Effects of Shinrin-Yoku (Forest Bathing) and Nature Therapy on Mental Health: a Systematic Review and Meta-analysis (original) (raw)

Abstract

Shinrin-yoku, immersing oneself in nature using one’s senses, has been receiving increased attention internationally. While most of the existing studies have focused on physical health, this systematic review and meta-analysis examined the mental health impacts of shinrin-yoku, using the PRISMA guidelines (PROSPERO registry: BLINDED). Of 497 articles retrieved on databases including PubMed/MEDLINE, PsycINFO, Science Direct and Google Scholar, twenty met the inclusion criteria. All studies were conducted in Asia and Europe, and used a variety of different bathing approaches (e.g. breathing, walking, yoga). While noting a need for more rigorous research and extensive follow-up assessments, the findings indicate that shinrin-yoku can be effective in reducing mental health symptoms in the short term, particularly anxiety. More careful examination of shinrin-yoku practices is needed; longer follow-up with participants from a range of countries along with greater examination of potential mechanisms is needed for shinrin-yoku to be accepted into mainstream interventions.

Methods

The present article followed the preferred reporting items for systematic review and meta-analysis (PRISMA; Moher et al. [2009](/article/10.1007/s11469-020-00363-4#ref-CR49 "Moher, D., Liberati, A., Tetzlaff, J., Altman, D. G., & Group, T. P. (2009). Preferred reporting items for systematic reviews and meta-analyses: The PRISMA statement. PLoS Medicine, 6(7), e1000097. https://doi.org/10.1371/journal.pmed.1000097

            .")) guidelines, to systematically review the literature and appraise the quality of evidence for the mental health effects of shinrin-yoku. Additionally, to maintain the validity of this systematic review, Klassen et al.’s ([1998](/article/10.1007/s11469-020-00363-4#ref-CR35 "Klassen, T. P., Jadad, A. R., & Moher, D. (1998). Guides for reading and interpreting systematic reviews: I. getting started. Archives of Pediatrics & Adolescent Medicine, 152(7), 700–704. 
              https://doi.org/10.1001/ARCHPEDI.152.7.700
              
            .")) framework was employed, focusing on question, criteria, missing articles, quality of the studies, assessment and results. The extended version of the PICO (population, intervention, control and outcomes) format (Boland et al. [2013](/article/10.1007/s11469-020-00363-4#ref-CR6 "Boland, A., Cherry, M. G., & Dickson, R. (2013). Doing a systematic review: a student’s guide. London: Sage.")) was used to construct a researchable question by dissecting the question into those components to help organise relevant information (Sackett et al. [1997](/article/10.1007/s11469-020-00363-4#ref-CR62 "Sackett, D., Richardson, W. S., Rosenburg, W., & Haynes, R. B. (1997). How to practice and teach evidence based medicine (2nd ed.). London: Churchill Livingstone.")). The main research questions of this review were the following: (i) how effective is shinrin-yoku in improving mental health outcomes? And (ii) what quantity and quality of evidence is reported?

Literature Search

The literature search was conducted clarifying (i) where (i.e. databases), (ii) when the literature was searched, (iii) who searched the literature, (iv) how (i.e. keywords), (v) what amount of articles retrieved at each combination of the keywords and (vi) why some articles were included/excluded (i.e. selection criteria) (Callahan [2010](/article/10.1007/s11469-020-00363-4#ref-CR9 "Callahan J. L. (2010). Constructing a Manuscript: Distinguishing Integrative Literature Reviews and Conceptual and Theory Articles. Human Resource Development Review, 9(3), 300–304. https://doi.org/10.1177/1534484310371492

            .")). Literature on PubMed/MEDLINE, PsycINFO, Science Direct and Google Scholar were searched, after a consultation with a subject librarian. Articles published before the 30 October 2019 were searched in November 2019\. The search terms ‘shinrin-yoku (including ‘shinrin yoku’) (_n_\=205), ‘forest bathing’ (_n_ \= 148) and ‘nature therapy’ (_n_ \= 129) retrieved 481 articles in total (‘nature therapy’ as synonymous with shinrin-yoku; Hansen et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR27 "Hansen, M. M., Jones, R., & Tocchini, K. (2017). Shinrin-Yoku (forest bathing) and nature therapy: a state-of-the-art review. International Journal of Environmental Research and Public Health, 14(8). 
              https://doi.org/10.3390/ijerph14080851
              
            .")).

Eligibility Criteria

To be eligible for further analysis, articles needed to (i) be published in a peer-reviewed academic journal using English language; (ii) employ a shinrin-yoku intervention; (iii) report an empirical intervention study, using pre- and post-intervention measures; and (iv) use mental health measures for depression, anxiety, stress and anger. Exclusion criteria were articles that (i) were not interventions, (ii) were case studies or qualitative studies and (iii) did not measure depression, anxiety, stress and anger (Table 1).

Table 1 Extended PICO for this review

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Outcome Measures

Outcome measures were instruments that evaluate the levels of depression, anxiety, stress and anger. Because there were various measurement tools used, we did not have principal summary measures set. Mental health outcomes measured in the included studies were depression (k = 19), anxiety (k = 22), anger (k = 14) and stress (k = 1).

Data Extraction and Synthesis

The lead author comprehensively examined the search results, and articles were shortlisted for possible inclusion if the title and abstract indicated that the study might satisfy the eligibility criteria. Additional manual reference searches on previous systematic reviews on shinrin-yoku (n = 15) identified 16 additional articles that might fit with the inclusion criteria, thus shortlisted (Rojon et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR61 "Rojon, C., McDowall, A., & Saunders, M. N. K. (2011). On the experience of conducting a systematic review in industrial, work, and organizational psychology. Journal of Personnel Psychology, 10(3), 133–138. https://doi.org/10.1027/1866-5888/a000041

            ."); Appendix [1](/article/10.1007/s11469-020-00363-4#MOESM1)). To counter any potential bias, the other co-authors independently reviewed the entire selection process. Full papers of shortlisted articles were independently reviewed by all co-authors. Lastly, a discussion was held among the co-authors to confirm whether the selected articles had met the eligibility criteria and revisited the excluded studies to ensure the reasons for exclusion were accurate (Appendix [2](/article/10.1007/s11469-020-00363-4#MOESM1)). Forward and backward reference searches of relevant articles revealed no additional studies.

Data were extracted focusing on study aims, characteristics, participants, intervention details, outcome measures and main findings (Table 2). Data were synthesised by the mental health outcomes examined in the selected articles, further categorised into the four measures of mental health—depression, anxiety, stress and anger (Table 3).

Table 2 Study details of selected articles exploring mental health effects of shinrin-yoku (n = 20)

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Table 3 Included studies organised by mental health measures

Full size table

Meta-analyses were conducted focusing on depression, anxiety and anger; meta-analysis of stress data was not possible (k = 1). We compared Pearson’s product-moment correlations to determine effect size for the shinrin-yoku intervention. Data were entered into Meta-Essentials (Van Rhee et al. [2015](/article/10.1007/s11469-020-00363-4#ref-CR72 "Van Rhee, H. J., Suurmond, R., & Hak, T. (2015). User manual for meta-essentials: workbooks for meta-analysis (version 1.0). Rotterdam: Erasmus Research Institute of Management http://www.erim.eur.nl/research-support/meta-essentials

            .")).

Variability was examined using Cochran’s Q and _I_2. Heterogeneity among effect sizes was determined by a significant Q value (p < 0.10). The _I_2 statistic indicates the degree of variability in effect sizes (low heterogeneity, 1–49; moderate heterogeneity, 50–74; high heterogeneity, 75–100). In the case of significant heterogeneity, subgroup and moderator analyses were undertaken.

Quality Appraisal: Risk of Bias

The quality of the included non-randomised studies was assessed using the Newcastle-Ottawa Scale (NOS; Wells et al. [2000](/article/10.1007/s11469-020-00363-4#ref-CR75 "Wells, G. A., Shea, B., O'Connell, D., Peterson, J., Welch, V., Losos, M., & Tugwell, P. (2000). The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analyses. Retrieved, 15 June 2017, from http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp

            ")). Using a star system, three assessors rate the quality of studies from 0 to 9 stars (high risk, 0–3; medium risk, 4–6; low risk, 7–9) in three domains: (i) representativeness of study group selection (max. 4), (ii) comparability of groups (max. 2) and (iii) ascertainment of either the exposure or outcome of interest (max. 3). Some adjustments were made to NOS because many of the included studies recruited samples who had no mental disorders (while NOS was originally developed for medical research attended by clinical samples): (i) the word ‘exposure’ was changed to ‘intervention’, (ii) the fourth scale item was changed from ‘Demonstration that outcome of interest was not present at start of study’ to ‘Demonstration that the measured outcome was assessed before the intervention’ (because some mental health outcomes exist before intervention, e.g. stress) and (iii) in respect of the first item in the outcome assessment, a star was awarded if the outcome was assessed using a validated scale (instead of medical records).

Randomised controlled trials were appraised using the Quality Assessment Table of Randomised Controlled Trials (Brown et al. [2013](/article/10.1007/s11469-020-00363-4#ref-CR8 "Brown, T., Pilkington, G., Bagust, A., Boland, A., Oyee, J., Tudur-Smith, C., Blundell, M., Lai, M., Martin Saborido, C., Greenhalgh, J., Dundar, Y., & Dickson, R. (2013). Clinical effectiveness and cost-effectiveness of first-line chemotherapy for adult patients with locally advanced or metastatic non-small cell lung cancer: a systematic review and economic evaluation. Health Technology Assessment, 17(31), 1–278. https://doi.org/10.3310/hta17310

            .")).

Results

Search Results

The article selection process was illustrated in Fig. 1. Of the 497 articles (481 from the databases and 16 from manual reference search), 167 articles were removed for duplication. The remaining 330 articles were screened for their title and abstract by authors. Sixty articles were selected for full-text review, of which 40 were excluded (Appendix 2) and 20 were included (Table 2).

Fig. 1

PRISMA flow diagram of the article selection process

Full size image

Characteristics of Included Studies

Twenty included studies were relatively recent, the oldest one being published in 2007 (Morita et al.). The majority of the studies were conducted in Asia (n = 18; 86%): ten in Japan (Furuyashiki et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR20 "Furuyashiki, A., Tabuchi, K., Norikoshi, K., Kobayashi, T., & Oriyama, S. (2019). A comparative study of the physiological and psychological effects of forest bathing (Shinrin-yoku) on working age people with and without depressive tendencies. Environmental Health and Preventive Medicine, 24(46), 46. https://doi.org/10.1186/s12199-019-0800-1

            ."); Horiuchi et al. [2013](/article/10.1007/s11469-020-00363-4#ref-CR29 "Horiuchi, M., Endo, J., Akatsuka, S., Uno, T., Hasegawa, T., & Seko, Y. (2013). Influence of forest walking on blood pressure, profile of mood states and stress markers from the viewpoint of aging. Journal of Aging and Gerontology. 
              https://doi.org/10.12974/2309-6128.2013.01.01.2
              
            ."); Lee et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR38 "Lee, J., Park, B.-J., Tsunetsugu, Y., Ohira, T., Kagawa, T., & Miyazaki, Y. (2011). Effect of forest bathing on physiological and psychological responses in young Japanese male subjects. Public Health, 125(2), 93–100. 
              https://doi.org/10.1016/j.puhe.2010.09.005
              
            ."); Morita et al. [2007](/article/10.1007/s11469-020-00363-4#ref-CR50 "Morita, E., Fukuda, S., Nagano, J., Hamajima, N., Yamamoto, H., Iwai, Y., Nakashima, T., Ohira, H., & Shirakawa, T. (2007). Psychological effects of forest environments on healthy adults: Shinrin-yoku (forest-air bathing, walking) as a possible method of stress reduction. Public Health, 121(1), 54–63. 
              https://doi.org/10.1016/j.puhe.2006.05.024
              
            ."); Ochiai et al. [2015](/article/10.1007/s11469-020-00363-4#ref-CR52 "Ochiai, H., Ikei, H., Song, C., Kobayashi, M., Miura, T., Kagawa, T., Li, Q., Kumeda, S., Imai, M., & Miyazaki, Y. (2015). Physiological and psychological effects of a forest therapy program on middle-aged females. International Journal of Environmental Research and Public Health, 12(12), 15222–15232. 
              https://doi.org/10.3390/ijerph121214984
              
            ."); Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. 
              https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005
              
            ."); Song et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR66 "Song, C., Ikei, H., Park, B.-J., Lee, J., Kagawa, T., & Miyazaki, Y. (2018). Psychological benefits of walking through forest areas. International Journal of Environmental Research and Public Health, 15(12), 2804. 
              https://doi.org/10.3390/ijerph15122804
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR67 "Song, C., Ikei, H., Kagawa, T., & Miyazaki, Y. (2019). Effects of walking in a forest on young women. International Journal of Environmental Research and Public Health, 16(2). 
              https://doi.org/10.3390/ijerph16020229
              
            ."); Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR70 "Takayama, N., Morikawa, T., Bielinis, E., Takayama, N., Morikawa, T., & Bielinis, E. (2019). Relation between psychological restorativeness and lifestyle, quality of life, resilience, and stress-coping in forest settings. International Journal of Environmental Research and Public Health, 16(8), 1456. 
              https://doi.org/10.3390/ijerph16081456
              
            .")), four in Korea (Chun et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            ."); Lee et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR39 "Lee, K. J., Hur, J., Yang, K.-S., Lee, M.-K., & Lee, S.-J. (2018). Acute biophysical responses and psychological effects of different types of forests in patients with metabolic syndrome. Environment and Behavior, 50(3), 298–323. 
              https://doi.org/10.1177/0013916517700957
              
            ."); Han et al. [2016](/article/10.1007/s11469-020-00363-4#ref-CR26 "Han, J.-W., Choi, H., Jeon, Y.-H., Yoon, C.-H., Woo, J.-M., & Kim, W. (2016). The effects of forest therapy on coping with chronic widespread pain: physiological and psychological differences between participants in a forest therapy program and a control group. International Journal of Environmental Research and Public Health, 13(3). 
              https://doi.org/10.3390/ijerph13030255
              
            ."); Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            .")), two in Taiwan (Chen et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR10 "Chen, H.-T., Yu, C.-P., Lee, H.-Y., Chen, H.-T., Yu, C.-P., & Lee, H.-Y. (2018). The effects of forest bathing on stress recovery: evidence from middle-aged females of Taiwan. Forests, 9(7), 403. 
              https://doi.org/10.3390/f9070403
              
            ."); Yu et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR80 "Yu, C.-P., Lin, C.-M., Tsai, M.-J., Tsai, Y.-C., & Chen, C.-Y. (2017). Effects of short forest bathing program on autonomic nervous system activity and mood states in middle-aged and elderly individuals. International Journal of Environmental Research and Public Health, 14(8), 897. 
              https://doi.org/10.3390/ijerph14080897
              
            .")) and one in China (Guan et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR24 "Guan, H., Wei, H., He, X., Ren, Z., & An, B. (2017). The tree-species-specific effect of forest bathing on perceived anxiety alleviation of young-adults in urban forests. Annals of Forest Research, 0(0), 327–341. 
              https://doi.org/10.15287/afr.2017.897
              
            ")). Three studies were conducted in Europe: two in Poland (Bielinis et al. [2018a](/article/10.1007/s11469-020-00363-4#ref-CR2 "Bielinis, E., Omelan, A., Boiko, S., & Bielinis, L. (2018a). The restorative effect of staying in a broad-leaved forest on healthy young adults in winter and spring. Baltic Forestry, 24(2), 218–227 Retrieved from 
              https://www.balticforestry.mi.lt/ojs/index.php/BF/article/view/204
              
            ."), [b](/article/10.1007/s11469-020-00363-4#ref-CR3 "Bielinis, E., Takayama, N., Boiko, S., Omelan, A., & Bielinis, L. (2018b). The effect of winter forest bathing on psychological relaxation of young Polish adults. Urban Forestry & Urban Greening, 29, 276–283. 
              https://doi.org/10.1016/J.UFUG.2017.12.006
              
            ."); Bielinis et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR4 "Bielinis, E., Bielinis, L., Krupińska-Szeluga, S., Łukowski, A., Takayama, N., Bielinis, E., et al. (2019). The effects of a short forest recreation program on physiological and psychological relaxation in young Polish adults. Forests, 10(1), 34. 
              https://doi.org/10.3390/f10010034
              
            .")), and one in Serbia (Vujcic et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            .")) (Table [2](/article/10.1007/s11469-020-00363-4#Tab2)). No studies were identified in Africa, Oceania and South and North America. Eight studies were non-randomised trials (Table [5](/article/10.1007/s11469-020-00363-4#Tab5)), and twelve were randomised controlled trials (RCT; Table [6](/article/10.1007/s11469-020-00363-4#Tab6)). All the non-randomised studies employed a pre-post design; two studies had a comparator condition. Twelve RCTs included six studies using crossover (e.g. a forest group walked in a city, while a city group walked in a forest on the second day; Lee et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR38 "Lee, J., Park, B.-J., Tsunetsugu, Y., Ohira, T., Kagawa, T., & Miyazaki, Y. (2011). Effect of forest bathing on physiological and psychological responses in young Japanese male subjects. Public Health, 125(2), 93–100. 
              https://doi.org/10.1016/j.puhe.2010.09.005
              
            ."); Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. 
              https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005
              
            ."); Song et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR66 "Song, C., Ikei, H., Park, B.-J., Lee, J., Kagawa, T., & Miyazaki, Y. (2018). Psychological benefits of walking through forest areas. International Journal of Environmental Research and Public Health, 15(12), 2804. 
              https://doi.org/10.3390/ijerph15122804
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR67 "Song, C., Ikei, H., Kagawa, T., & Miyazaki, Y. (2019). Effects of walking in a forest on young women. International Journal of Environmental Research and Public Health, 16(2). 
              https://doi.org/10.3390/ijerph16020229
              
            ."); Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."); Takayama et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR70 "Takayama, N., Morikawa, T., Bielinis, E., Takayama, N., Morikawa, T., & Bielinis, E. (2019). Relation between psychological restorativeness and lifestyle, quality of life, resilience, and stress-coping in forest settings. International Journal of Environmental Research and Public Health, 16(8), 1456. 
              https://doi.org/10.3390/ijerph16081456
              
            .")), and one study where groups were categorised by different types of trees (Guan et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR24 "Guan, H., Wei, H., He, X., Ren, Z., & An, B. (2017). The tree-species-specific effect of forest bathing on perceived anxiety alleviation of young-adults in urban forests. Annals of Forest Research, 0(0), 327–341. 
              https://doi.org/10.15287/afr.2017.897
              
            ")). Interventions included walking and meditation, and time duration ranged from 15 min to 9 days. While all studies involved paying attention to the five senses (Table [1](/article/10.1007/s11469-020-00363-4#Tab1) for eligibility criteria), 18 (90%) studies involved walking (Bielinis et al. [2018a](/article/10.1007/s11469-020-00363-4#ref-CR2 "Bielinis, E., Omelan, A., Boiko, S., & Bielinis, L. (2018a). The restorative effect of staying in a broad-leaved forest on healthy young adults in winter and spring. Baltic Forestry, 24(2), 218–227 Retrieved from 
              https://www.balticforestry.mi.lt/ojs/index.php/BF/article/view/204
              
            ."), [b](/article/10.1007/s11469-020-00363-4#ref-CR3 "Bielinis, E., Takayama, N., Boiko, S., Omelan, A., & Bielinis, L. (2018b). The effect of winter forest bathing on psychological relaxation of young Polish adults. Urban Forestry & Urban Greening, 29, 276–283. 
              https://doi.org/10.1016/J.UFUG.2017.12.006
              
            ."); Chen et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR10 "Chen, H.-T., Yu, C.-P., Lee, H.-Y., Chen, H.-T., Yu, C.-P., & Lee, H.-Y. (2018). The effects of forest bathing on stress recovery: evidence from middle-aged females of Taiwan. Forests, 9(7), 403. 
              https://doi.org/10.3390/f9070403
              
            ."); Chun et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            ."); Furuyashiki et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR20 "Furuyashiki, A., Tabuchi, K., Norikoshi, K., Kobayashi, T., & Oriyama, S. (2019). A comparative study of the physiological and psychological effects of forest bathing (Shinrin-yoku) on working age people with and without depressive tendencies. Environmental Health and Preventive Medicine, 24(46), 46. 
              https://doi.org/10.1186/s12199-019-0800-1
              
            ."); Guan et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR24 "Guan, H., Wei, H., He, X., Ren, Z., & An, B. (2017). The tree-species-specific effect of forest bathing on perceived anxiety alleviation of young-adults in urban forests. Annals of Forest Research, 0(0), 327–341. 
              https://doi.org/10.15287/afr.2017.897
              
            "); Han et al. [2016](/article/10.1007/s11469-020-00363-4#ref-CR26 "Han, J.-W., Choi, H., Jeon, Y.-H., Yoon, C.-H., Woo, J.-M., & Kim, W. (2016). The effects of forest therapy on coping with chronic widespread pain: physiological and psychological differences between participants in a forest therapy program and a control group. International Journal of Environmental Research and Public Health, 13(3). 
              https://doi.org/10.3390/ijerph13030255
              
            ."); Horiuchi et al. [2013](/article/10.1007/s11469-020-00363-4#ref-CR29 "Horiuchi, M., Endo, J., Akatsuka, S., Uno, T., Hasegawa, T., & Seko, Y. (2013). Influence of forest walking on blood pressure, profile of mood states and stress markers from the viewpoint of aging. Journal of Aging and Gerontology. 
              https://doi.org/10.12974/2309-6128.2013.01.01.2
              
            ."); Lee et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR38 "Lee, J., Park, B.-J., Tsunetsugu, Y., Ohira, T., Kagawa, T., & Miyazaki, Y. (2011). Effect of forest bathing on physiological and psychological responses in young Japanese male subjects. Public Health, 125(2), 93–100. 
              https://doi.org/10.1016/j.puhe.2010.09.005
              
            ."); Lee et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR39 "Lee, K. J., Hur, J., Yang, K.-S., Lee, M.-K., & Lee, S.-J. (2018). Acute biophysical responses and psychological effects of different types of forests in patients with metabolic syndrome. Environment and Behavior, 50(3), 298–323. 
              https://doi.org/10.1177/0013916517700957
              
            ."); Morita et al. [2007](/article/10.1007/s11469-020-00363-4#ref-CR50 "Morita, E., Fukuda, S., Nagano, J., Hamajima, N., Yamamoto, H., Iwai, Y., Nakashima, T., Ohira, H., & Shirakawa, T. (2007). Psychological effects of forest environments on healthy adults: Shinrin-yoku (forest-air bathing, walking) as a possible method of stress reduction. Public Health, 121(1), 54–63. 
              https://doi.org/10.1016/j.puhe.2006.05.024
              
            ."); Ochiai et al. [2015](/article/10.1007/s11469-020-00363-4#ref-CR52 "Ochiai, H., Ikei, H., Song, C., Kobayashi, M., Miura, T., Kagawa, T., Li, Q., Kumeda, S., Imai, M., & Miyazaki, Y. (2015). Physiological and psychological effects of a forest therapy program on middle-aged females. International Journal of Environmental Research and Public Health, 12(12), 15222–15232. 
              https://doi.org/10.3390/ijerph121214984
              
            ."); Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. 
              https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005
              
            ."); Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            ."); Song et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR67 "Song, C., Ikei, H., Kagawa, T., & Miyazaki, Y. (2019). Effects of walking in a forest on young women. International Journal of Environmental Research and Public Health, 16(2). 
              https://doi.org/10.3390/ijerph16020229
              
            ."); Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR70 "Takayama, N., Morikawa, T., Bielinis, E., Takayama, N., Morikawa, T., & Bielinis, E. (2019). Relation between psychological restorativeness and lifestyle, quality of life, resilience, and stress-coping in forest settings. International Journal of Environmental Research and Public Health, 16(8), 1456. 
              https://doi.org/10.3390/ijerph16081456
              
            ."); Vujcic et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            ."); Yu et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR80 "Yu, C.-P., Lin, C.-M., Tsai, M.-J., Tsai, Y.-C., & Chen, C.-Y. (2017). Effects of short forest bathing program on autonomic nervous system activity and mood states in middle-aged and elderly individuals. International Journal of Environmental Research and Public Health, 14(8), 897. 
              https://doi.org/10.3390/ijerph14080897
              
            .")), four (20%) involved meditative activities (Furuyashiki et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR20 "Furuyashiki, A., Tabuchi, K., Norikoshi, K., Kobayashi, T., & Oriyama, S. (2019). A comparative study of the physiological and psychological effects of forest bathing (Shinrin-yoku) on working age people with and without depressive tendencies. Environmental Health and Preventive Medicine, 24(46), 46. 
              https://doi.org/10.1186/s12199-019-0800-1
              
            ."); Lee et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR39 "Lee, K. J., Hur, J., Yang, K.-S., Lee, M.-K., & Lee, S.-J. (2018). Acute biophysical responses and psychological effects of different types of forests in patients with metabolic syndrome. Environment and Behavior, 50(3), 298–323. 
              https://doi.org/10.1177/0013916517700957
              
            ."); Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            ."); Ochiai et al. [2015](/article/10.1007/s11469-020-00363-4#ref-CR52 "Ochiai, H., Ikei, H., Song, C., Kobayashi, M., Miura, T., Kagawa, T., Li, Q., Kumeda, S., Imai, M., & Miyazaki, Y. (2015). Physiological and psychological effects of a forest therapy program on middle-aged females. International Journal of Environmental Research and Public Health, 12(12), 15222–15232. 
              https://doi.org/10.3390/ijerph121214984
              
            .")) and three (15%) involved recreational activities (Bielinis et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR4 "Bielinis, E., Bielinis, L., Krupińska-Szeluga, S., Łukowski, A., Takayama, N., Bielinis, E., et al. (2019). The effects of a short forest recreation program on physiological and psychological relaxation in young Polish adults. Forests, 10(1), 34. 
              https://doi.org/10.3390/f10010034
              
            ."); Chen et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR10 "Chen, H.-T., Yu, C.-P., Lee, H.-Y., Chen, H.-T., Yu, C.-P., & Lee, H.-Y. (2018). The effects of forest bathing on stress recovery: evidence from middle-aged females of Taiwan. Forests, 9(7), 403. 
              https://doi.org/10.3390/f9070403
              
            ."); Han et al. [2016](/article/10.1007/s11469-020-00363-4#ref-CR26 "Han, J.-W., Choi, H., Jeon, Y.-H., Yoon, C.-H., Woo, J.-M., & Kim, W. (2016). The effects of forest therapy on coping with chronic widespread pain: physiological and psychological differences between participants in a forest therapy program and a control group. International Journal of Environmental Research and Public Health, 13(3). 
              https://doi.org/10.3390/ijerph13030255
              
            .")).

A total of 2257 participants (M = 1478, F = 779; age range 18–79 years old) were involved in these included studies, indicating shinrin-yoku’s wide applicability. Six studies involved clinical samples: metabolic syndrome (Lee et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR39 "Lee, K. J., Hur, J., Yang, K.-S., Lee, M.-K., & Lee, S.-J. (2018). Acute biophysical responses and psychological effects of different types of forests in patients with metabolic syndrome. Environment and Behavior, 50(3), 298–323. https://doi.org/10.1177/0013916517700957

            .")), chronic stroke (Chun et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            .")), psychiatric disorders (Vujcic et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            .")), chronic diseases (Yu et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR80 "Yu, C.-P., Lin, C.-M., Tsai, M.-J., Tsai, Y.-C., & Chen, C.-Y. (2017). Effects of short forest bathing program on autonomic nervous system activity and mood states in middle-aged and elderly individuals. International Journal of Environmental Research and Public Health, 14(8), 897. 
              https://doi.org/10.3390/ijerph14080897
              
            .")), chronic pain (Han et al. [2016](/article/10.1007/s11469-020-00363-4#ref-CR26 "Han, J.-W., Choi, H., Jeon, Y.-H., Yoon, C.-H., Woo, J.-M., & Kim, W. (2016). The effects of forest therapy on coping with chronic widespread pain: physiological and psychological differences between participants in a forest therapy program and a control group. International Journal of Environmental Research and Public Health, 13(3). 
              https://doi.org/10.3390/ijerph13030255
              
            .")) and alcoholism (Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            .")).

Measures

Table 3 presents all included studies organised by the mental health measures. POMS was frequently used in shinrin-yoku research (n = 14); other measures used in more than one paper were the State-Trait Anxiety Inventory (STAI; n = 6), and the Beck Depression Inventory (BDI; n = 3).

Outcomes

Depression was measured using six scales: POMS, BDI, the Depression Anxiety and Stress Scale (DASS), the EuroQol Visual Analog Scale (EQVAS), the Hamilton Depression Rating Scale (HDR). Anxiety was also measured using six scales: POMS, STAI, the Anti-Anxiety Questionnaire (AAQ), DASS, EQVAS and the Multiple Mood Scale (MMS). Anger was measured using a single scale: POMS.

Meta-analyses

The depression subscales in POMS, DASS21, MMS and BDI were considered for meta-analysis of depression; EQVAS was excluded as it measures depression and anxiety together. Chun, Chang and Lee’s study ([2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. https://doi.org/10.3109/00207454.2016.1170015

            .")) used HDR and BDI, so HDR was removed because HDR emphasises physical symptoms (Hamilton [1960](/article/10.1007/s11469-020-00363-4#ref-CR25 "Hamilton, M. (1960). A rating scale for depression. Journal of Neurology, Neurosurgery, and Psychiatry, 23, 56–62.")). The anxiety subscales in POMS, STAI and DASS21 were considered for meta-analysis of anxiety, and the anger subscale in POMS was considered for meta-analysis of anger (\* in Table [3](/article/10.1007/s11469-020-00363-4#Tab3)). The anxiety score in STAI from Chun et al.’s study ([2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            .")) was not included as whether the 20 items used were related to state or trait anxiety was not reported. Likewise, the anxiety score in AAQ (Guan et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR24 "Guan, H., Wei, H., He, X., Ren, Z., & An, B. (2017). The tree-species-specific effect of forest bathing on perceived anxiety alleviation of young-adults in urban forests. Annals of Forest Research, 0(0), 327–341. 
              https://doi.org/10.15287/afr.2017.897
              
            ")) was not included as whether this scale has been validated was not reported. For studies that employed two anxiety subscales, namely POMS and STAI (Chen et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR10 "Chen, H.-T., Yu, C.-P., Lee, H.-Y., Chen, H.-T., Yu, C.-P., & Lee, H.-Y. (2018). The effects of forest bathing on stress recovery: evidence from middle-aged females of Taiwan. Forests, 9(7), 403. 
              https://doi.org/10.3390/f9070403
              
            ."); Song et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR66 "Song, C., Ikei, H., Park, B.-J., Lee, J., Kagawa, T., & Miyazaki, Y. (2018). Psychological benefits of walking through forest areas. International Journal of Environmental Research and Public Health, 15(12), 2804. 
              https://doi.org/10.3390/ijerph15122804
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR67 "Song, C., Ikei, H., Kagawa, T., & Miyazaki, Y. (2019). Effects of walking in a forest on young women. International Journal of Environmental Research and Public Health, 16(2). 
              https://doi.org/10.3390/ijerph16020229
              
            ."); Yu et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR80 "Yu, C.-P., Lin, C.-M., Tsai, M.-J., Tsai, Y.-C., & Chen, C.-Y. (2017). Effects of short forest bathing program on autonomic nervous system activity and mood states in middle-aged and elderly individuals. International Journal of Environmental Research and Public Health, 14(8), 897. 
              https://doi.org/10.3390/ijerph14080897
              
            .")), POMS was considered as it is more commonly used. The random effects models were used as the included studies included diverse populations; thus, heterogeneity was assumed (mean effect sizes = small, 0.10–0.29; moderate, 0.30–0.49; high, ≧ 0.50; Cohen [1992](/article/10.1007/s11469-020-00363-4#ref-CR12 "Cohen, J. (1992). A power primer. Psychological Bulletin, 112, 155–159.")).

First, data from six RCT studies were analysed (Bielinis et al. [2018a](/article/10.1007/s11469-020-00363-4#ref-CR2 "Bielinis, E., Omelan, A., Boiko, S., & Bielinis, L. (2018a). The restorative effect of staying in a broad-leaved forest on healthy young adults in winter and spring. Baltic Forestry, 24(2), 218–227 Retrieved from https://www.balticforestry.mi.lt/ojs/index.php/BF/article/view/204

            ."), [b](/article/10.1007/s11469-020-00363-4#ref-CR3 "Bielinis, E., Takayama, N., Boiko, S., Omelan, A., & Bielinis, L. (2018b). The effect of winter forest bathing on psychological relaxation of young Polish adults. Urban Forestry & Urban Greening, 29, 276–283. 
              https://doi.org/10.1016/J.UFUG.2017.12.006
              
            ."); Chun et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            ."); Lee et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR38 "Lee, J., Park, B.-J., Tsunetsugu, Y., Ohira, T., Kagawa, T., & Miyazaki, Y. (2011). Effect of forest bathing on physiological and psychological responses in young Japanese male subjects. Public Health, 125(2), 93–100. 
              https://doi.org/10.1016/j.puhe.2010.09.005
              
            ."); Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            ."); Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR70 "Takayama, N., Morikawa, T., Bielinis, E., Takayama, N., Morikawa, T., & Bielinis, E. (2019). Relation between psychological restorativeness and lifestyle, quality of life, resilience, and stress-coping in forest settings. International Journal of Environmental Research and Public Health, 16(8), 1456. 
              https://doi.org/10.3390/ijerph16081456
              
            .")). Song et al.’s RCT studies ([2018](/article/10.1007/s11469-020-00363-4#ref-CR66 "Song, C., Ikei, H., Park, B.-J., Lee, J., Kagawa, T., & Miyazaki, Y. (2018). Psychological benefits of walking through forest areas. International Journal of Environmental Research and Public Health, 15(12), 2804. 
              https://doi.org/10.3390/ijerph15122804
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR67 "Song, C., Ikei, H., Kagawa, T., & Miyazaki, Y. (2019). Effects of walking in a forest on young women. International Journal of Environmental Research and Public Health, 16(2). 
              https://doi.org/10.3390/ijerph16020229
              
            .")) were excluded as these studies only reported post-intervention scores. Lee et al. ([2018](/article/10.1007/s11469-020-00363-4#ref-CR39 "Lee, K. J., Hur, J., Yang, K.-S., Lee, M.-K., & Lee, S.-J. (2018). Acute biophysical responses and psychological effects of different types of forests in patients with metabolic syndrome. Environment and Behavior, 50(3), 298–323. 
              https://doi.org/10.1177/0013916517700957
              
            .")) and Guan et al. ([2017](/article/10.1007/s11469-020-00363-4#ref-CR24 "Guan, H., Wei, H., He, X., Ren, Z., & An, B. (2017). The tree-species-specific effect of forest bathing on perceived anxiety alleviation of young-adults in urban forests. Annals of Forest Research, 0(0), 327–341. 
              https://doi.org/10.15287/afr.2017.897
              
            ")) were excluded as their RCTs compared different types of forest. Lastly, Vujcic et al. ([2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            .")) was excluded as their RCT did not employ a comparable control group.

Second, 16 studies that reported pre-intervention and post-intervention scores were analysed (Bielinis et al. [2018a](/article/10.1007/s11469-020-00363-4#ref-CR2 "Bielinis, E., Omelan, A., Boiko, S., & Bielinis, L. (2018a). The restorative effect of staying in a broad-leaved forest on healthy young adults in winter and spring. Baltic Forestry, 24(2), 218–227 Retrieved from https://www.balticforestry.mi.lt/ojs/index.php/BF/article/view/204

            ."), [b](/article/10.1007/s11469-020-00363-4#ref-CR3 "Bielinis, E., Takayama, N., Boiko, S., Omelan, A., & Bielinis, L. (2018b). The effect of winter forest bathing on psychological relaxation of young Polish adults. Urban Forestry & Urban Greening, 29, 276–283. 
              https://doi.org/10.1016/J.UFUG.2017.12.006
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR4 "Bielinis, E., Bielinis, L., Krupińska-Szeluga, S., Łukowski, A., Takayama, N., Bielinis, E., et al. (2019). The effects of a short forest recreation program on physiological and psychological relaxation in young Polish adults. Forests, 10(1), 34. 
              https://doi.org/10.3390/f10010034
              
            ."), Chen et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR10 "Chen, H.-T., Yu, C.-P., Lee, H.-Y., Chen, H.-T., Yu, C.-P., & Lee, H.-Y. (2018). The effects of forest bathing on stress recovery: evidence from middle-aged females of Taiwan. Forests, 9(7), 403. 
              https://doi.org/10.3390/f9070403
              
            ."), Chun et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            ."), Furuyashiki et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR20 "Furuyashiki, A., Tabuchi, K., Norikoshi, K., Kobayashi, T., & Oriyama, S. (2019). A comparative study of the physiological and psychological effects of forest bathing (Shinrin-yoku) on working age people with and without depressive tendencies. Environmental Health and Preventive Medicine, 24(46), 46. 
              https://doi.org/10.1186/s12199-019-0800-1
              
            ."), Horiuchi et al. [2013](/article/10.1007/s11469-020-00363-4#ref-CR29 "Horiuchi, M., Endo, J., Akatsuka, S., Uno, T., Hasegawa, T., & Seko, Y. (2013). Influence of forest walking on blood pressure, profile of mood states and stress markers from the viewpoint of aging. Journal of Aging and Gerontology. 
              https://doi.org/10.12974/2309-6128.2013.01.01.2
              
            ."), Lee et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR38 "Lee, J., Park, B.-J., Tsunetsugu, Y., Ohira, T., Kagawa, T., & Miyazaki, Y. (2011). Effect of forest bathing on physiological and psychological responses in young Japanese male subjects. Public Health, 125(2), 93–100. 
              https://doi.org/10.1016/j.puhe.2010.09.005
              
            ."), [2018](/article/10.1007/s11469-020-00363-4#ref-CR39 "Lee, K. J., Hur, J., Yang, K.-S., Lee, M.-K., & Lee, S.-J. (2018). Acute biophysical responses and psychological effects of different types of forests in patients with metabolic syndrome. Environment and Behavior, 50(3), 298–323. 
              https://doi.org/10.1177/0013916517700957
              
            ."), Morita et al. [2007](/article/10.1007/s11469-020-00363-4#ref-CR50 "Morita, E., Fukuda, S., Nagano, J., Hamajima, N., Yamamoto, H., Iwai, Y., Nakashima, T., Ohira, H., & Shirakawa, T. (2007). Psychological effects of forest environments on healthy adults: Shinrin-yoku (forest-air bathing, walking) as a possible method of stress reduction. Public Health, 121(1), 54–63. 
              https://doi.org/10.1016/j.puhe.2006.05.024
              
            ."), Ochiai et al. [2015](/article/10.1007/s11469-020-00363-4#ref-CR52 "Ochiai, H., Ikei, H., Song, C., Kobayashi, M., Miura, T., Kagawa, T., Li, Q., Kumeda, S., Imai, M., & Miyazaki, Y. (2015). Physiological and psychological effects of a forest therapy program on middle-aged females. International Journal of Environmental Research and Public Health, 12(12), 15222–15232. 
              https://doi.org/10.3390/ijerph121214984
              
            ."), Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. 
              https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005
              
            ."), Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            ."), Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR70 "Takayama, N., Morikawa, T., Bielinis, E., Takayama, N., Morikawa, T., & Bielinis, E. (2019). Relation between psychological restorativeness and lifestyle, quality of life, resilience, and stress-coping in forest settings. International Journal of Environmental Research and Public Health, 16(8), 1456. 
              https://doi.org/10.3390/ijerph16081456
              
            ."), Yu et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR80 "Yu, C.-P., Lin, C.-M., Tsai, M.-J., Tsai, Y.-C., & Chen, C.-Y. (2017). Effects of short forest bathing program on autonomic nervous system activity and mood states in middle-aged and elderly individuals. International Journal of Environmental Research and Public Health, 14(8), 897. 
              https://doi.org/10.3390/ijerph14080897
              
            ."), Vujcic et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            .")).

As significant heterogeneity was identified in each symptom, three moderator analyses (i, ii and iii) and three subgroup analyses (iv, v and vi) were conducted to appraise whether (i) crossover of the two groups, (ii) participants being Asian, (iii) participants being Japanese, (iv) the length of the intervention, (v) gender (female-male ratio) and (vi) age accounted for the variability made difference in the effects. The first moderator (i) crossover was not examined in the 16 pre-post studies, as it was not applicable. Lastly, publication bias was examined. Table 4 summarises the results of our meta-analysis.

Table 4 Effect sizes (g) and p values for moderators and subgroups in each variable (depression, anxiety and anger)

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Depression in RCT

The total sample size for RCTs measuring depression was 417 (range 12–47) from six studies. Figure 2 shows the forest plot for the meta-analysis with depression in RCT. There was a small mean negative effect size, g = − 2.54, 95% CI(− 3.56, − 1.52), which was significant. Heterogeneity of effects was significant (Q = 38.84, p < 0.001) and inconsistency was high (_I_2 = 87.13%); in all studies, the effect was negative—depression decreased more in the forest setting compared to the urban setting.

Fig. 2

Effect size for depression in RCT

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The three moderators—(i) whether group crossover was done or not (p = 0.27), (ii) whether participants were Asian or not (p = 0.26) and (iii) whether participants were Japanese or not (p = 0.27)—were not significant predictors. In subgroup analyses, the length of the intervention (iv) was a significant predictor of effect size for depression (slope = 0.04, p = 0.002). However, the intervention length was not significant (p = 0.11) when one extreme value (Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. https://doi.org/10.1007/s12199-011-0215-0

            .")) was removed. The female-male ratio (v) was not a significant (slope = 0.09, _p_ \= 0.97), whereas average age (vi) was a significant predictor for depression (slope = 0.04, _p_ \= 0.04). Possible evidence of publication bias was identified (Appendix [3](/article/10.1007/s11469-020-00363-4#MOESM1)).

Depression in Studies Reported Pre-Post Scores

The total sample size for pre-post scores measuring depression was 1449 (range 12–498) from 16 studies. Figure 3 shows the forest plot for the meta-analysis with depression in studies that reported pre-post scores. There was a medium mean negative effect size, g = − 1.04, 95% CI(− 1.47, − 0.60), which was significant. Heterogeneity of effects was significant (Q = 331.57, p < 0.001) and inconsistency was high (_I_2 = 95.48%); in all studies, apart from Lee et al. ([2011](/article/10.1007/s11469-020-00363-4#ref-CR38 "Lee, J., Park, B.-J., Tsunetsugu, Y., Ohira, T., Kagawa, T., & Miyazaki, Y. (2011). Effect of forest bathing on physiological and psychological responses in young Japanese male subjects. Public Health, 125(2), 93–100. https://doi.org/10.1016/j.puhe.2010.09.005

            .")), the effect was negative—depression decreased from pre-shinrin-yoku to post-shinrin-yoku.

Fig. 3

Effect size for depression in studies reported pre-post scores

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The two moderators—(ii) whether participants were Asian or not (p = 0.12), and (iii) whether participants were Japanese or not (p = 0.20)—were not significant predictors. In subgroup analyses, the length of intervention (iv) was not significant (slope = − 0.02, p = 0.24); however, after removing an extreme value (Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005

            .")), it became significant and it was negative; i.e. the longer the intervention, the smaller the effects (slope = − 0.03, _p_ \= 0.01). The female-male ratio (v) was significant (slope = 1.04, _p_ \= 0.03); however, after removing an extreme value (Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. 
              https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005
              
            .")), it became non-significant (slope = 0.58, _p_ \= 0.15). Lastly, average age (vi) was non-significant (slope = 0.01, _p_ \= 0.22). Possible evidence of publication bias was identified (Appendix [4](/article/10.1007/s11469-020-00363-4#MOESM1)).

Anxiety in RCT

The total sample size for studies measuring anxiety was 327 (range 12–46) from five studies. Figure 4 shows the forest plot for the meta-analysis with anxiety. There was a large mean negative effect size, g = − 8.81, 95% CI(− 21.91, 3.57), which was not significant. Variability across samples was significant (Q = 125.03, p < 0.001) and high (_I_2 = 96.80%).

Fig. 4

Effect size for anxiety in RCT

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All moderators—crossover (p = 0.003), Asian (p = 0.007) and Japanese (p = 0.003)—were significant predictors of effect size for anxiety. The length of the intervention (iv) was not a significant predictor (slope = 0.35, p = 0.19). Female-male ratio (v) was a significant predictor; i.e. the effects were smaller when there were fewer female participants (slope = − 30.84, p < 0.001), whereas average age (vi) was not a significant predictor (slope = 0.13, p = 0.20). Possible evidence of publication bias was not identified (Appendix 5).

Anxiety in Studies Reported Pre-Post Scores

The total sample size for pre-post scores measuring anxiety was 1371 (range 12–498) from 16 studies. Figure 5 shows the forest plot for the meta-analysis with anxiety in studies that reported pre-post scores. There was a large mean negative effect size, r = − 1.83, 95% CI(− 3.07, − 0.58), which was significant. Heterogeneity of effects was significant (Q = 611.89, p < 0.001) and inconsistency was high (_I_2 = 97.55%); in all studies, anxiety decreased from pre-shinrin-yoku to post-shinrin-yoku.

Fig. 5

Effect size for anxiety in studies reported pre-post scores

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The two moderators—(ii) whether participants were Asian or not (p = 0.88), and (iii) whether participants were Japanese or not (p = 0.75)—were not significant predictors. In subgroup analyses, (iv) the length of the intervention was significant; i.e. the longer the intervention, the more effects observed (slope = 0.13, p = 0.02); however, after removing one extreme value (Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005

            .")), it was not significant (slope = 0.05, _p_ \= 0.24). The female-male ratio (v) was not significant (slope = 1.26, _p_ \= 0.12). Lastly, the average age (vi) was a significant moderator (slope = 0.04, _p_ \= 0.01); however, after removing an extreme value (Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. 
              https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005
              
            .")), it became non-significant (slope = 0.01, _p_ \= 0.52). Possible evidence of publication bias was identified (Appendix [6](/article/10.1007/s11469-020-00363-4#MOESM1)).

Anger in RCT

The total sample size for studies measuring anger was 268 (range 12–46) from four studies. Figure 6 shows the forest plot for the meta-analysis with anger. There was a medium mean negative effect size, g = − 1.63, 95% CI(− 13.25, − 0.01), which was significant. Variability across samples was significant (Q = 25.52, p < 0.001) and high (_I_2 = 88.25%).

Fig. 6

Effect size for anger in RCT

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For anger, all the three moderators—crossover (p = 0.13), Asian (p = 0.13) and Japanese (p = 0.13)—were not significant predictors. Likewise, the intervention length (data unidentified), female-male ratio (slope = − 1.61, p = 0.47) and average age (slope = − 3.40, p = 0.06) were not significant. Possible evidence of publication bias was identified (Appendix 7).

Anger in Studies Reported Pre-Post Scores

The total sample size for pre-post scores measuring anger was 1365 (range 12–498) from 12 studies. Figure 7 shows the forest plot for the meta-analysis with anger in studies that reported pre-post scores. There was a medium mean negative effect size, g = − 0.81, 95% CI(− 1.17, − 0.45), which was significant. Heterogeneity of effects was significant (Q = 153.52, p < 0.001) and inconsistency was high (_I_2 = 92.83%); in all studies, anger decreased from pre-shinrin-yoku to post-shinrin-yoku.

Fig. 7

Effect size for anger in studies reported pre-post scores

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The two moderators—(ii) whether participants were Asian or not (p = 0.46), and (iii) whether participants were Japanese or not (p = 0.15)—were not significant predictors. In subgroup analyses, all subgroups were significant: the length of the intervention (iv) slope = − 0.13, p < 0.001; female-male ratio (v) slope = − 0.65, p < 0.001, and average age (vi) slope = − 0.01, p < 0.001. Possible evidence of publication bias was identified (Appendix 8).

Risk of Bias

The risk of bias in the non-randomised studies was deemed to be medium for all eight studies (Bielinis et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR4 "Bielinis, E., Bielinis, L., Krupińska-Szeluga, S., Łukowski, A., Takayama, N., Bielinis, E., et al. (2019). The effects of a short forest recreation program on physiological and psychological relaxation in young Polish adults. Forests, 10(1), 34. https://doi.org/10.3390/f10010034

            ."); Chen et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR10 "Chen, H.-T., Yu, C.-P., Lee, H.-Y., Chen, H.-T., Yu, C.-P., & Lee, H.-Y. (2018). The effects of forest bathing on stress recovery: evidence from middle-aged females of Taiwan. Forests, 9(7), 403. 
              https://doi.org/10.3390/f9070403
              
            ."); Furuyashiki et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR20 "Furuyashiki, A., Tabuchi, K., Norikoshi, K., Kobayashi, T., & Oriyama, S. (2019). A comparative study of the physiological and psychological effects of forest bathing (Shinrin-yoku) on working age people with and without depressive tendencies. Environmental Health and Preventive Medicine, 24(46), 46. 
              https://doi.org/10.1186/s12199-019-0800-1
              
            ."); Han et al. [2016](/article/10.1007/s11469-020-00363-4#ref-CR26 "Han, J.-W., Choi, H., Jeon, Y.-H., Yoon, C.-H., Woo, J.-M., & Kim, W. (2016). The effects of forest therapy on coping with chronic widespread pain: physiological and psychological differences between participants in a forest therapy program and a control group. International Journal of Environmental Research and Public Health, 13(3). 
              https://doi.org/10.3390/ijerph13030255
              
            ."); Horiuchi et al. [2013](/article/10.1007/s11469-020-00363-4#ref-CR29 "Horiuchi, M., Endo, J., Akatsuka, S., Uno, T., Hasegawa, T., & Seko, Y. (2013). Influence of forest walking on blood pressure, profile of mood states and stress markers from the viewpoint of aging. Journal of Aging and Gerontology. 
              https://doi.org/10.12974/2309-6128.2013.01.01.2
              
            ."); Morita et al. [2007](/article/10.1007/s11469-020-00363-4#ref-CR50 "Morita, E., Fukuda, S., Nagano, J., Hamajima, N., Yamamoto, H., Iwai, Y., Nakashima, T., Ohira, H., & Shirakawa, T. (2007). Psychological effects of forest environments on healthy adults: Shinrin-yoku (forest-air bathing, walking) as a possible method of stress reduction. Public Health, 121(1), 54–63. 
              https://doi.org/10.1016/j.puhe.2006.05.024
              
            ."); Ochiai et al. [2015](/article/10.1007/s11469-020-00363-4#ref-CR52 "Ochiai, H., Ikei, H., Song, C., Kobayashi, M., Miura, T., Kagawa, T., Li, Q., Kumeda, S., Imai, M., & Miyazaki, Y. (2015). Physiological and psychological effects of a forest therapy program on middle-aged females. International Journal of Environmental Research and Public Health, 12(12), 15222–15232. 
              https://doi.org/10.3390/ijerph121214984
              
            ."); Yu et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR80 "Yu, C.-P., Lin, C.-M., Tsai, M.-J., Tsai, Y.-C., & Chen, C.-Y. (2017). Effects of short forest bathing program on autonomic nervous system activity and mood states in middle-aged and elderly individuals. International Journal of Environmental Research and Public Health, 14(8), 897. 
              https://doi.org/10.3390/ijerph14080897
              
            .")). All of these studies assessed the mental health outcomes before and after shinrin-yoku (for non-clinical samples, participation eligibility of no mental health disorder was reported). None of these eight studies commented on the representativeness of the cohort or conducted follow-up assessments (Table [5](/article/10.1007/s11469-020-00363-4#Tab5)).

Table 5 Assessment of quality of studies based on mental health outcome (non-randomised trials)

Full size table

In the randomised controlled trials, the risk of bias was deemed high to medium: all the studies scored from two (Bielinis et al. [2018a](/article/10.1007/s11469-020-00363-4#ref-CR2 "Bielinis, E., Omelan, A., Boiko, S., & Bielinis, L. (2018a). The restorative effect of staying in a broad-leaved forest on healthy young adults in winter and spring. Baltic Forestry, 24(2), 218–227 Retrieved from https://www.balticforestry.mi.lt/ojs/index.php/BF/article/view/204

            ."), [b](/article/10.1007/s11469-020-00363-4#ref-CR3 "Bielinis, E., Takayama, N., Boiko, S., Omelan, A., & Bielinis, L. (2018b). The effect of winter forest bathing on psychological relaxation of young Polish adults. Urban Forestry & Urban Greening, 29, 276–283. 
              https://doi.org/10.1016/J.UFUG.2017.12.006
              
            .")) to six (Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            .")). For the studies that employed crossover (Park et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR53 "Park, B.-J., Furuya, K., Kasetani, T., Takayama, N., Kagawa, T., & Miyazaki, Y. (2011). Relationship between psychological responses and physical environments in forest settings. Landscape and Urban Planning, 102(1), 24–32. 
              https://doi.org/10.1016/J.LANDURBPLAN.2011.03.005
              
            ."); Song et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR67 "Song, C., Ikei, H., Kagawa, T., & Miyazaki, Y. (2019). Effects of walking in a forest on young women. International Journal of Environmental Research and Public Health, 16(2). 
              https://doi.org/10.3390/ijerph16020229
              
            ."); Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR70 "Takayama, N., Morikawa, T., Bielinis, E., Takayama, N., Morikawa, T., & Bielinis, E. (2019). Relation between psychological restorativeness and lifestyle, quality of life, resilience, and stress-coping in forest settings. International Journal of Environmental Research and Public Health, 16(8), 1456. 
              https://doi.org/10.3390/ijerph16081456
              
            .")), blinding administration and participants were both graded as ‘not applicable (NA)’ as it was impossible for participants to be unaware of the condition they were assigned to at each time. All studies reported the number of participants allocated to different groups, and inclusion criteria apart from Song et al. ([2019](/article/10.1007/s11469-020-00363-4#ref-CR67 "Song, C., Ikei, H., Kagawa, T., & Miyazaki, Y. (2019). Effects of walking in a forest on young women. International Journal of Environmental Research and Public Health, 16(2). 
              https://doi.org/10.3390/ijerph16020229
              
            .")). The baseline comparability of different groups was reported in seven studies (Chun et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            ."); Guan et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR24 "Guan, H., Wei, H., He, X., Ren, Z., & An, B. (2017). The tree-species-specific effect of forest bathing on perceived anxiety alleviation of young-adults in urban forests. Annals of Forest Research, 0(0), 327–341. 
              https://doi.org/10.15287/afr.2017.897
              
            "); Lee et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR38 "Lee, J., Park, B.-J., Tsunetsugu, Y., Ohira, T., Kagawa, T., & Miyazaki, Y. (2011). Effect of forest bathing on physiological and psychological responses in young Japanese male subjects. Public Health, 125(2), 93–100. 
              https://doi.org/10.1016/j.puhe.2010.09.005
              
            ."), [2018](/article/10.1007/s11469-020-00363-4#ref-CR39 "Lee, K. J., Hur, J., Yang, K.-S., Lee, M.-K., & Lee, S.-J. (2018). Acute biophysical responses and psychological effects of different types of forests in patients with metabolic syndrome. Environment and Behavior, 50(3), 298–323. 
              https://doi.org/10.1177/0013916517700957
              
            ."); Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            ."); Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR70 "Takayama, N., Morikawa, T., Bielinis, E., Takayama, N., Morikawa, T., & Bielinis, E. (2019). Relation between psychological restorativeness and lifestyle, quality of life, resilience, and stress-coping in forest settings. International Journal of Environmental Research and Public Health, 16(8), 1456. 
              https://doi.org/10.3390/ijerph16081456
              
            .")), and achieved in seven studies (Chun et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            ."); Guan et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR24 "Guan, H., Wei, H., He, X., Ren, Z., & An, B. (2017). The tree-species-specific effect of forest bathing on perceived anxiety alleviation of young-adults in urban forests. Annals of Forest Research, 0(0), 327–341. 
              https://doi.org/10.15287/afr.2017.897
              
            "); Lee et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR38 "Lee, J., Park, B.-J., Tsunetsugu, Y., Ohira, T., Kagawa, T., & Miyazaki, Y. (2011). Effect of forest bathing on physiological and psychological responses in young Japanese male subjects. Public Health, 125(2), 93–100. 
              https://doi.org/10.1016/j.puhe.2010.09.005
              
            ."), [2018](/article/10.1007/s11469-020-00363-4#ref-CR39 "Lee, K. J., Hur, J., Yang, K.-S., Lee, M.-K., & Lee, S.-J. (2018). Acute biophysical responses and psychological effects of different types of forests in patients with metabolic syndrome. Environment and Behavior, 50(3), 298–323. 
              https://doi.org/10.1177/0013916517700957
              
            ."); Shin et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR65 "Shin, W. S., Shin, C. S., & Yeoun, P. S. (2012). The influence of forest therapy camp on depression in alcoholics. Environmental Health and Preventive Medicine, 17(1), 73–76. 
              https://doi.org/10.1007/s12199-011-0215-0
              
            ."); Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."); Vujcic et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            .")): Vujcic et al. ([2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            .")) did not present demographic details; however, they noted that the gender and diagnosis distribution were equal. Unsurprisingly, given the type of intervention, no study maintained allocation concealment and blinding of assessors, administration and participants; hence, the blinding procedure was poor (Table [6](/article/10.1007/s11469-020-00363-4#Tab6)).

Table 6 Assessment of quality of RCT studies based on mental health outcomes

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Discussion

This systematic review and meta-analysis examined the quality and extent of evidence reported in studies investigating the effects of shinrin-yoku on mental health. Twenty studies (eight non-randomised, and twelve randomised controlled trials), involving 2257 participants, satisfied all of the eligibility criteria for in-depth review and assessment. Shinrin-yoku was deemed to have a greater effect on anxiety, than depression and anger, and the effects on anxiety could be predicted by many of the moderators examined, including the gender and Japanese or Asian participants (greater proportions of females, Japanese or Asian participants were associated with larger effects). Potential publication bias was identified in all analyses apart from RCTs on anxiety. While some studies demonstrated rigorous design and reporting, our conclusions are tempered by a number of weaknesses concerning study design and outcomes. Accordingly, in this discussion, we elucidate a number of areas of improvement.

Shinrin-yoku was reported effective for depression, anxiety, stress and anger in both clinical and non-clinical samples, especially for anxiety. The results reported in the selected studies were in line with relevant theories: spending time in nature increased restoration (Bielinis et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR4 "Bielinis, E., Bielinis, L., Krupińska-Szeluga, S., Łukowski, A., Takayama, N., Bielinis, E., et al. (2019). The effects of a short forest recreation program on physiological and psychological relaxation in young Polish adults. Forests, 10(1), 34. https://doi.org/10.3390/f10010034

            .")) aligning with Attention Restoration Theory (Kaplan and Kaplan [1989](/article/10.1007/s11469-020-00363-4#ref-CR33 "Kaplan, R., & Kaplan, S. (1989). The experience of nature :a psychological perspective. Cambridge University Press.")). Stress was reduced through shinrin-yoku (Vujcic et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            ."); Morita et al. [2007](/article/10.1007/s11469-020-00363-4#ref-CR50 "Morita, E., Fukuda, S., Nagano, J., Hamajima, N., Yamamoto, H., Iwai, Y., Nakashima, T., Ohira, H., & Shirakawa, T. (2007). Psychological effects of forest environments on healthy adults: Shinrin-yoku (forest-air bathing, walking) as a possible method of stress reduction. Public Health, 121(1), 54–63. 
              https://doi.org/10.1016/j.puhe.2006.05.024
              
            .")), supporting Stress Reduction Theory (Ulrich et al. [1991](/article/10.1007/s11469-020-00363-4#ref-CR71 "Ulrich, R. S., Simons, R. F., Losito, B. D., Fiorito, E., Miles, M. A., & Zelson, M. (1991). Stress recovery during exposure to natural and urban environments. Journal of Environmental Psychology, 11(3), 201–230. 
              https://doi.org/10.1016/S0272-4944(05)80184-7
              
            .")). The role of nature in affect regulation is often overlooked (Korpela et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR36 "Korpela, K., Nummi, T., Lipiäinen, L., De Bloom, J., Sianoja, M., Pasanen, T., & Kinnunen, U. (2018). Nature exposure predicts well-being trajectory groups among employees across two years. Journal of Environmental Psychology, 52, 81–91. 
              https://doi.org/10.1016/J.JENVP.2017.06.002
              
            .")), and although not explicitly explored, findings accord with the three emotion regulatory systems model (threat, drive, and soothing; Richardson et al. [2016](/article/10.1007/s11469-020-00363-4#ref-CR59 "Richardson, M., McEwan, K., Maratos, F., & Sheffield, D. (2016). Joy and calm: how an evolutionary functional model of affect regulation informs positive emotions in nature. Evolutionary Psychological Science, 2(4), 308–320. 
              https://doi.org/10.1007/s40806-016-0065-5
              
            .")); being in nature may activate our soothing system, endorsing compassion, safety and connection, protecting our mental health. Psychological constructs relevant to the soothing system such as self-compassion and psychological safety need to be examined in shinrin-yoku research. Further, although likely to activate the pathways to nature connection (Lumber et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR42 "Lumber, R., Richardson, M., & Sheffield, D. (2017). Beyond knowing nature: contact, emotion, compassion, meaning, and beauty are pathways to nature connection. PLoS One, 12(5), e0177186. 
              https://doi.org/10.1371/journal.pone.0177186
              
            .")), none of the studies explored the psychological construct of nature connectedness—oneness with nature (Nisbet et al. [2009](/article/10.1007/s11469-020-00363-4#ref-CR51 "Nisbet, E. K., Zelenski, J. M., & Murphy, S. A. (2009). The nature relatedness scale. Environment and Behavior, 41(5), 715–740. 
              https://doi.org/10.1177/0013916508318748
              
            ."))—which is positively associated with psychological well-being (Pritchard et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR58 "Pritchard, A., Richardson, M., Sheffield, D., & McEwan, K. (2019). The relationship between nature connectedness and Eudaimonic well-being: a meta-analysis. Journal of Happiness Studies, 21, 1–23. 
              https://doi.org/10.1007/s10902-019-00118-6
              
            .")). Future shinrin-yoku research should also explore nature connectedness.

Although all included studies demonstrated promising results, the risk of bias was deemed medium to high, and potential publication bias was identified in almost all analyses. This may explain why benefits were greater for Japanese and Asian participants: people in a culture that accords with nature’s healing effects may receive greater benefits of shinrin-yoku (e.g. Shintoism, perceptions of nature differ cross-culturally; Gierlach et al. [2010](/article/10.1007/s11469-020-00363-4#ref-CR21 "Gierlach, E., Belsher, B. E., & Beutler, L. E. (2010). Cross-cultural differences in risk perceptions of disasters. Risk Analysis, 30(10), 1539–1549. https://doi.org/10.1111/j.1539-6924.2010.01451.x

            .")). Furthermore, none of the RCTs compared shinrin-yoku with other major therapeutic approaches such as CBT (while there was a study that combined CBT and nature; Kim et al. [2009](/article/10.1007/s11469-020-00363-4#ref-CR34 "Kim, W., Lim, S. K., Chung, E. J., & Woo, J. M. (2009). The effect of cognitive behavior therapy-based psychotherapy applied in a forest environment on physiological changes and remission of major depressive disorder. Psychiatry Investigation, 6(4), 245–254. 
              https://doi.org/10.4306/pi.2009.6.4.245
              
            .")): shinrin-yoku was only compared with spending time in urban settings (Vujcic et al. [2017](/article/10.1007/s11469-020-00363-4#ref-CR73 "Vujcic, M., Tomicevic-Dubljevic, J., Grbic, M., Lecic-Tosevski, D., Vukovic, O., & Toskovic, O. (2017). Nature based solution for improving mental health and well-being in urban areas. Environmental Research, 158, 385–392. 
              https://doi.org/10.1016/j.envres.2017.06.030
              
            .") compared with art therapy, not a major approach). Given that being in an urban setting has negative health effects (Lederbogen et al. [2011](/article/10.1007/s11469-020-00363-4#ref-CR37 "Lederbogen, F., Kirsch, P., Haddad, L., Streit, F., Tostet, H., et al. (2011). City living and urban upbringing affect neural social stress processing in humans. Nature, 474, 498–501. 
              https://doi.org/10.1038/nature10190
              
            ."); Marques and Lima [2011](/article/10.1007/s11469-020-00363-4#ref-CR43 "Marques, S., & Lima, M. L. (2011). Living in grey areas: industrial activity and psychological health. Journal of Environmental Psychology, 31(4), 314–322. 
              https://doi.org/10.1016/j.jenvp.2010.12.002
              
            .")), shinrin-yoku should be compared with other major therapeutic approaches. Indeed, in our RCT meta-analyses, all control/urban groups, apart from depression score in Chun, Chang and Lee’s study ([2017](/article/10.1007/s11469-020-00363-4#ref-CR11 "Chun, M. H., Chang, M. C., & Lee, S.-J. (2017). The effects of forest therapy on depression and anxiety in patients with chronic stroke. International Journal of Neuroscience, 127(3), 199–203. 
              https://doi.org/10.3109/00207454.2016.1170015
              
            .")), reported increases in mean scores. These points suggest the need for shinrin-yoku research in Oceania, Africa and North and South America, and the need to compare shinrin-yoku with other major approaches. Moreover, shinrin-yoku’s effects were particularly salient for anxiety, which is the most common mental health problem in the world (Ritchie and Roser [2018](/article/10.1007/s11469-020-00363-4#ref-CR60 "Ritchie, H. & Roser, M. (2018). Mental health. Retrieved from 
              https://ourworldindata.org/mental-health
              
            .")), again suggesting more shinrin-yoku research is needed.

Other limitations in shinrin-yoku research included a lack of follow-up assessments and consideration for sample representativeness. A lack of follow-up assessments can compromise the validity of clinical research because whether the effects of shinrin-yoku can last or not remains uncertain (Dettori [2011](/article/10.1007/s11469-020-00363-4#ref-CR14 "Dettori, J. (2011). Loss to follow-up. Evidence-Based Spine-Care Journal 2(01), 7–10. https://doi.org/10.1055/s-0030-1267080

            .")). The representativeness of the sample was not addressed; therefore, whether the study recruited people who were interested in, and positively interpreted/reported the effects of shinrin-yoku or not, was not clarified. In the RCTs, randomisation and blinding were not addressed. This may be again related to participants’ expectations (Antonelli et al. [2019](/article/10.1007/s11469-020-00363-4#ref-CR1 "Antonelli, M., Barbieri, G., & Donelli, D. (2019). Effects of forest bathing (shinrin-yoku) on levels of cortisol as a stress biomarker: a systematic review and meta-analysis. International Journal of Biometeorology, 63, 1117–1134. 
              https://doi.org/10.1007/s00484-019-01717-x
              
            .")): revealing the allocation of the group, participants who were interested in shinrin-yoku might have become more susceptible to placebo effects. In addition, many RCTs used a crossover design with no interval (the groups were swapped on the next day), which may violate the accuracy of the results: the impacts of the first intervention need to be washed out before swapping the groups (Enck and Zipfel [2019](/article/10.1007/s11469-020-00363-4#ref-CR15 "Enck, P., & Zipfel, S. (2019). Placebo effects in psychotherapy: a framework. Frontiers in Psychiatry, 10. 
              https://doi.org/10.3389/fpsyt.2019.00456
              
            .")). Furthermore, failing to blind the researchers can lead to placebo effects in participants; this may be particularly important when many of the reviewed studies included a prominent shinrin-yoku figure (e.g. Miyazaki, Lee). Finally, the included RCTs did not conduct intention-to-treat analysis and did not clarify whether other outcomes were measured or not. Similar to a previous systematic review (Kamioka et al. [2012](/article/10.1007/s11469-020-00363-4#ref-CR32 "Kamioka, H., Tsutani, K., Mutoh, Y., Honda, T., Shiozawa, N., Park, S., et al. (2012). A systematic review of randomized controlled trials on curative and health enhancement effects of forest therapy. Psychology Research and Behavior Management, 5, 85. 
              https://doi.org/10.2147/PRBM.S32402
              
            .")), lack of these research items needs to be addressed in the future studies.

Lastly, though we defined that nature-based practice must include integration with nature engaging with five sensory experience to be recognised as shinrin-yoku (Table 1), shinrin-yoku practice included diverse forms: most commonly walking, meditation and recreational activities such as handcrafts. While this indicates high applicability of shinrin-yoku, it could also leave shinrin-yoku practice rather unguided. This may resemble mindfulness, which can be practiced in many ways (Williams and Penman 2011), but its flexibility may make practitioners feel that they were just sitting or sleeping (Bojic and Becerra [2017](/article/10.1007/s11469-020-00363-4#ref-CR5 "Bojic, S., & Becerra, R. (2017). Mindfulness-based treatment for bipolar disorder: a systematic review of the literature. Europe’s Journal of Psychology, 13(3), 573–598. https://doi.org/10.5964/ejop.v13i3.1138

            .")) and lead to biased reporting (Schumer et al. [2018](/article/10.1007/s11469-020-00363-4#ref-CR63 "Schumer, M. C., Lindsay, E. K., & Creswell, J. D. (2018). Brief mindfulness training for negative affectivity: a systematic review and meta-analysis. Journal of Consulting and Clinical Psychology, 86(7), 569–583. 
              https://doi.org/10.1037/ccp0000324
              
            .")). As with other alternative approaches, shinrin-yoku can benefit from more guidance in practice, to be more accepted as a reliable clinical approach. Accredited training packages are emerging (e.g. the European Forest Therapy Institute [2019](/article/10.1007/s11469-020-00363-4#ref-CR16 "European Forest Therapy Institute. (2019). Become a guide. Retrieved from 
              https://www.europeanforesttherapyinstitute.com/become-a-guide/
              
            ")).

While this article offers useful insights, limitations need to be noted. Firstly, unpublished studies, qualitative studies (e.g. Sonntag-Öström et al. [2015](/article/10.1007/s11469-020-00363-4#ref-CR68 "Sonntag-Öström, E., Stenlund, T., Nordin, M., Lundell, Y., Ahlgren, C., Fjellman-Wiklund, A., Järvholm, L. S., & Dolling, A. (2015). ‘Nature’s effect on my mind’ – patient’s qualitative experiences of a forest–based rehabilitation programme. Urban Forestry and Urban Greening, 14, 607–614. https://doi.org/10.1016/j.ufug.2015.06.002

            .")) or studies not published in English language were excluded (Appendix [9](/article/10.1007/s11469-020-00363-4#MOESM1) for articles in Japanese satisfying the other criteria). Also, some studies examined many variables (Bielinis et al. [2018a](/article/10.1007/s11469-020-00363-4#ref-CR2 "Bielinis, E., Omelan, A., Boiko, S., & Bielinis, L. (2018a). The restorative effect of staying in a broad-leaved forest on healthy young adults in winter and spring. Baltic Forestry, 24(2), 218–227 Retrieved from 
              https://www.balticforestry.mi.lt/ojs/index.php/BF/article/view/204
              
            ."), [b](/article/10.1007/s11469-020-00363-4#ref-CR3 "Bielinis, E., Takayama, N., Boiko, S., Omelan, A., & Bielinis, L. (2018b). The effect of winter forest bathing on psychological relaxation of young Polish adults. Urban Forestry & Urban Greening, 29, 276–283. 
              https://doi.org/10.1016/J.UFUG.2017.12.006
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR4 "Bielinis, E., Bielinis, L., Krupińska-Szeluga, S., Łukowski, A., Takayama, N., Bielinis, E., et al. (2019). The effects of a short forest recreation program on physiological and psychological relaxation in young Polish adults. Forests, 10(1), 34. 
              https://doi.org/10.3390/f10010034
              
            ."); Takayama et al. [2014](/article/10.1007/s11469-020-00363-4#ref-CR69 "Takayama, N., Korpela, K., Lee, J., Morikawa, T., Tsunetsugu, Y., Park, B.-J., Li, Q., Tyrväinen, L., Miyazaki, Y., & Kagawa, T. (2014). Emotional, restorative and vitalizing effects of forest and urban environments at four sites in Japan. International Journal of Environmental Research and Public Health, 11(7), 7207–7230. 
              https://doi.org/10.3390/IJERPH110707207
              
            ."), [2019](/article/10.1007/s11469-020-00363-4#ref-CR70 "Takayama, N., Morikawa, T., Bielinis, E., Takayama, N., Morikawa, T., & Bielinis, E. (2019). Relation between psychological restorativeness and lifestyle, quality of life, resilience, and stress-coping in forest settings. International Journal of Environmental Research and Public Health, 16(8), 1456. 
              https://doi.org/10.3390/ijerph16081456
              
            .")); however, the multiple comparisons problem was not addressed. These could exaggerate the effects of shinrin-yoku. Lastly, the included studies were conducted only in two continents. Considering the serious nature of mental health globally, and different views on nature, research in other continents should be conducted.

Conclusion

The twenty studies included reported that shinrin-yoku is effective for mental health, particularly anxiety. Shinrin-yoku can be practiced in 15 min to 9 days, and reduce negative mental health symptoms. While promising results were reported, medium-high risk of bias and publication bias were identified. Some of the key constructs related to mental health (e.g. self-compassion, isolation, nature connectedness) have not been explored in shinrin-yoku research and mechanisms of benefits have not been determined. Additionally, the duration of benefits and how they compare with other established therapeutic approaches need to be examined for shinrin-yoku to be accepted as mainstream intervention.

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Yasuhiro Kotera, Miles Richardson & David Sheffield

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Yasuhiro Kotera
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Kotera, Y., Richardson, M. & Sheffield, D. Effects of Shinrin-Yoku (Forest Bathing) and Nature Therapy on Mental Health: a Systematic Review and Meta-analysis.Int J Ment Health Addiction 20, 337–361 (2022). https://doi.org/10.1007/s11469-020-00363-4

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Published: 28 July 2020
Issue Date: February 2022
DOI: https://doi.org/10.1007/s11469-020-00363-4

Effects of Shinrin-Yoku (Forest Bathing) and Nature Therapy on Mental Health: a Systematic Review and Meta-analysis (original) (raw)

Abstract

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Methods

Literature Search

Eligibility Criteria

Outcome Measures

Data Extraction and Synthesis

Quality Appraisal: Risk of Bias

Results

Search Results

Characteristics of Included Studies

Measures

Outcomes

Meta-analyses

Depression in RCT

Depression in Studies Reported Pre-Post Scores

Anxiety in RCT

Anxiety in Studies Reported Pre-Post Scores

Anger in RCT

Anger in Studies Reported Pre-Post Scores

Risk of Bias

Discussion

Conclusion

References

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Authors and Affiliations

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