Several systematic reviews support the use of 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 mixed method umbrella review aims to synthesise evidence on the effectiveness of 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 will be performed from inception to present. The search strategy will aim to find published systematic reviews of nature-based interventions (NBIs) where improving health and wellbeing is an explicit goal. This is a mixed method review, and systematic reviews with both quantitative and qualitative data synthesis will be considered. Two authors will independently perform the literature search, record screening, data extraction, and quality assessment of each included systematic review and meta-analysis. The individual qualitative and quantitative syntheses will be conducted in parallel and combined in an overarching narrative synthesis. The quantitative evidence will be used to assess the strength and direction of the effect of nature-based interventions on mental health and wellbeing outcomes. Evidence drawn from qualitative studies will be analysed and synthesised to understand the various pathways to engagement, involvement process and experiential factors that may mediate experiences. The risk of bias of the systematic reviews will be assessed using a 16-item Assessment of Multiple Systematic Reviews 2 (AMSTAR2) checklist. Trail registration: This review is registered on PROSPERO (CRD42022329179).
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