Factors influencing the effectiveness of nature-based interventions (NBIs) aimed at improving mental health and wellbeing: An umbrella review

Several systematic reviews support nature-based interventions (NBIs) as a mechanism of enhancing mental health and wellbeing. However, the available evidence for the effectiveness of these interventions is fragmentary and mixed. The heterogeneity of existing evidence and significant fragmentation of knowledge within the field make it difficult to draw firm conclusions regarding the effectiveness of NBIs. This can only limit the development of interventions to support personal and planetary wellbeing, potentially resulting in ineffective and ill- -targeted investment decisions. These concerns have hindered the translation of research evidence into practice and guidelines are urgently required to assist researchers, practitioners and policymakers in developing interventions to promote environmental stewardship and meet the health needs of diverse communities. A higher- order or meta-level synthesis is required to make sense of this evidence. This umbrella review synthesises evidence on the barriers and enablers to nature-based interventions through a summative review of existing published systematic reviews and meta-analyses. A systematic search in PsycINFO, PubMed, Greenfile, Web of Science, Embase, Scopus, Academic Search Complete (EBSCO), Environment Complete (EBSCO), Cochrane Library, CINAHL, Health Policy Reference Centre and Google Scholar was performed. This is a mixed method review, and systematic reviews with both quantitative and qualitative data synthesis were included. 64 systematic reviews were included in the synthesis. The descriptive data, extracted from each study, forms an overview of the characteristics of available evidence on nature-based interventions. A narrative synthesis is used to present the potential factors influencing the outcomes of NBIs. The risk of bias of the systematic reviews was assessed using a 16-item Assessment of Multiple Systematic Reviews 2 (AMSTAR2) checklist. Directions for future research, potential quality needs and recommendations for future research, policymaking, and practice are discusssed.