Better Forests, Better Cities

Appendix A: Research Methodology

The Better Forests, Better Cities publication reviewed the latest research on the effects of forests at three levels (inner, nearby, and faraway) on cities and their residents. The report summarizes the findings of research gathered through a series of systematic “reviews of reviews” (surveys of published review papers; e.g., van den Bosch and Sang [2017]) and scoping literature reviews complemented by “snowballing” (reviewing reference lists and/or expert recommendation), reviews of the primary literature, and literature recommended by topical experts, conducted for each of the four main sections: health and well-being, water, climate, and biodiversity.

The body of relevant work in each of these section topics spans multiple decades and encompasses an array of disciplines. Many topics were thus well suited to using the “review of reviews” method, which we found by developing a list of key search terms and performing a series of searches in scholarly databases (see individual sections for details). Throughout the publication, the formal review process was supplemented with primary literature, reports and gray literature, consultations with and recommendations from topical experts (see details of people consulted for each section below), and from the amassed experience of WRI and Pilot Projects in implementing various city-related projects (including Cities4Forests; UrbanShift;52 Wood at Work;53 Urban Water Resilience Initiative;54 the Global Commission on Adaptation,55 specifically the cities and NBS tracks; Brooklyn Bridge Forest;56 and various other projects through WRI’s Ross Center for Sustainable Cities,57 Water Program,58 and Forests Program59) and their discussions and engagements with city representatives over the years. These findings have been used in the report as brief in-text illustrations and in-depth case studies to exemplify forest-related ecosystem services and their nuances, highlight success stories, portray other important ideas, and fill gaps in knowledge where insufficient reviews were available. Drafts of each section were also reviewed by experts in the respective fields prior to the formal review process.

Health and Well-Being

To equip urban leaders with an overview of the immense array of information connecting forests to human health and well-being, we conducted a systematic review of reviews—including peer-reviewed syntheses, meta-analyses, and summary reports in gray literature—identified using multiple databases. We supplemented our findings with empirical research from individual cities to provide additional context and geographic representation. Because human health and well-being research that focuses specifically on forests remains limited, we broadened our scope from urban forests and trees to include urban green space, green infrastructure, and urban nature in some portions of this section to provide a more comprehensive overview where forest-specific science was lacking; this shift in emphasis is indicated in the corresponding report text when applicable.

We began by conducting a systematic literature review of existing review papers, including other literature reviews, meta-analyses, and gray literature, later supplemented with additional information from primary sources.

We began a systematic literature review by identifying relevant search terms and seminal works in consultation with topic experts, including Dr. Kathleen Wolf60 and Dr. Dexter Locke.61 After the initial list was created, exploration of existing reviews and reviews of reviews led to the addition of many terms. The search terms were designed to identify review papers that explored forests, urban areas, and at least one issue of social, physical, mental, or economic health and well-being simultaneously (Box A1).

Box A1 | Search Terms For Health and Well-Being

((agroforest* OR “ecosystem services” OR forest* OR green-space* OR “nature-based solution*” OR “natural infrastructure” OR tree* OR woodland*) AND (“meta analysis” OR review* OR meta-analysis OR meta-synthesis OR synthesis OR synopsis) AND (city OR cities OR metropolitan OR metro OR urban OR peri-urban OR sprawl) AND (adiposity OR allerg* OR anxiety OR asthma OR “attention restoration” OR attention-deficit OR birth weight OR “blood pressure” OR BMI OR cancer OR cardiovascular OR Child* OR cognitive OR cortisol OR diabetes OR depression OR disease OR disorder OR elderly OR epidemiology OR exercise OR health OR healing OR immune OR immunolog* OR inflammat* OR infant OR mental OR Microbiome OR mindfulness OR Morbidity OR Mortality OR noise OR nutrition OR obesity OR perception OR “physical activity” OR physiological OR prescription OR psycholog* OR public OR PTSD OR “quality of life” OR recreation OR respiratory OR restor* OR risk OR sedentary OR sleep OR sound OR Stress OR “UV radiation” OR vector OR well-being OR wellbeing OR wellness OR youth OR academic OR access OR activity OR aggression OR cohesion OR community OR creativity OR crime OR cultur* OR disparit* OR ethnicity OR equity OR “food security” OR forag* OR fuelwood OR gentrification OR “green streets” OR inequalit* OR “land tenure” OR neighborhood OR race OR recreation OR “soil remediation” OR residential OR resilienc* OR safety OR school* OR social OR societ* OR stewardship OR vacant OR value OR visibility OR walkability OR cooling OR “heat island*” OR heat* OR irradiation OR microclimate OR refuge OR shade OR temperature OR thermal OR UV OR wind OR air OR allergen* OR BVOC OR “carbon monoxide” OR contamin* OR CO OR “Nitrogen Oxide” OR NOx OR ozone OR O3 OR particulate* OR pollen OR pollut* OR smog OR SO2 OR “sulfur dioxide” OR VOC* OR “Volatile Organic Compound*”))

Using a systematic review of three databases (Scopus, Web of Science, and PubMed), as well as Google Scholar, we identified peer-reviewed literature and other publications (e.g., book chapters and summary reports). The first phase of the systematic search returned 1,953 results, supplemented by the first 200 results from two Google Scholar searches (for a total of 2,353). During the first phase, we scanned titles and abstracts to determine relevance. During the second phase, following removal of duplicates, 384 full texts were reviewed. A small number of publications could not be accessed and were thus excluded. A total of 146 peer-reviewed articles, book chapters, and reports met inclusion criteria (i.e., written in English; needed to be a systematic review, meta-analysis, or narrative review; addressed one of the main research questions). In addition to the results of the systematic review, 36 publications identified by snowballing were included. Thus, in total, 182 texts were initially reviewed in full for inclusion in this publication. The returned documents represented a broad variety of disciplines, including urban forestry, landscape planning, environmental health, epidemiology, psychology, and political ecology.

In areas of interest where few or no review articles exist, we consulted relevant empirical studies in the primary literature (nonreview papers), identified via scholarly databases, review of reference lists, and additional recommendations from internal and external experts, including Elleni Ashebir,62 Dr. Beatriz Cardenas,63 Dr. Jessica Seddon,64 Dr. Theodore Eisenman,65 Dr. Viniece Jennings,66 Dr. Nick Hewitt,67 and Dr. David Rojas-Rueda.68

As a relatively young field, few studies on the specific health benefits of urban forests exist. Many of the reviews we identified referenced the same seminal studies (e.g., Ulrich 1984 or Kuo 2003). Similarly, many narrowly focused review articles (e.g., reviews of forest bathing) survey and dissect the same small body of literature. Aware of this potential for bias, we consciously sought to minimize repetition of these findings in our report.

To understand potential biases in the research of the 182 texts identified in the original systematic review, we recorded the location of the organization with which the first author of a publication was affiliated. Most of the first authors on the publications originated from North America, Europe, and Australia (Figure A1), which reflects the general pattern of urban ecosystem service research originating primarily in the global North (Haase et al. 2014).

Figure A1 | Geographic Locations of Primary Authors of Publications Reviewed During the Original Systematic Review (according to Institutional Affiliation)

Source: Authors. Adapted from Haase et al. (2014).


To understand the connections between forests, cities, and their water supplies, we first conducted a scoping review of reviews, later supplemented with findings from empirical studies and reports. A comprehensive discussion of the vast literature on the relationship between forests and hydrological cycles (e.g., Guswa et al. 2020) at local and global scales is beyond the scope of this publication. Instead, we provide an overview of key points of interest for cities, including the effects of urban forests on stormwater and flooding, the importance of forests in maintaining healthy watersheds, and the emerging research on the global interconnections between large, intact forests and water.

As with the health and well-being section, to identify relevant search terms for our scoping review, we consulted experts at WRI (Suzanne Ozment,69 Todd Gartner,70 Paige Langer,71 and Sara Walker72) and Conservation International (Aarin Gross73 and Robin Abell74). The initial list was created by combining forest terms, review terms, city terms, and water terms (such as stormwater, drought, flood, and water quality; Box A2). The results from the initial searches using forest terms, review terms, and city terms were complemented by relevant papers that had been identified to inform the broader literature search and from the suggestions of topical experts from WRI and other organizations. As with the health and well-being section, review of primary literature was used as necessary to supplement and illustrate trends, generalizations, and caveats and considerations.

Box A2 | Search Terms for Water

(Forest* Plantation* “ecosystem service*” “urban forest*” “Street tree*” agroforest* reforest* “tree plant*” mangrove* bamboo silvicult* “natural regenerat*” Bioswale* “green infrastructure” “natural infrastructure” Arboriculture “green space” “Riparian Vegetat*” Afforest* woodland* “filter strip*” “forest buffer*” “green roo*” “rain garden*” “natural capital” “nature based solutions” “nature-based solutions” “ecosystem-based adaptation” “ecosystem based adaptation” AND “flying river*” *hydrolog* *transpiration evapo* hydroclimat* Precipit* moisture “cloud forest*” vapor* rain* Stormwater Flood* *filtrat* filter* Regulat* Recharge Erosion Pollut* Flow* Runoff Sewage Drainage Rainwater Throughfall “Surface water” “ground water” “groundwater” Interflow Intercept* Catchment* Canopy Mitigat* Permeab* stemflow uptake “coastal flood*” Drought River* Lake* Streamflow Discharge “water yield*” “water quantity*” Wetland* stream* Watershed* “water scarce*” “water suppl*” provis* “blue water” “green water” “water resource” “water security” “water availability” “water storage” “water balance” “water table” “water production” aquifer “aquifer recharge” hydroelectric* hydropower “water quality” Nutrient* Freshwater Sediment* Nonpoint purifi* nitr* phosph* eutrophic* leach* siltation “water temperature” “stream temperature” “bioindicators” spawn* “fish habitat” “indicator species” “invertebrate*” “EPT index” flyfish* AND “meta analysis” Review* Meta-analys* Meta-synthes* Review Synthesis Synopsis “systematic review*” “weight of evidence” “evidence map” AND Cit* Urban Metropolitan Peri-urban Sprawl)


To explore the connection between climate change mitigation and forests, we conducted two reviews for the climate section: a systematic review of reviews on forests outside cities and a review of reviews and empirical studies on forests inside cities, as the latter is an emerging field with few reviews available.

To identify relevant search terms, we consulted experts within WRI (Frances Seymour,75 David Gibbs,76 Nancy Harris,77 Alexander Rudee78) and externally (David Nowak79). The initial list combined forest terms, review terms, and carbon and climate terms (Box A3). A secondary search using forest terms, review terms, and city terms yielded few results and encouraged researchers to pursue a different methodology to assess the role of forests inside cities on climate mitigation.

We tested the initial list of search terms for forests outside cities on Scopus and Web of Science. Similar to the health and well-being search, we simplified the forest terms, removing references to “ecosystem services” and “nature-based solutions” that greatly increased the number of reviews and were not as directly focused on trees or forests.

Box A3 | Search Terms for Carbon Storage in Forests outside Cities

((forest* OR “tree plantation” OR “timber plantation” OR “monoculture forest*” OR “urban forest*” OR “street tree*” OR agroforest* OR reforest* OR “tree plant*” OR afforest* OR woodland*) AND (“meta analysis” OR meta-analys* OR “evidence synthesis” OR “synthesis of evidence” OR “literature synthesis” OR “synthesis of literature” OR synopsis OR “evidence review” OR “literature review” OR “review of literature” OR “systematic review*” OR “evidence map” OR “review of evidence”) AND (carbon OR CO2 OR biomass OR “greenhouse gas*” OR GHG* OR sequester* OR sequestr* OR REDD*))

Google Scholar search for top 100 articles for both:

  • “review tree forest carbon”
  • “meta-analysis tree forest carbon”

For forests outside cities, we conducted a systematic review of reviews on the Web of Science and Scopus databases as well as Google Scholar to identify peer-reviewed literature and other publications relevant to our research topic. The search returned 1,123 results from Web of Science and 854 from Scopus, in addition to the first 100 results from two Google Scholar searches (for a total of 2,177). During the first phase, we scanned titles and abstracts to determine relevance. During the second phase, following removal of duplicates and inclusion of snowballed articles, 410 full texts were flagged for review. A total of 27 peer-reviewed articles, book chapters, and reports met inclusion criteria (i.e., written in English; needed to be a systematic review, meta-analysis, or narrative review; addressed one of the main research questions). We then supplemented our review of these findings with additional recommendations from other colleagues and experts.

For forests inside cities, we conducted a scoping literature review of studies related to forest carbon storage and cooling benefits. Because carbon storage, carbon sequestration, and cooling/avoided emissions are typically classified as ecosystem services, many studies relevant to this section had already been identified in the systematic review conducted for the health and well-being section. We drew from these works and supplemented with additional exploration of the primary literature and of existing reviews.

To provide a comparison of urban forest carbon density per hectare with that of forests outside cities, we used data from seven peer-reviewed publications that were identified in our reviews on carbon storage inside cities that cover a range of climates and geographies. Six of the peer-reviewed studies provided citywide averages (Intasen et al. 2016; Moussa et al. 2019; Speak et al. 2020), and one study provided a national average for the United States (Nowak et al. 2013). (The researchers did not include data from Kumasi, Ghana, where the citywide average was 228 tC per ha, as this data point was considered to be an outlier; see Nero et al. [2018]). These data points for forests inside cities were compared to averages of carbon density in temperate, tropical, and boreal forests outside of cities, using ranges estimated from Pan et al. (2011) and Goldstein et al. (2020).


To understand the connections between forests, cities, and city water supplies, we conducted a review of the ways that forests at all levels may support biodiversity using an initial list of search terms (Box A4). Each subtopic covered an extensive body of literature, and so by necessity a more general literature review approach was employed rather than a more systematic review of reviews.

Biodiversity often appeared in the searches for the preceding three sections (for example, 56 articles returned for the health and well-being search address some aspect of biodiversity). These papers were reviewed and summarized in the biodiversity section. Additional subtopics of interest identified by the authors through previous experience working with cities and research from preceding sections were covered via a scoping “review of reviews” search, and cases and empirical work were included to fill gaps or to develop fields of research where few or no review papers had been published. Several important sources were found through references from other papers or reviews and experts (Robin Chazdon80 and Patricia Balvanera81) were consulted regarding seminal works on many of the topics.

Box A4 | General Search Terms for Biodiversity

(“Biodiversity”, “global biodiversity”, “review”, “meta analysis”, “forests”, (urban OR city), (species OR species richness OR diversity) combined with specific terms including: measuring, resilience, function, ecosystem services, carbon, carbon storage, carbon sequestration, invasive species, endemic species, generalist species, health, benefits, well-being, mental health, medicin*, pharmaceutical, pollination, pollinator*, global food production, urban agriculture, peri-urban agriculture, zoonotic disease, pandemic, urban forests)

Case studies on urban forest biodiversity were chosen based on preexisting knowledge of world-renowned urban forests and availability of information. To compare biodiversity in urban areas with areas outside cities, areas outside of the urban bounds were chosen based upon availability of data, proximity to the urban area (to maintain similarity of biome), and size (as close to urban forest area as possible). Specific searches were conducted to provide comparable species richness information for forested areas inside and outside of cities, and sources included peer-reviewed journal articles, gray literature, species lists reported by government or park web resources, and publicly available databases.

Recommendations for Policy and Action

Our recommendations in this section are derived in equal part from direct and indirect suggestions made by authors of the articles reviewed for the four main sections, and from engagements with city representatives (through project implementation, at conferences and during workshops, and through formal and informal discussions) and the experience that the Cities4Forests team has accumulated over the years from implementing forest-related projects with cities, including through the development of our Cities4Forests Toolbox82 and two learning guides focused on decision-makers, “Urban Forests for Healthier Cities: Policy, Planning, Regulations, and Institutional Arrangements” (Juno and Virsilas 2019) and “Social Equity Considerations for Cities’ Decision Making Related to Inner, Nearby, and Faraway Forests” (Trivedi et al. 2020), which represent the culmination of additional research on existing urban forest policy environments and consultation with experts and practitioners. We also relied on the experience of a team of colleagues who work on issues related to forests in cities, namely Todd Gartner, Terra Virsilas,83 Lisa Beyer,84 James Anderson,85 Lizzie Marsters,86 Suzanne Ozment, Ayushi Trivedi,87 Natalie Elwell,88 and Frances Seymour, to develop recommendations and general guidance based on best practices and lessons learned.

Recommendations were developed for several areas of policy and action, including measuring and monitoring, planning, partnerships, finance, and markets. For each forest level, we developed recommendations under each type of recommendation (for example, we outlined specific actions that cities could take to identify financing opportunities for inner forest initiatives). Communication was included as a cross-cutting theme for all forest levels.

Gaps and Limitations of the Methodology

The body of evidence exploring the roles that forests inside and outside of cities play in supporting our four topic areas of focus is expansive and rapidly growing. As a result, our reviews may not reflect some of the most recent developments in these areas. Language barriers and publishing biases may have also presented geographical bias in the reviews. To attempt to remedy this issue, we specifically sought to include findings from primary literature and highlight case studies from under-represented areas in the supplemental stages of our reviews.

Despite these limitations, we believe these reviews provide policymakers; city leaders; practitioners from civil society, community-based, and nonprofit organizations that work hand in hand with cities; and researchers with a broad, interdisciplinary overview of current research connecting forests and trees to the health and well-being of city residents and to cities’ goals related to water, climate change mitigation, and biodiversity preservation.