and socialization. They can enhance health by reducing blood pressure and heart rates, lowering stress and negative emotions, increasing perceived resilience, and decreasing the risk of mortality from all causes [14][15][16][17][18]. However, providing large-scale green infrastructure in high-density urban areas where land resources are scarce and real estate costs are high can be a challenging task [19]. In this context, small and affordable micro-parks such as pocket parks are becoming crucial elements of green infrastructure and the primary means of expanding outdoor recreation spaces in densely populated urban areas [20][21][22].
Pocket parks, also known as vestibule parks, vestpocket parks, mini-parks, green pockets, or street corner parks, originated in European cities' reconstruction after World War II. Some war-damaged sites were transformed into small parks during that time. The idea of pocket parks came to the United States in the 1950s, and one of the most famous pocket parks is Zion's design, Paley Park, located in Manhattan [19]. However, the concept of pocket parks lacks a uniform definition among scholars, who mostly define it based on its location, spatial scale, attributes, and functions (table 1). For instance, Seymour [23] describes pocket parks as 'parks smaller than most urban parks, less than half an acre in size, and often created out of vacant lots, rooftops, and other forgotten and unused spaces.' Meanwhile, Blake [24] suggests the following definition: 'Pocket parks are urban open spaces at a very small scale. Usually only a few house lots in size or smaller, pocket parks can be tucked into and scattered throughout the urban fabric, serving the immediate local population.' Recently, pocket parks have diversified, with the emergence of pop-up parks [25] and guerrilla gardening [26]. Pop-up parks aim to transform vacant land into a functional space or park temporarily, while guerrilla gardening involves growing food and other plants in unused urban spaces. As a result, the definition of pocket parks has expanded significantly [27][28][29]. In sum, pocket parks' only criterion is scale, in addition to their public character as a park. Public green spaces, regardless of their shape, location, or boundary environment, can be classified as pocket parks as long as they meet the area requirements. Pocket parks in high-density areas are characterized by flexible layouts, proximity to services, daily accessibility, low costs, and convenient maintenance. These features make them well-suited to complex, high-density environments. Pocket parks can provide ecosystem services, fill gaps created by the absence of large parks, and offer a space where residents can engage in outdoor recreational activities and form diverse social and natural connections [30][31][32][33]. Therefore, they are often used as a precise planning tool to revitalize urban communities, alleviate the imbalance between the supply and demand of open spaces, and are also regarded as valuable places that can aid in restoring and promoting mental health, as well as contributing to social and ecological sustainability [28].
Although creating pocket parks in high-density urban areas where socio-environmental issues prevail is necessary, its implementation is still in its early stages. This is due to a lack of awareness among frontline stakeholders, as knowledge and awareness are critical 'departure points' for the implementation of sustainable practices [42]. Despite the presence of several review studies on pocket parks, there is still a gap in knowledge that limits the ability to make informed decisions about adopting pocket parks. For example, Zhang and Han [1] undertook a review of 32 English and 33 Chinese papers to summarize the definition of pocket parks, research locations, research topics, and trends. They compared the similarities and differences between Chinese and English papers. In another study, Kerishnan and Maruthaveeran [2] reviewed 15 papers to summarize the factors that lead people to use pocket parks. These articles provide an interpretation and analysis of the literature on a particular topic, but the current body of knowledge lacks comprehensive research on the influencing factors, design methods, and benefits associated with pocket park construction. Therefore, updating stakeholders on the current research status of pocket parks is necessary.
Moreover, no research has been conducted to date using bibliometric methods to systematically review pocket parks. A quantitative analysis could provide a comprehensive understanding of the research field and establish a systematic framework for a critical review. Citespace, a free software program designed for bibliometric analysis and visualization, can aid this analysis. It offers powerful statistical capabilities that enable the quick and accurate summarization of information on the number of publications, authors, and countries. Through quantitative modeling techniques, an objective and comprehensive understanding of the knowledge landscape can be quickly formed. This is achieved by fragmenting a large amount of literature into subjective perspectives and presenting the resulting information in tables, figures, and knowledge maps. Such an approach is widely utilized in bibliometric analysis to gain insights into the research area [43].
The purpose of this study is to conduct a bibliometric analysis and critical review of literature on pocket parks, in order to provide a comprehensive overview of global research trends, hotspots, and knowledge gaps. This will be achieved by addressing the following questions: (1) where has research on pocket parks been conducted? (2) What are the major research topics and themes, and where are the gaps in knowledge? (3) What factors contribute to the usage of pocket parks? (4) What are the planning and design methods for pocket parks, and how do they differ regionally? (5) What is the focus of research on the benefits of pocket parks, and what are the priorities for future research? Section 2 outlines the methodology used to search the literature, while section 3 provides a comprehensive overview of research on pocket parks, which has been conducted based on the bibliometric analysis. Sections 4-6 will offer a detailed Where the need for outdoor space is greatest, small vacant lots and land occupied  by derelict buildings can be acquired, usually at low cost, to be developed into small  parks  1969 Seymour [23] Parks smaller than most urban parks; less than half an acre in size; and are often created out of vacant lots, rooftops and otherwise forgotten and unused spaces 2005 Forsyth and Musacchio [35] Small parks are a key part of most neighborhoods, but they typically provide mostly recreational benefits. They have limited areas, so they cannot meet all the potential demands for space for varied human activities and multiple natural processes 2013 Blake [24] Pocket parks are urban open spaces at the very small scale. Usually only a few house lots in size or smaller, pocket parks can be tucked into and scattered throughout the urban fabric where they serve the immediately local population 2019 Labuz [36] The concept of a pocket park emerged from the need to improve the quality of urban life in the aspect of accessibility of public spaces, including green areas 2020 The Trust for Public Land [37] Pocket parks are generally recognized as public park spaces that occupy less than one acre of land, the shape and dimensions of a pocket park will vary depending on the site 2020 Babalis [34] Pocket parks are well-designed small urban spaces with, or without, green elements, for urban quality, and urban resilience, wellbeing, connectivity, diversity and flexibility of use 2020 Salih et al [38] Pocket parks can be defined as low-cost, small green open spaces for various activities and benefits 2019 Liu and Wang [39] Pocket parks are tiny urban parks created from vacant lots that are accessible to the public. They can be in the urban fabric of different areas and are typically smaller than 1-3 acres or between 1 and 4 house lots in size 2021 Naghibi et al [20] Pocket parks are defined as small green spaces embedded in, but not separate from, the urban landscape which offer small spaces of solace amidst the busy city streets with profound impact 2021 Ding and Zhang [40] Pocket parks are parks formed by further adding leisure and entertainment facilities on the basis of planting greenery on smaller plots. Pocket park construction areas are generally one to three times the area of housing construction 2022 Ministry of Housing and Urban-Rural Development of the People's Republic of China [41] A park greening activity site open to the public, with a small scale, diverse shapes, and certain recreational functions, generally covering an area of between 400 and 10 000 square meters 2022 Hou et al [22] The pocket parks are usually small green urban spaces that are flexibly distributed in high-density urban areas as patches critical review of pocket park research from three major perspectives: influencing factors, design methods, and benefits. Furthermore, in section 7, we offer a discussion on future research and provide recommendations based on our findings. Finally, section 8 presents the conclusion of this study. This study highlights the importance of examining pocket parks from a macroscopic and broad perspective, providing researchers with knowledge to promote their general acceptance. Additionally, it can assist urban planners, practitioners, policymakers, and government administrators in the broader application of pocket parks to create more resilient and livable cities in the future.

Literature search
We conducted a comprehensive review by searching electronically for all possible articles on pocket parks in two journal databases: Web of Science (WOS) and Scopus. These databases contain reliable and accurate literature citation data. WOS database includes high-quality academic journals, and the number of cited references has reached 1.9 billion. Scopus is the largest database of literature abstracts and citations in the world, encompassing over 20 500 source publications from more than 5000 publishers worldwide. We gathered significant research on pocket parks, including relevant theoretical, conceptual, and applied papers, and systematically sorted out the titles, abstracts, and keywords of these studies to identify their most common keywords.
As the theme of this paper, original definitions such as pocket parks and miniparks are of priority when selecting search terms. However, as pocket park forms have evolved, researchers use related keywords such as pop-up parks and guerrilla gardening to explore the topic, which we also included in our search criteria. We eventually opted for a more inclusive query, which looked for publications related to the topics 'pocket park * ' , 'minipark * ' , 'small neighborhood park * ' , 'small park * ' , 'small green space * ' , 'small urban park * ' , 'vest pocket park * ' , 'pop-up park * ' , 'guerrilla gardening * ' , 'tactical urbanism' , 'urban acupuncture' , and 'DIY urbanism' .

Data processing
This paper utilized the Citespace 6.2 tool to analyze the key characteristics of research on pocket parks, including the co-citation network and dynamic changes in the field. Citespace enables quick examination of basic information, including number of publications and countries, and analyses the cooperation network to understand research institutions and author collaborations. Keywords' word frequency, betweenness centrality, and half-life were used to visualize the research universe in the field. The 'burst term' function aided in exploring the frontier's direction and progress and helped examine the current research hotspots [43]. The timeline view visualized the evolution of prevalent topics and sorted closely connected keywords into groups. The resulting clusters revealed crucial subgroups with distinct research directions and exposed the connections between said clusters, which contributed to the research field's overall structure. Therefore, the paper conducted a bibliometric analysis to critically review the main research topics based on the outcomes.

Geographic pattern
Authors from 51different countries or regions contributed to the research on pocket parks. The United States, the United Kingdom, and Canada were the first countries to conduct research on pocket parks before 2009. According to figure 2, the United States (67), China (52), United Kingdom (18), Canada (17), Australia (16), Italy (13), Japan (11), and Germany (10) had the highest number of publications. Relevant studies are conducted in these countries due to the impact of the rapid process of urbanization, biodiversity, and sustainable development. Since pocket park research was conducted in China in 2017, they have undergone rapid development. A search and analysis of the Chinese CNKI database revealed that research on pocket parks in China has primarily focused on planning and design strategies and mitigation of the heat island effect. Some underdeveloped countries have conducted prospective research, recognizing that pocket parks are a highly effective, feasible, and low-cost intervention to improve environmental quality. Nonetheless, research on pocket parks has been conducted in few  regions globally, resulting in knowledge gaps and disparities, particularly between developed and underdeveloped nations.

Authors
The research on pocket parks involved 367 authors, and figure 4 shows the co-authorship network generated by Citespace. An increase in connections among the different circles indicates greater collaboration

Basic research field analysis
After using the keyword co-occurrence function, we were able to identify 438 nodes and 1134 connections, leading to a network density of 0.0118 ( figure 5). The node and font size reflected the respective frequency of keywords used in studies. Those with a betweenness centrality of over 0.1 were regarded as critical nodes. As for the longevity of keywords, the longer the half-life, the greater the interest they garnered. Our analysis revealed that the top 20 frequently researched keywords related to pocket parks (table 2) with centrality greater than 0.1 include: 'pocket park' (0.25), 'urban green space' (0.16), 'city' (0.26), 'urban parks' (0.16), 'public space' (0.14), and 'urbanization' (0.11). Further examination indicated that all but one of these keywords, namely 'urban  related to pocket parks may vary over time, considerations of pocket park construction in terms of public space, urban planning, and the urban thermal environment have remained constant. With the growing scarcity of urban land resources and the outbreak of the COVID-19 pandemic in 2019, people are increasingly prioritizing leisure, fitness, and physical activities, and the need for green spaces has become more pressing. At the same time, pocket parks have the characteristics of low cost, daily accessibility, and appeal to the activity needs of the elderly, making 'physical activity' , 'urban parks' , 'heat island' , 'park use' , 'older adults' , 'thermal comfort' , and 'small green space' become hotspots of pocket park research.

Research topic evolution analysis
The timeline view provides an overview of the relationship between clusters and the historical span of keywords in those clusters. By combining an analysis of basic research fields and phased hotspots with the timeline view, the major topic of pocket park research can be determined. The cluster analysis by timeline view yielded a modularity Q of 0.5983 and a mean silhouette of 0.8416 (figure 7), indicating a significant division of the community structure and scientifically reasonable cluster results. This outcome contributes to the structure of a critical review. The primary keywords in the #0 park use cluster include 'urban green space,' 'physical activity,' 'ecosystem service,' 'design,' 'public health,' 'environmental justice,' 'walking,' and 'perception' . The cluster's focus is to examine the various factors affecting the utilization of pocket parks, specifically exploring how their internal spatial characteristics and external social environment impact physical activity and social interaction. The goal is to identify and use these influences to create effective interventions that can guide the spatial design of pocket parks. Meeting the individual preferences of different groups is a key factor in pocket park use since the service functions of pocket parks have evolved from general public needs to focusing more on the elderly and adults. Since 2015, the factors influencing the development of pocket parks have shifted towards considerations of environmental equity and walkability. Researchers have started utilizing social media and questionnaires since 2021 to capture peoples' perceptions of the pocket park environment. New technological tools accounting for human factors have been applied and developed in this field.
The main keywords within the #1 public space cluster include 'pocket park,' 'public space,' 'urban design,' 'urban planning,' 'green space,' 'small green space,' 'urban development,' 'urban politics,' and 'spatiotemporal analysis' . Pocket parks are crucial as public spaces, and they can effectively catalyze the urban regeneration process. Therefore, the spatial layout and design of pocket parks have received continuous attention from both macro urban planning and micro spatial design perspectives. The cluster involves utilizing multiple sources of data, along with spatiotemporal analysis, street view images, and other techniques, to examine the environmental features of pocket parks, investigate their usage, and quantify the factors that influence them. This information can be used to plan and design pocket parks scientifically. Since 2022, researchers have begun to focus on the impact of urban political dimensions on emerging urban public spaces such as pocket parks. Temporary and staged land use changes brought about by short-term interventions such as pop-up parks and guerrilla gardening disrupt traditional and authorized land use patterns. There is an urgent need to create policies that support long-term transformations to establish a sustainable model of progressive community guidance, government policydriven collaboration, and socially inclusive efforts to promote the development of 'informal' green spaces.
Clusters #2 diversity, #3 urban heat island, and #4 protected areas have been relatively well-developed and have continued from the early days of pocket park research to the present. These clusters reflect the numerous benefits provided by pocket parks, particularly in the context of continuous encroachment on green spaces caused by the rapid process of urbanization. Pocket parks are gradually becoming an essential tool for biodiversity conservation in highdensity urban areas. Furthermore, the figure displays colored circles indicating the yearly progression of keyword rings with high betweenness centrality. Each color represents the year in which the keyword appeared, reflecting the iconic topic and the duration of sustained interest. Under the #3 urban heat island cluster, 'city' and 'heat island' have consistently been research hotspots, and the number of publications related to this topic has increased since 2005. The keyword 'interval' within this cluster is becoming smaller, indicating the popularity of using pocket parks to mitigate the urban heat island effect caused by climate change. Recently, researchers have shifted their focus towards improving human thermal comfort.
Based on the bibliometric analysis and paper content mentioned above, sections 4-6 provide a critical review of three major research topics that affect pocket park adoption decisions-influencing factors, design methods, and benefits. Figure 8 illustrates the roadmap for this critical review.

Influencing factors of pocket parks
Good pocket parks generally meet the criteria of authenticity, specificity, functionality, adaptability, security, attractiveness, and accessibility [44,45]. In summary, the factors influencing the use of pocket parks can be categorized into three aspects, namely internal factors, external physical factors, and socioeconomic factors (table 3).

Internal factors
Internal factors are primarily derived from the fundamental elements that make up a pocket park, including site management, site attributes, landscape design, and facility completeness [33]. These factors have a direct impact on the park's attractiveness and usage. For instance, site management affects the safety of facilities and the quality of park usage; a study conducted in Gyeongsan City, South Korea revealed that inadequate management is the primary cause of infrequent visits to parks [33,47]. Salih et al's survey found that safety and security, as well as management and maintenance, are factors impacting the effective utilization of pocket parks [48]. While the topography of the park may be appealing to residents and encourage them to use the pocket park, the impact of noise within the park can vary from person to person [50]. Grass, flowers, water features, and seating all contribute to the restorative function of environments [36,46,51]. In terms of potential health promotion applications, Nordh et al demonstrated that structural elements such as grass and trees are more appealing to individuals for stopping and resting than decorative environmental elements such as flowers and water features [14]. Peschardt et al demonstrated that incorporating green cover and green façades can contribute to creating open spaces in pocket parks [52]. Therefore, it is important to pay attention to the design of walking paths, play areas, and sports facilities while minimizing obstacles to ensure the quality of pocket parks [44].

External physical factors
External physical factors refer to the structural characteristics of the physical space and environmental elements surrounding pocket parks. These factors include adjacent commercial facilities, green spaces, traffic conditions, land use, and surrounding street forms. Pocket parks face challenges in meeting the broader service needs of surrounding residents, due to their limited space and lack of internal amenities. Therefore, external physical factors play a crucial role and are necessary to compensate for inherent deficiencies. Accessibility and connectivity in traffic conditions are critical factors that affect the efficiency of entry for tourists [56]. Therefore, placing pocket parks in the city center can be an effective way to increase the utilization rate of parks [47].
Accessibility also has an impact on participation in physical activity [33,57]. The surrounding street forms not only impact the microclimate and overall appearance of pocket parks but also influence potential users of these spaces. Sun et al conducted a study on 74 small public spaces located on street corners and found that the height of surrounding buildings and street width significantly affect the utilization of these spaces by the elderly [54]. Labuz's research showed that commercial facilities positively impact the number of tourists. This is because these facilities provide potential visitors with access to pocket parks and also offer accompanying services such as public toilets, which provide necessary daily support [36].

Socio-economic factors
Socio-economic factors encompass personal factors, land value, social support, and public policies. Age, gender, income level, and ethnicity are among the personal factors that can lead to differences in the pocket park utilization. Kerishnan et al found that women tend to utilize parks more than men [33]. Meanwhile, Kaczynski et al showed that park participation rates are higher for female occupants, young adults aged 18-34, and older adults aged 55 and above [57]. Wang et al demonstrated that children under seven are more likely to spend time in pocket parks [55]. Park use is lower in low-income neighborhoods compared to high-income neighborhoods areas [59]. Additionally, neighborhoods with lower incomes and more residents of color place greater importance on park accessibility, while those with higher incomes and more white residents prioritize park size and amenities [49]. The value of the land reflects the location of the lot and the completeness of supporting facilities. These factors influence the design quality, maintenance level, and characteristics of potential users, thereby affecting the appeal and use of pocket parks. For example, Zhou et al found that higher housing prices correspond to a higher number of visits to pocket parks [21]. Furthermore, Kerishnan et al discovered that improved management and maintenance in the city center increase pocket park visitation among urban dwellers [33]. Social support can be manifested in two ways: engaging in solitary activities for recuperation and rest, or participating in social activities with others. Studies show that group travel enhances the sense of security and participation while using pocket parks [55]. It is crucial to ensure safety while engaging in independent activities, which promotes rest and relaxation [60]. Given the wide array of forms and uses for urban spaces, public policy is necessary to provide guidance and regulation for pocket parks. Mexico has focused on the politics of public space investment site selection to investigate how governments and agencies configure distributional criteria for pocket parks [62]. The implementation of pop-up parks, initiated by businesses, students, and practitioners, requires not only public feedback but also policy choices to gain council approval [61]. Several studies have comprehensively analyzed the factors that influence the frequency of visits to pocket parks, seeking reasons that either promote or hinder such visits. A study conducted in Shanghai, China revealed that the factors affecting the frequency of visits to small parks are, in order of their degree of explanation of total variance: residential space attributes, socio-demographic attributes, and park characteristic elements [55]. This means that external physical factors take precedence over socio-economic factors and internal factors. Zhou et al [21] found a positive correlation between larger open spaces and visitor numbers, from an internal perspective. From an external perspective, the density of restaurants, commercial and office facilities, and housing prices showed a positive correlation, whereas the green coverage rate exhibited a negative correlation. Another study conducted by Zhou et al [21] demonstrated that visitation is facilitated by internal factors such as open spaces and public toilets, as well as socio-economic factors such as resident population. Conversely, external physical factors such as traffic, large parks, and distance from the city center hinder visitation. Although these studies have demonstrated the influence of internal, external physical, and socioeconomic factors on the use of pocket parks, further research is needed to understand the mechanisms and interactions of these factors. This will help to guide the planning and construction of pocket parks in a more scientific manner.

Design methods of pocket parks
Contemporary pocket park research and practices primarily concentrate on two levels of planning and design: microscopic landscape design and macroscopic spatial planning. Current microscopic landscape design methods include traditional design methods that leverage expert knowledge and experience, identification of design elements through questionnaire survey methods, microclimate simulation methods that target environmental enhancement, and mixed methods. On the other hand, macroscopic techniques for spatial planning include spatiotemporal analysis driven by multi-source data.

Microscopic landscape design methods
Currently, traditional design methods, including the case study method and the expert experience method, are the mainstream design methods for pocket parks. The case study method generally identifies specific design elements, such as location, function, size, and configuration, by studying and analyzing information from photographs, documents, and interviews, thus summarizing general design principles for pocket parks. For example, a Danish study analyzed nine pocket parks in Copenhagen and identified healthrelated design elements, such as trails, historical features, seating, and flowerbeds to provide a basis for health-oriented pocket park design [52]. A study in Italy summarized the spatial configurations and elements of 13 types of pocket parks in Europe, from which the design principles for pocket parks were formulated [34]. The expert experience method is an empirical judgment method that relies on the knowledge and experience of experts to evaluate and select design schemes. For instance, experts rated the quality of pocket parks in terms of accessibility of features, attractiveness of the park, functionality of the features, and perception of safety, which summarizes the design guidelines for high-quality pocket parks [44].
The questionnaire method is a human-centered design approach that focuses on the perception of the environment by the public. This method is employed in designing pocket parks by extracting design elements based on investigation of the general public's usage preferences. For instance, Naghibi et al utilized computer-generated scenes of 25 pocket parks, each having a different enclosure level of planting, to explore the preferences of 318 residents in terms of coherence, legibility, and refuge through a visual online questionnaire. This completes the pocket park design practice by identifying spatial configurations relative to enclosure levels [20]. Similarly, Hadavi et al introduced a novel method for understanding resident preferences which involved participant-generated photo groupings. The study revealed an interconnection between environmental affordances and pocket park design attributes through the participants' selection of their preferred landscape design elements and outdoor scenes. The findings provide useful insights for designers to create livable pocket parks [63].
The microclimate simulation method guides the rational design of pocket parks by conducting urban microclimate simulations. For example, Zwangsleitner provided on-site 3D models and weather condition parameters based on design task requirements to obtain design information such as orientation, shading, wind shelter, materials, and the use of trees and shrubs for pocket parks that effectively improve human thermal comfort [64]. By combining multiple methods, multi-angle and allround data can be obtained through various type of methods including observation, interviews, questionnaires, and electronic data collection. These mixed methods help determine design principles for natural landscapes, leisure facilities and more. For instance, Naghibi employed mixed methods like procedure, case study introduction, expert interviews, and BWM methods to identify the design principles for a natural landscape with diverse vegetation and other facilities, gentle leisure activities, and community gardening. This allows for the creation of an ideal environment in pocket parks that meet the needs of the elderly [65].

Macroscopic spatial planning methods
The spatiotemporal analysis method, driven by multisource data, prioritizes the installation location of pocket parks. This is achieved by analyzing the factors that influence their installation, building an index system, and evaluating the suitability of pocket park installation. For instance, Hu conducted research in Shichahai, Beijing, China, by building a GIS database using historical conservative elements, existing parks, and demographic information. This helped evaluate the suitability of pocket park construction and ultimately determine the layout of the pocket parks [66]. Similarly, Zhou et al chose Yancheng city center, China, as a research area where they built a demandsupply framework to quantitatively assess the priority of installing pocket parks on each piece of land [19]. These novel technologies that consider human factors and the social environment have effectively guided the planning and design of pocket parks by breaking through spatial and temporal constraints. However, compared to micro-scale landscape design, macro-scale planning methods require further indepth exploration.
Additionally, according to global design method statistics (figure 9), the United States has conducted extensive design research in different cities mainly using traditional design methods. The reason for this may be due to the relatively mature development of pocket park design practices in the United States, which has formed a more complete design system that can scientifically guide the construction of pocket parks. On the other hand, Norway, Iraq, and Iran mostly rely on the questionnaire method. Meanwhile, in China, macro-level spatial planning has risen and developed rapidly due to the need to solve environmental degradation and low spatial quality problems caused by rapid urban expansion in the context of urbanization from high-speed to high-quality development. The application of various micro-level landscape design methods is generally more diversified and comprehensive in Europe, while research on pocket park design methods is relatively inadequate in Africa and Oceania.

Benefits of pocket parks
Over the years, numerous studies conducted in various countries have highlighted the benefits of pocket parks. These non-traditional green spaces have been proven to offer ecological and socio-economic benefits that are comparable to those offered by other forms of urban green infrastructure. This is especially true in high-density urban areas where resources are scarce, and populations are dense [1,67,68].

Socio-economic benefits
Pocket parks are regarded as important 'outdoor living rooms' that provide small urban spaces where residents can cultivate intimacy, engage in outdoor recreation, satisfy their daily need for contact with nature, and improve their relationship with their local environment. They play a crucial role in promoting socialization, relaxation, and recreation for users of all genders, ages, races, and cultures, strengthening their sense of belonging and happiness, and contributing to human health and well-being [19,38,69]. Gibson and Canfield [70] demonstrated that pocket parks promote community interactions and are often seen as a valuable community asset. Similarly, the perspectives of community residents collected by Winter et al before, during, and after the installation of the popup parks show that the parks create a vibrant space in an otherwise underutilized area, which is enjoyed by a variety of people in many ways, like children playing, family relaxing, engaging in physical activity, and social interactions [25]. Cohen et al conducted a survey on park usage among users and residents of three pocket parks. They found that pocket parks were as effective as existing playground spaces in nearby parks in promoting moderate-to-vigorous physical activity. Additionally, they found that pocket parks were cost-effective. In Japan, where earthquakes happen frequently, pocket parks are built as evacuation sites [39]. Research has shown that green elements such as groundcovers, shrubs, trees, and water can stimulate mental restoration in small urban parks [71]. Future research in the health field could experiment with interdisciplinary cross-sectional studies, such as conducting health education and health assessments by nurses.

Ecological benefits
Pocket parks complement and expand traditional green spaces' network by providing various ecological benefits that aid in regulating microclimates, reducing stormwater runoff, mitigating urban heat island effects, supporting biodiversity, and improving air quality [2,19,[72][73][74]. For instance, Rosso et al [75] conducted a study that involved objective and subjective assessments to validate that individuals perceive higher comfort levels in pocket parks than on streets. The study revealed a shift from a 'neutral' perception on nearby streets to a 'good/very good' perception in the pocket park. Ding and Zhang [40] integrated the concept of sponge cities into the construction of pocket parks. Their study showed that renovated pocket parks successfully enhanced rainwater system function and mitigated water pollution.
Pocket parks, as public green spaces, have the potential to reduce the environmental stress caused by the urban heat island effect and improve the thermal environment [22,76,77]. According to Liu et al [78] pocket parks have lower temperatures compared to their surrounding streets during both daytime and nighttime, and can mitigate the intensity of micro-scale heat islands in high-rise, high-density urban areas. Additionally, Ma et al [79] conducted a study on two pocket parks located in Xi'an, China, demonstrating that these parks have a cooling effect that can extend up to 100 meters beyond their boundaries. Various factors influence the thermal mitigation effect of pocket parks, including their size, internal and surrounding landscape features (such as vegetation types, patch density, and landscape shape index), and other environmental conditions. Wu et al [80] found that the land surface temperature of pocket parks decreases logarithmically with their area, whereas the maximum local cool island intensity and the maximum cooling area increase logarithmically with the area of pocket parks. Moreover, pocket parks dominated by tree-shrubgrass landscapes exhibit the highest cooling efficiency. The cooling effect of pocket parks has a negative correlation with the patch density in the surroundings, while it has a positive correlation with the landscape shape index.
Pocket parks can also serve as habitats for wildlife, promoting biodiversity conservation [71,81,82]. Jasmani et al [71] conducted research on the potential of small parks as urban habitats in tropical regions. Their findings indicate that the percentage of canopy cover, park size, and presence of native vegetation are the strongest predictors of bird richness and abundance. Based on their findings, they provided recommendations to enhance the biodiversity of small parks. In a separate study, Ikin et al [83] investigated the total species richness and abundance of all birds, as well as the richness of wood-landdependent, insectivorous, and hollow-nesting species in pocket parks located in Canberra, Australia. They found that pocket parks with high greenspace cover support more bird species and individuals overall, as well as woodland-dependent species, insectivores, and hollow-nesters. The study also suggested that greenspace areas are more crucial than the greenspace patch configuration, such as the number, average size, and connectivity, in supporting bird populations. Ama-ya-Espinel et al [84] demonstrated that bird richness and abundance, particularly native and insectivorous species, decrease as building density and road coverage increased in the area surrounding the pocket park. Furthermore, the size of the pocket park, proximity to the nearest one, and diversity of native vegetation have significant impacts on several bird species.

Discussion
Using the WOS and Scopus databases, 276 publications related to pocket parks were obtained. There is a large gap compared to other types of green infrastructure, such as green roofs and rain gardens. It is imperative to further explore and expand the research filed of pocket parks. As the number of relevant research papers continues to grow in the future, the systematic reviews that combine bibliometric methods and critical review could offer a more comprehensive view of this research field. The United States and the United Kingdom were the first countries to conduct research on pocket parks, while China has the second highest number of publications in the world and is significantly ahead of the United Kingdom. The dramatic increase in the number of publications in China is due to the growing interest in pocket parks in recent years, as China's urban development has transitioned from rapid economic growth to a new normal of highquality development, and pocket parks and other small green spaces have become a necessary choice in the trend of high-density construction in large cities. Although studies have been conducted in some underdeveloped countries, there is still a research gap compared to developed countries. The studies in undeveloped countries primarily focus on the environmental benefit assessment of urban parks, of which pocket parks are only a part, and there is a lack of targeted studies on pocket parks. In addition, a total of 271 institutions and 367 authors have contributed to research on pocket parks. Early research institutions focused on analyzing the importance of pocket parks [85]. While recent influential institutions and authors have focused on exploring factors influencing the service status of pocket parks and the quantification of construction priority assessment methods from an urban macro-scale perspective [19,28]. In general, the cooperation between institutions and authors is not yet sufficient. Breaking the barriers of cooperation between authors from different institutions can help facilitate the sharing of knowledge and information.
After a period of slow development from 1977 to 2008, pocket parks have gradually gained attention, and their basic research fields are centered around topics such as city, urban parks, public space, urbanization, and landscape. With the rise of tactical urbanism, DIY urbanism, and urban acupuncture, various forms of low-cost and short-term interventions have emerged, including pop-up parks and guerrilla gardening. These interventions bring about nontraditional land use changes that must be supported and regulated by appropriate policies. New hotspot fields since 2020 have focused on the great potential of pocket parks to promote physical and mental health, as well as enhance environmental benefits. Additionally, new technical tools considering human activities have been applied and developed.
The clustering results show that the research on pocket parks can be summarized into three main themes: influencing factors, design methods, and benefits that affect pocket park adoption decisions. Internal factors, external physical factors, and socioeconomic factors influence the attractiveness and usage of pocket parks. Unlike large parks with welldeveloped facilities, the service status of pocket parks is more susceptible to changes in the external physical and social environment, and thus the interactions among these influencing factors deserve further exploration. Pocket park design methods include microscopic landscape design and macroscopic spatial planning, and there is a need for further research on the macro-scale of cities [30,86]. It is noteworthy that all relevant studies using macroscopic spatiotemporal analysis have been conducted in China, and the application of various methods for microscopic landscape design is more diverse and comprehensive in Europe. Additionally, current research focuses on human health and well-being, mitigation of urban heat island effects, and biodiversity conservation. While the pocket parks, like other forms of urban green infrastructure, can provide large amount of benefits such as rainfall runoff regulation and carbon sequestration. It is important to conduct quantitative assessments of these benefits for the scientific planning and construction of pocket parks.

Recommendations
At present, research into the characteristics, influencing factors, design methods, benefits, and policies of pocket parks has been carried out to varying degrees. However, there is still a disparity in the depth and breadth of research on other types of green infrastructure. Future research can focus on the following three aspects to bridge this gap.

Comprehensive analysis of benefits
The benefits of pocket parks encompass socioeconomic and ecological advantages [1]. Due to their small size, the emphasis of pocket parks is usually on their social dimension in both their design and management. Apart from the quantitative studies on the ecological and economic benefits of pocket parks in terms of breadth, there is a need for in-depth studies comparing the benefits of pocket parks in different climatic zones. Particular emphasis should be laid on exploring the amplification mechanisms of ecosystem service functions. In general, current research mainly conducted in the United States and China. Nevertheless, regions with varying climatic conditions and environmental characteristics demand onsite studies, as the results seem to be sensitive to local conditions. Therefore, future studies should include similar analyses from countries in different climatic zones to obtain standardized conclusions. Furthermore, measurable ecological benefits, such as rainfall runoff regulation, carbon sequestration and oxygen release, air purification, and noise reduction, can serve as research subjects. Based on these, the ecological benefits of pocket parks can be transformed into economic values, which can raise public awareness and provide feedback to optimize public policies related to pocket parks.

Planning and design strategy
With the rapid process of urbanization, climate change, and increasing emphasis on physical and mental health by residents, major cities have been implementing the planning and development of pocket parks and related practices one after another. Various countries and organizations have proposed design principles for pocket parks accordingly. For instance, in 2019, the Ministry of Housing and Urban-Rural Development of the People's Republic of China released the 'Urban Habitat Creation Series Manual-Pocket Parks.' This manual provides a comprehensive overview of the planning and construction of pocket parks, including conceptual understanding, site characteristics, design elements, and technical aspects of management and operation. 'The Trust for Public Land' in the United States, issued the 'Pocket Park Toolkit,' a guide to construct pocket parks that offers practical advice. However, the limited availability of land resources and the exorbitant cost of land in densely populated urban regions create a pressing need for efficient pocket park services. Unfortunately, the absence of comprehensive research that takes into account the external environment has resulted in haphazard installation of new pocket parks in numerous cities, failing to fully realize their potential or optimize the value of urban land. Many innovative cities face issues with the unequal distribution of pocket parks, as well as problems with the design and management of these spaces. These challenges can impede the efficient planning and construction of urban pocket parks [19,87]. Future research should strive to instill a planning-focused mindset and conduct comprehensive assessments of the built environment, encompassing social, urban, and ecological elements. This methodology can establish a basis for investigating spatial planning tactics that satisfy the requirements of the community while accommodating the current built environment and optimizing the benefits of pocket parks.

Public policy
The primary challenge of implementing pocket parks on a large scale in high-density urban areas is their integration into urban green space planning and master planning. Accordingly, promoting pocket parks is closely tied to the development of relevant policies, which necessitate a well-designed top-level mechanism. While the design of pocket parks in Mexico has the potential to create significant urban spaces, a misalignment between politics, policy, and design has impeded the implementation of the proposed 150 new pocket parks [88]. International research and practical applications of new green infrastructure, such as green roofs, which are similar to pocket parks, have demonstrated the effectiveness of policy interventions in producing the intended results [13]. Therefore, public policy plays a critical role in promoting the implementation of urban pocket parks. Unlike traditional urban green spaces, pocket parks have a diverse range of land ownership, resulting in various stakeholders who control them. These stakeholders can be categorized into three groups: those controlled by the public sector, those controlled by institutional groups, and those controlled independently by residents. Pocket parks provide public benefits that are not solely enjoyed by the stakeholder responsible for their installation. Therefore, policy development and public intervention are necessary to coordinate among multiple stakeholders [75,88]. Furthermore, short-term interventions, such as pop-up parks and guerrilla gardening, require legitimate policy development, and the bibliometric analysis demonstrates that researchers have only recently begun to focus on the impact of policies on pocket parks, with most studies emerging after 2022. Therefore, there is ample opportunity for further research to enhance our understanding of policy impacts in this area. In summary, future work should focus on the policy system and benefit guarantee mechanism related to the promotion of pocket parks.

Conclusion
Pocket parks are a cost-effective and efficient green solution for addressing the shortage of open spaces in high-density urban areas. This study conducted a bibliometric analysis of 276 papers published in WOS and Scopus databases between 1977 and 2023, aiming to understand the overall development of pocket park research. Additionally, it provided a detailed critical review of three major research themes, which are the influencing factors, design methods, and benefits obtained from the bibliometric analysis. The main findings of this study are as follows: Firstly, the amount of research on pocket parks has significantly increased over the past few decades, with the number of publications from 2020 to 2023 already surpassing the total number of papers published on this topic in the previous 43 years. A total of 367 authors from 51 countries and regions, as well as 271 research institutions, have contributed to research on pocket parks. The United States and China have published the largest number of articles, accounting for 43% of the total. However, there is a significant global knowledge gap, and collaborative exchange between institutions in different countries is urgently needed. Secondly, the factors that influence the use of pocket parks include internal, external physical, and socio-economic factors. The mechanisms of these factors and their interactions warrant further investigation. Thirdly, there is a need to further explore the planning methods for pocket parks in greater depth. Planning and design methods in different country contexts can be expanded and compared to provide more scientific guidance for the construction of pocket parks. Fourthly, current research on the benefits of pocket parks focuses on human health and well-being, mitigation of the urban heat island effect, and biodiversity conservation. However, to gain a comprehensive understanding of these benefits, future cross-sectional comparisons across different climatic zones in terms of depth and width are necessary. Fifth, the future large-scale application of pocket parks will require policy research, particularly regarding the legalization of short-term interventions such as pop-up parks and guerrilla gardening. This review can serve as a roadmap for researchers who are interested in studying pocket parks in the future. It can also assist urban planners, policy makers, and government managers in the broader scientific application of pocket parks.