What are nature-based interventions?

A common feature of NBIs is the use of natural environments to improve health and wellbeing across a range of contexts including healthcare [4,14,15], education [16,17], and community-based settings [18]. The mode of delivery for NBIs is highly heterogenous, varying by the type of nature [19], duration of exposure [20], level of interaction [21], intensity of physical activity [22], and presence of other people [23] or non-human animals [16,24].

In an umbrella review on NBIs, Harper et al. [25] created a list of five distinct categories of nature-based therapies based on past work, which included nature-based therapy, forest therapy, horticultural therapy, wilderness therapy, and adventure therapy. Whilst informative, this review was conducted within the context of clinician-led therapies and thus was largely constrained to measurements of individual wellbeing, such as addressing substance abuse, ill health, and cognitive performance. Consequently, Harper et al.’s review [25] excluded non-clinician-led studies and investigations with a broader focus including indicators of collective and planetary wellbeing, concepts that will be introduced below. Nonetheless, these five categories may be helpfully applied to the broader scope of interest identified in this protocol. The term ‘intervention,’ rather than ‘therapy’ will be adopted for the purposes of this review, whereby ‘intervention’ may be understood as action taken towards either ameliorating an illness, dysfunction, or unwanted symptoms, or otherwise promoting wellbeing and positive psychological experience [1,26,27]. We consider ‘intervention’ to be a broader term that does not necessitate direct interaction with a therapist, and therefore will allow the review to capture impacts of therapy-based as well as non-therapy-based NBIs. Thus, the altered categories presented by Harper et al. [25] will serve as a framework for categorising NBIs identified in the review process.

What is wellbeing?

Dictionary definitions of wellbeing focus on the state of being comfortable, healthy, and happy. Dominant scholarly accounts of wellbeing also emphasise the immediate experience of the individual, focusing on constructs such as life satisfaction [e.g., 28] or happiness [e.g., 29], often to the exclusion of important system-wide contextual factors such as social relationships and the health of the non-human environment [13,30,31, but see 32]. Recent developments in the field characterise wellbeing more holistically, with a much wider focus that includes the self, others, and nature [e.g., 13,33]. These developments highlight individual, collective, and even planetary wellbeing, inspired by social ecological models [34,35]. In this regard, individual wellbeing has been defined as the capacity to feel good and function well, exemplified by positive emotions, physical health, and achieving personal goals [36]. Collective wellbeing has been described as feeling good and functioning well, interdependently, signified, for example, by high group cohesion, mutual support, and a sense of belonging [37]. The more recently coined phrase ‘planetary wellbeing’ has been defined as ‘the highest attainable standard of wellbeing for human and non-human beings and their social and natural systems’ [2], p. 4. Encompassing pro-environmental behaviours, such as reducing waste and conserving biodiversity, planetary wellbeing is a systems-oriented perspective that integrates ecosystem health and human wellbeing. The JYU.Wisdom Community [38] defines planetary wellbeing as a state in which the Earth’s natural systems are able to sustain life whilst simultaneously fulfilling organisms’ needs, allowing them to actualise their inherent capacities.

Researchers have typically focused on these different aspects of wellbeing in parallel. The recently developed GENIAL framework [11,13,33,39,40] defines wellbeing from a biopsychosocial-ecological perspective, encompassing the individual, community, and natural environment. By building upon concepts presented in biopsychosocial models [41], ecological systems theory [34,35], and sustainable wellbeing [13], GENIAL integrates evidence from diverse fields – including positive psychology, epidemiology, and environmental health – to provide a unified, holistic understanding of wellbeing that recognises the interdependence of individual, collective, and planetary domains of wellbeing [40].

This model has encouraged the development of innovative interventions, such as a community-supported agriculture initiative to support the wellbeing of local communities through the lens of sustainable food (Pereira, Blackburn et al., [unpublished]). This and related work [e.g., 12,39] characterise what has been described as an emerging third wave of positive psychology, underpinned by an ‘epistemological broadening’ [42, p. 660] that spans beyond the individual and incorporates broader and system-wide contextual factors, including the groups, cultures, and systems in which people are embedded [42–45]. This broadening draws attention to the interdependence of ecological systems and human wellbeing [12], acknowledging that environmental challenges are also challenges to human wellbeing [46].

Recognising wellbeing as a multidimensional construct provides a clear attempt to move beyond the concept of wellbeing as a ‘wicked problem’, an intractable issue that allows for neither simple definitions nor clear (and reproducible) solutions. Part of the ‘wickedness’ of wellbeing lies in often-contradictory approaches to addressing wellbeing at different levels of scale and the intersectionality of complex societal problems, including inequalities and climate change. Actions aimed at enhancing wellbeing within one domain may have system-wide effects that either facilitate or inhibit wellbeing in other domains. Acknowledging the systematic interaction of problems—and potential solutions—across a diversity of domains is essential when attempting to resolve wicked problems [47,48]. We suggest that Nature-Based Interventions (NBIs) may be uniquely placed to influence wellbeing across individual, social, and environmental domains, whilst GENIAL provides a holistic framework well-suited to conceptualising and assessing the reciprocal and cumulative effects of promoting wellbeing at multiple levels.

So why is another review on NBIs needed?

Several reviews on NBIs have now been published [1,49–51], as have multiple ‘reviews of reviews’ [25,52,53, see also 54]. Yet, previous reviews have failed to investigate the effects of NBIs across multiple domains relevant to a more holistic understanding of wellbeing. Past reviews have largely focused on aspects relating to individual wellbeing (and ‘illbeing’), while a focus on collective or planetary wellbeing is less frequently reported. For instance, the meta-review published by Antonelli et al. [52] identified 16 reviews that investigated the effects of forest bathing on wellbeing. Of these, only a single review reported on a measure of social wellbeing [55], and no reviews have yet focused on the construct of planetary wellbeing, despite the proliferation of research investigating the effects of human connection to - and interdependence with - nature and the implications such connection has for pro-environmental attitudes and behaviours [56–59].

Our focus will be a scoping review, an increasingly popular method of systematic literature review [60–62] that provides the opportunity to identify the volume and characteristics of primary research, including key themes and knowledge gaps, in the subject area of interest [62,63]. Previously published scoping reviews on NBIs have focused on a diversity of issues such as mental health [64], bereavement care [65], disease [66,67], and health-related behaviours [5]. There are also several published protocols for scoping reviews that intend to focus on impacts of NBIs on environmental behaviour [68], and parent-child relationships [69]. In a scoping review on NBIs for vulnerable youth [16], the majority of original studies (N = 68) reported on measures of individual and social outcomes, yet none of the studies provided a measure relating to planetary health such as changes in pro-environmental behaviours (e.g., waste reduction and recycling). In contrast, a systematic review presented by Silva et al. [51] included 38 studies where NBIs were used to enhance wellbeing and nature-related outcomes (e.g., nature affinity). Silva et al. [51] identified only four studies which provided a measure of social wellbeing [70–73]. Several factors may have contributed to the limited number of social outcomes observed in this review. Notably, Silva et al. [51] employed only a single search term related to social outcomes, ‘social,’ selected after a brief review of the literature. This narrow selection of search terms might have restricted the scope of their findings.

The present scoping review aims to address this limitation by incorporating a broader and more diverse set of search terms related to NBIs and wellbeing, particularly social wellbeing. For instance, while Silva et al. [51] used only the term ‘social,’ our review includes additional related terms such as ‘loneliness,’ ‘affiliation,’ ‘community,’ ‘belonging,’ and ‘neighbourhoods.’ Our review also offers a greater variety of terms relating to different types of NBIs, such as forest therapy, horticultural therapy, and animal-assisted therapies in outdoor environments. This approach is intended to capture a wider array of relevant studies, thereby providing a more comprehensive synthesis of the available evidence.

Moreover, Silva et al. [51] excluded studies involving virtual reality, participants with serious mental health conditions or severe physical disorders, and self-guidance (i.e., without researcher or practitioner presence). These exclusions might have further limited the scope of their review. By contrast, our review encompasses a broader range of studies, including those with virtual reality components and diverse participant profiles, to ensure a more inclusive and comprehensive understanding of holistic wellbeing outcomes. Finally, Silva et al.’s review included articles published until July 2020 [51]. The present review, therefore, offers the opportunity to include more recent literature in what is a rapidly growing field of investigation. By incorporating studies published after this date, our review aims to provide an up-to-date synthesis of current evidence.

Collectively, previously published reviews, including the work of Silva et al. [51], have demonstrated a great variety of evidence relating to NBIs. However, no single review has comprehensively synthesised the available research on the effects of NBIs on an epistemologically broadened conceptualization of wellbeing that includes a focus on self, others and nature. This scoping review aims to build on Silva et al.’s epistemological breadth by incorporating a more extensive set of search terms, broader inclusion criteria, and more recent literature. In doing so, we will advance understanding of the extent to which different types of NBIs influence wellbeing-related outcomes across (1) individual, (2) collective, and (3) planetary domains, and identify current gaps in knowledge.

Stage 1: Identifying the research question

Research questions were developed in adherence to the population, context, concept (PCC) framework as recommended by JBI [61; see Table 2]. The primary research question is as follows:

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Table 2. The population, concept, and context relating to the primary research question.

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1. How has wellbeing been conceptualised in work on nature-based interventions?

Additional questions to guide the review are presented below:

1. What are the theoretical frameworks that underpin the published research in this area?
2. What types of nature-based interventions have been delivered?
3. What populations have been studied?
4. What outcomes (e.g., psychological, social, environmental) have been measured?
5. How and when does wellbeing arise through nature-based interventions?

Stage 2: Identifying relevant studies eligibility criteria

Search strategy.

The search strategy was informed by JBI guidelines [61,62] and the methodology outlined by Arksey and O’Malley [76]. To ensure a comprehensive list of search terms, the authors completed several consultation exercises to identify terms related to the key themes of wellbeing and NBIs. Development of the search term list proceeded in two phases. Firstly, an initial set of search terms was generated from existing literature by drawing upon the research team’s diverse expertise, including clinical practice (LJ), wellbeing science (AK), and sustainability and wellbeing research (AI). During the first consultation exercise, authors convened to exchange search terms. Discussion during this consultation stimulated additions to the list. In the second phase, which focused on broadening the list, search terms were inputted into the four target databases identified as relevant to this study (see ‘Search Strategy’), and the resulting publications were ‘term farmed.’ The list was expanded again by inputting each search term into APAPsycInfo’s Thesaurus of Psychological Index Terms tool, with the ‘explode’ function enabled. A second consultation allowed authors to discuss the expanded list, leading to additional term suggestions from the team. Search terms which repeatedly produced studies outside of the scope of this review were discarded or modified with prefixes or suffixes at this stage. For example, the term ‘purpose,’ chose for its relation to individual wellbeing, resulted in a large number of unrelated papers due the common phrase ‘the purpose of this study.’ The complete search strategy can be seen in Table 3.

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Table 3. Research concepts and associated search terms.

https://doi.org/10.1371/journal.pone.0314591.t003

Following consultation with experienced librarians, the following databases were identified as suitable for a search of the full search term list: APAPsycINFO, Web of Science, Medline, and Scopus. The ProQuest Dissertations & Theses Global database will be used to identify relevant unpublished research dissertations. Database-specific search strategies using the identified search terms are displayed in S2 Appendix. Any alterations made to search strategies during the review process will be reported in the full review. Data will be stored and screened using EndNote 20.6 [78] and Covidence systematic review software [79].

Eligibility criteria

Types of sources.

To be included in the review, studies must be published in English language with full text available to authors. To ensure a focus on validated sources of evidence, we have opted to exclude grey literature, focusing instead on theses and peer-reviewed journal articles. Whilst this may limit the diversity of evidence captured, this approach prioritises reliability and consistency. Future research could incorporate grey literature to explore additional perspectives and practice-based insights. In the absence of institutional access to the full text, the corresponding author of the study will be contacted via email to request a copy of their work, with a maximum of three attempts. No restrictions will apply to methodology or study design. The review will include randomised control trials, quasi-experimental designs, focus groups, and interviews. Additional study designs included during the review process will be reported in full in the review. Inclusion and exclusion criteria are presented in Table 4.

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Table 4. Inclusion and exclusion criteria.

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Stage 3: Study selection

Duplicates will be removed and, of the remaining texts, titles and abstracts will undergo a pilot screening. In line with JBI methodology, this pilot exercise will consist of each named author reviewing the title and abstracts of a random sample of 25 papers. Authors will convene to compare included and excluded papers. A minimum consensus 75% will be required before the full screening proceeds. Adjustments to the criteria will be made iteratively as required and disclosed along with any other amendments in the methodology of the full review. Two reviewers, JB and AP, will apply the final eligibility criteria to a review of all study titles and abstracts. For each study, each reviewer will independently vote ‘yes, include,’ ‘no, exclude,’ or ‘maybe, unknown.’ Studies receiving a unanimous ‘no’ will be excluded and studies receiving a unanimous ‘yes’ will be included. The inclusion or exclusion of studies with a unanimous ‘maybe’ or without a unanimous result will be determined following a discussion between the two reviewers. A third reviewer will offer input to break ties when needed. Following the recommendations of Tricco et al. [74], a PRISMA flow diagram will be generated through Covidence to record the number of studies included and excluded – and the rationale of these decisions – at each stage of the study selection process. The proposed flow of articles through this process can be seen in Fig 1.

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Fig 1. Proposed flow of articles through the scoping review process.

https://doi.org/10.1371/journal.pone.0314591.g001

Stage 4: Charting the data

Data will be extracted by two independent reviewers (JB and AP) with the use of a data charting form (see Table 5). Data extraction will be piloted by JB and AP on a randomly selected sample of 10 papers prior to a calibration exercise with the remaining authors (AI, LJ, AK). The calibrated form will be applied to all papers retained following screening. Data extraction will involve regular discussion between the two reviewers; disagreements will be resolved by input from a third reviewer. Data to be extracted will include general study information (e.g., title, first author name, year, population demographics, key findings) and the concepts (e.g., wellbeing outcomes and characteristics of NBI) and context (targeted population, geographic location) identified in the present review. NBI categorisation will broadly follow the five-category grouping presented by Harper et al. [25] but will remain an iterative process. Changes made to the categorisation system will be disclosed in the review. In the instance that the self-identification of an NBI is not recorded in a study or the authors’ self-identification of the NBI does not align with Harper et al.’s categories [25], a note describing the rationale of subsequent categorisation will be provided by the authors in the full review. Any information from the data charting form which is missing from the study will be coded as ‘not recorded,’ and unclear information as ‘unclear.’ The reviewers will attempt a maximum of three email contacts with the study’s corresponding author to request information or clarification. If the information remains inaccessible or unclear, this will be reported in the final review. Following data charting, a final consultation exercise with all authors will be held to review the suitability of the planned data analysis and presentation strategy, providing an opportunity to resolve unexpected findings. For example, review of the literature may reveal a sixth type of NBI not covered by Harper et al.’s [25] classification. In this instance, the authors will be able to amend the planned data presentation strategy. The details of any such amendments and their rationale will be reported in full in the review. Finally, given that the purpose of scoping reviews is to map the breadth of existing evidence rather than to assess methodological quality, and due to the typically heterogeneous nature of included sources, risk of bias assessments are generally considered unsuitable for scoping reviews [62,74] and will not be included in the present review.

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Table 5. Data charting form.

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Stage 5: Collating, summarising and reporting the results

Following data charting, a final consultation exercise with all authors will be conducted to develop a data analysis and presentation strategy. The details of this data analysis and presentation will be reported in full in the review. Expected data presentation related to each research question may be seen in Table 5. All data will be published in the full review. Data synthesis and presentation will be informed by the multi-domain approach to wellbeing imposed by the GENIAL framework, with several sub-group analyses to handle the expected heterogeneity of results. Quantitative synthesis will involve tabular and graphical summaries showing the frequency of studies that report data on each facet listed in Table 5 (a sample of one such table is presented as Table 6 below). This synthesis will highlight any imbalances in the distribution of studies across methodological approach (quantitative or qualitative), NBI type, theoretical framework (e.g., Attention Restoration Theory), psychotherapeutic approach (where relevant; e.g., Cognitive Behavioural Therapy), geographic location, population (clinical or non-clinical), and outcome by wellbeing domain, ultimately pointing to areas for future research.

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Table 6. Planned table of outcome measures and direction of change reported by quantitative investigations of NBIs by wellbeing domain.

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The expected heterogeneity of results from quantitative and qualitative methods precludes a direct statistically comparison of findings. However, a broad synthesis of heterogenous results can be achieved through narrative synthesis and dual-display tables. A narrative synthesis will examine how wellbeing is conceptualised and measured in NBI studies, reflecting on proposed mechanisms of change and the extent to which NBI research adopts multi-faceted conceptualisations of wellbeing. Dual-display tables, which provide a structured presentation of key findings across methodologies, will be used to synthesise quantitative and qualitative findings across the multiple domains of wellbeing as per the GENIAL framework (for a sample dual-display table, see Table 7). In summary, data synthesis will answer the research questions proposed in Stage 1, by presenting an overview of the following findings: (1) definitions and conceptualisations of wellbeing, (2) theoretical frameworks utilised, (3) key characteristics of NBIs, (4) how wellbeing is measured, and (5) populations that have been studied. The narrative synthesis will also identify any proposed mechanisms of change reported in the literature, and highlight any commonalities in terms of the specific domain of wellbeing to which the outcomes relate.

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Table 7. Planned dual-display table of the key findings from quantitative and qualitative investigations of NBIs by wellbeing domain.

https://doi.org/10.1371/journal.pone.0314591.t007