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Effects of Shinrin-Yoku (Forest Bathing) and Nature Therapy on Mental Health: A Systematic Review and Meta-Analysis

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Objectives The Japanese healing practice of shinrin-yoku (forest-bathing) involves immersing oneself in nature using one's senses; it has been receiving increased attention internationally. Many studies have reported health benefits but most have focused on physical health. Given the concerns regarding mental health worldwide, the purpose of this systematic review and meta- analysis was to evaluate the mental health benefits of shinrin-yoku, using the preferred reporting items for systematic review and meta-analysis (PRISMA) guidelines. Methods Academic articles in English were retrieved on research databases including PubMed/MEDLINE, PsycINFO, Science Direct, and Google Scholar. Of 481 articles retrieved (search terms: ‘shinrin-yoku [n = 204]’ ‘forest bathing [n = 148]’, and ‘nature therapy [n = 129]’), twenty met the inclusion criteria (eight non-randomised trials and twelve randomised controlled trials). Results All studies were conducted in Asia and Europe and used a variety of different bathing approaches; for example, in studies participants rested and relaxed in the forest whereas in others they engaged in activities including yoga and cooking. While noting a need for more rigorous research and for more extensive follow-up assessments, the findings indicate that shinrin-yoku can be effective in reducing negative mental health symptoms (i.e., depression, anxiety, anger) in the short-term. In particular, the effects on anxiety were largest (but not significant due to considerable heterogeneity of effects), but the effects for anxiety, depression and anger were all large (g > .80); a number of factors were correlated with effect sizes. Examination of RCTs also revealed large effects indicating improvements in mental health parameters. Conclusions Overall, in 20 studies, forest bathing improved depression, anxiety and anger in the short- term but there were a number of moderators of the effects. More careful examination of shinrin-yoku practices and its effects are needed; longer follow-up with participants from a range of countries along with greater examination of potential mechanisms of action are needed for shinrin-yoku to be accepted into mainstream mental health approaches.
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Effects of Shinrin-Yoku (Forest Bathing) and Nature Therapy on Mental Health: A
Systematic Review and Meta-Analysis
Yasuhiro Kotera1*, Miles Richardson1, David Sheffield1
1University of Derby, United Kingdom
*Corresponding author. Y.Kotera@derby.ac.uk, +44(0)1332 592670.
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Abstract
Objectives
The Japanese healing practice of shinrin-yoku (forest-bathing) involves immersing oneself in
nature using one's senses; it has been receiving increased attention internationally. Many
studies have reported health benefits but most have focused on physical health. Given the
concerns regarding mental health worldwide, the purpose of this systematic review and meta-
analysis was to evaluate the mental health benefits of shinrin-yoku, using the preferred
reporting items for systematic review and meta-analysis (PRISMA) guidelines.
Methods
Academic articles in English were retrieved on research databases including
PubMed/MEDLINE, PsycINFO, Science Direct, and Google Scholar. Of 481 articles
retrieved (search terms: ‘shinrin-yoku [n = 204]’ ‘forest bathing [n = 148]’, and ‘nature
therapy [n = 129]’), twenty met the inclusion criteria (eight non-randomised trials and twelve
randomised controlled trials).
Results
All studies were conducted in Asia and Europe and used a variety of different bathing
approaches; for example, in studies participants rested and relaxed in the forest whereas in
others they engaged in activities including yoga and cooking. While noting a need for more
rigorous research and for more extensive follow-up assessments, the findings indicate that
shinrin-yoku can be effective in reducing negative mental health symptoms (i.e., depression,
anxiety, anger) in the short-term. In particular, the effects on anxiety were largest (but not
significant due to considerable heterogeneity of effects), but the effects for anxiety,
depression and anger were all large (g > .80); a number of factors were correlated with effect
sizes. Examination of RCTs also revealed large effects indicating improvements in mental
health parameters.
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Conclusions
Overall, in 20 studies, forest bathing improved depression, anxiety and anger in the short-
term but there were a number of moderators of the effects. More careful examination of
shinrin-yoku practices and its effects are needed; longer follow-up with participants from a
range of countries along with greater examination of potential mechanisms of action are
needed for shinrin-yoku to be accepted into mainstream mental health approaches.
Keywords: shinrin-yoku, forest bathing, nature therapy, mental health, meta-analysis
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Introduction
Shinrin-yoku (known as forest bathing) is a healing practice in Japan, where people
immerse themselves in nature, while mindfully paying attention to their senses. Often
involving a walk in a forest, it aims to integrate and harmonise humans with a forest
(Miyazaki, 2018). Contents of shinrin-yoku programmes include breathing exercises, yoga,
meditation, walking, aromatherapy, and other recreational activities (e.g., cooking), that are
often aimed at producing relaxation effects (Forest Therapy Society, 2005). The word
‘shinrin-yoku (“森林浴”)’ was coined by the Ministry of Agriculture, Forestry and Fisheries
of Japan in 1982, for ‘yoku (bathing)’ implies the holistic nature of our health. Shinrin-yoku
then began to be introduced into Japanese clinical fields (Hansen, Jones & Tocchini, 2017).
Literature reviews reported diverse health benefits of shinrin-yoku: on immune
system functioning by increasing natural killer cells (preventing cancer), and the
cardiovascular and respiratory systems (Williams, 2016). The health benefits of shinrin-yoku
are not limited to physical wellbeing; improvements have been described in mood disorders
(e.g., depression and anxiety) and stress, and mental relaxation (preventing disorders such as
attention deficit/hyperactivity disorder) (Park, Tsunetsugu, Lee, Kagawa & Miyazaki, 2012).
There are a number of theories that account for the health benefits of exposure to
nature which could explain the benefits of forest-bathing. Kaplan’s Attention Restoration
Theory claims that spending time in nature restores our concentration through practice of
effortless attention (performed by being immersed with nature) (Kaplan & Kaplan, 1989).
Stress Reduction Theory asserts that being in an unthreatening natural environment reduces
stress and improves relevant physiological functions such as heart-rate and blood pressure
(Ulrich et al., 1991). Indeed, Song, Ikei, and Miyazaki noted that natural stimuli help to
reduce stress and strengthen our immune system, while recognising individual differences.
More recent studies explored the mechanism of shinrin-yoku and found that the benefits of
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shinrin-yoku accord with Gilbert’s (2014) model of affect regulation (Richardson, McEwan,
Maratos & Sheffield, 2016). Although the benefits of nature for affect regulation are often
overlooked (Korpella et al., 2018) it is essential to health and well-being (Gross, 2013).
Forest bathing and connecting with nature can help us regulate our emotions, through
soothing and calming (i.e. the parasympathetic system), instead of fear, anxiety, and drive
(i.e., sympathetic system) (Richardson et al., 2016).
Historically, humans are more familiar with spending time in nature than urban
environments: over seven million years of human history, we have spent 99.99% of the time
in nature (Miyazaki, 2018); this may partially explain why we feel better in nature (Miyazaki,
2018). Women living in a green-rich area had a 12% lower rate of mortality (excluding
accidental deaths) than those living in a green-poor area (James, Hart, Banay & Laden, 2016).
Spending time in nature is related to lower rates of depression and high blood pressure, and
the frequent visits to nature was related to social cohesion (Shanahan et al., 2016).
Participants who viewed a towering tree for one minute scored high awe scores, associated
with more prosocial helping behaviours, than people who viewed a building with the same
height as the tree for one minute (Piff, Dietze, Feinberg, Stancato & Keltner, 2015). A three-
day shinrin-yoku programme increased the number and activity of natural killer cells
compared with three days of walking in a city (Li, 2010). Likewise, a 90-minute walk in
nature reduced the level of rumination (negative repetitive thoughts, linking with mental
health problems) and the activities of subgenual prefrontal cortex (part of brain that is related
with mental health problems) (Bratman, Hamilton, Hahn, Daily & Gross, 2015). Finally,
there is recent evidence that visits and time in nature may be acting as proxy measures for
nature connectedness as individual factors are often overlooked in the research (Martin et al.
2020).
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While our physical health has markedly improved in the 20th century (e.g., improved
longevity), our mental health has arguably worsened (Mental Health Foundation, 2016). In
1995 the World Health Organization (WHO) launched an initiative called ‘Nations for
Mental Health’ aiming to raise awareness of mental health and innovate mental health
treatment (WHO, 2002). Mental health action plan 2013-2020 was passed at the 66th WHO
Summit in 2012, promoting universal mental health care (WHO, 2013). Approximately 1.1
billion people were estimated to have a mental health or substance use disorder in the world
in 2016, equating with 15% of the population: most prominent disorders being anxiety (4%),
depression (3%), and alcohol use disorders (1%; Ritchie & Roser, 2018). The global costs of
mental illness were estimated about £2.5 trillion in 2010, which was projected to increase to
six trillion pounds by 2030. About two-thirds of those costs are not directly associated with
mental health issues, for example, reduced productivity and income, or due to disability or
death (Marquez & Saxena, 2016). Among developed countries, the costs related to mental
disorders are 2-4% of gross domestic product (GDP) (Hewlett, 2014), and they were
estimated to be substantially higher in developing countries (Patel, 2007). Unsurprisingly,
many countries have enacted government-led initiatives. For example, in the UK, mental
health has been high on the national agenda and the budget for mental health care has been
increasing (Department of Health, 2011). In Japan, poor mental health has also been a major
national issue (e.g., the high suicide rates), and new policies for supporting people with
mental health problems was established in 2004, improving citizens’ mental health awareness
and mental health care (Ministry of Health, Labour and Welfare [MHLW], 2004). This led to
a revision of the law on mental health care, further augmenting mental health care in the
country in 2013 (MHLW, 2014).
These reports suggest that mental health is a worldwide concern, and affordable,
accessible, and effective mental health solutions are needed. Treatment and care using nature
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may be one solution that can satisfy those needs (Hunter, Gillespie, & Chen, 2019). Some of
the benefits have been reviewed recently (Richardson et al., 2016; Farrow & Washburn,
2019; Payne & Delphinus, 2019), but no review has focused on the range of possible mental
health benefits. Accordingly, the present review systematically evaluated empirical findings
noting the effects of shinrin-yoku on mental health. The most prevalent mental health
problems are depression, anxiety and stress, therefore these were our foci (Farmer & Dyer,
2016). In addition, we also examined whether shinrin-yoku was associated with changes in
anger, as it is associated with depression, anxiety and stress (Walsh, Wolk, Haimes, Jensen-
Doss & Beidas, 2018).
Methods
PROSPERO register has been obtained (CRD42019157829). The present article
followed the preferred reporting items for systematic review and meta-analysis (PRISMA;
Moher, Liberati, Tetzlaff, Altman, & The PRISMA Group, 2009) guidelines, to
systematically review the literature and appraise the quality of evidence for the mental health
effects of shinrin-yoku. Additionally, in order to help maintain the validity of this systematic
review, Klassen, Jadad and Moher’s (1998) framework was employed; this focuses 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, Cherry & Dickson, 2013) was used to construct a researchable question by
dissecting the question into those four components to help organise relevant information
(Sackett et al., 1997). The main research questions of this review were i) how effective is
shinrin-yoku in helping to improve mental health outcomes? and ii) what quantity and quality
of evidence is reported?
Literature Search
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The literature search was conducted clarifying (i) where (i.e., databases used), (ii)
when the literature was searched, (iii) who searched the literature, (iv) how (i.e., keywords
used in the databases), (v) what amount of articles retrieved at each combination of the
keywords, and the final number of included articles, and (vi) why some articles were
included, and not others (i.e., selection criteria), as recommended by Callahan (2010).
Literature on PubMed/MEDLINE, PsycINFO, Science Direct, and Google Scholar were
searched via EBSCO, 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 (we included ‘nature therapy’ as it often synonymous with
shinrin-yoku; Hansen et al., 2017).
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 or forest
bathing intervention, (iii) report an empirical intervention study, using pre- and post-
intervention measures of dependent variables, and (iv) use mental health measures for
depression, anxiety, stress, and anger. Exclusion criteria were articles that (i) were not
interventions (e.g., articles that only introduced shinrin-yoku, or its relevant theories), (ii)
were case studies or qualitative studies, and (iii) did not measure depression, anxiety, stress,
and anger (see Table 1 for full details of the eligibility criteria).
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).
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Table 1. Extended PICO for this review
Review questions
How effective is shinrin-yoku in helping to improve mental health
outcomes? What quantity and quality of evidence is reported?
Inclusion criteria
Exclusion criteria
Population
Any population
-
Intervention
Shinrin-yoku (forest bathing) and
nature therapy
Others (e.g., only examining one
or few sensory stimulus such as
smell of trees or view of forest
from a window; combined with
other therapies, except for
mindfulness and walking*).
Comparator
Any comparator including no
intervention
-
Outcomes
Mental health measures
(depression, anxiety, stress, and
anger) used at least at pre- and
post-intervention
Other measures used
Study design
Empirical intervention study
Single case studies, qualitative
studies, reviews, discussion
articles, articles introducing
theories/concepts/models/applic
ations
Other
Published in a peer-reviewed
academic journal in English
Conference proceedings, theses,
dissertations.
*Interventions combining shinrin-yoku and mindfulness and/or walking were included as
they are common in shinrin-yoku practice, immersing with nature while paying attention to
five senses (Miyazaki, 2018).
Data Extraction and Synthesis
First, the lead author comprehensively examined the search results, and articles were
shortlisted for possible inclusion if the title and abstract of the article indicated that the study
might satisfy the eligibility criteria of this review. Additional manual reference searches on
previous systematic reviews on shinrin-yoku (n = 15), forest bathing, and nature therapy
identified 22 additional articles that might fit with the inclusion criteria, thus shortlisted
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(Rojon et al., 2011; Table 2). To counter any potential bias, the other co-authors
independently reviewed the entire selection process. Second, full-papers of shortlisted articles
were independently reviewed by all the co-authors. Third, a discussion was held among the
co-authors to confirm whether the selected articles had met the eligibility criteria and revisit
the excluded studies to ensure the reasons for exclusion were accurate. 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 (Appendix 1). Reasons for exclusion of full-
texts reviewed articles were noted (Appendix 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).
Separate meta-analyses were conducted focusing on depression, anxiety and anger;
meta-analysis of stress data was not possible as k = 1. For each outcome, we compared
Pearson’s product-moment correlations (r) were used to determine effect size for the shinrin-
yoku intervention. Data were entered into Meta-Essentials (van Rhee, Suurmond & Hak,
2015), and random-effects models were used to calculate mean effect sizes, which were
interpreted as small for 0.10 to 0.29, moderate for 0.30 to 0.49, and high for 0.50 and above
(Cohen, 1992).
Variability was examined using Cochran’s Q and I2. Heterogeneity among effect sizes
was determined by a significant Q value (p < 0.10). The I2 statistic indicates the degree of
variability in effect sizes; low heterogeneity for 1 to 49, moderate heterogeneity for 50 to 74,
and high heterogeneity for 75 to 100. In the case of significant heterogeneity, subgroup and
moderator analyses were undertaken.
Figure 1. PRISMA flow diagram of the article selection process
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Quality Appraisal: Risk of Bias
The quality of the included non-randomised studies were assessed using the
Newcastle-Ottawa Scale (NOS). NOS is a credible tool to evaluate the risk of bias in non-
randomised interventions (Wells et al., 2000). Using a star system, two 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 (maximum of four stars), (ii)
comparability of groups (maximum of two stars), and (iii) ascertainment of either the
exposure or outcome of interest (maximum of three stars). Some adjustments were made to
NOS in this review because many of the included studies recruited samples who had no
mental health disorders (while NOS was originally developed for medical research attended
Records identified through
database searching (n =
481)
Excluded as
duplicates (n = 173)
Records after duplicates removed.
Screened for title and abstract. (n =
330)
through other sources (n =
Full text articles
reviewed (n = 60)
Studies included in
the review (n = 20)
Records excluded
(n = 270)
Identification
Screening
Eligibility
Included
Excluded (n = 40): Not shinrin-yoku
(26), Not English (4), Combined with
other approaches (3), Intervention
unclear (3), No mental health
measure (3), Not empirical (1)
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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 the intervention, e.g., stress), and (iii) in
respect of the first item in the outcome assessment section, a star was awarded if the outcome
was assessed using a validated psychometric scale (i.e., as opposed to medical records).
Randomised controlled trials included in this review, were appraised using the Quality
Assessment Table of Randomised Controlled Trials (Brown et al., 2013). To be consistent
with the NOS rating and our primary interest of whether each research item was reported
adequately or not, rating was done using a star (*) for ‘yes’, blank for ‘no’, and ‘NA’ for ‘not
applicable.
Results
Search Results
The article selection process was illustrated in Figure 1. Of the 503 articles (481 from
the databases and 22 from manual reference search), 173 articles were removed due to
duplication. The remaining 330 articles were screened for their title and abstract by authors
for possible inclusion. After screening, 60 articles were selected for full-text review, of which
40 were excluded and 20 were included (Table 2). Reasons for exclusion were: studies that
did not examine shinrin-yoku (n = 26); articles not written in English (n = 4); interventions
combined shinrin-yoku with other approaches (n = 3); unclear interventions (n = 3); studies
that did not use mental health measure (n = 3); and non-empirical article (n = 1) (Appendix 2
for the details of excluded articles).
Table 2. Study details of selected articles exploring mental health effects of shinrin-yoku
(n=20).
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Sample and setting Intervention Measures Findings
1.Furuyashik
i et al., 2019
Japanese workers with
depressive tendencies
(n=58, M=28, F=30,
Age=42.3 ± 9.3) and
without (n=97, M=41,
F=56, Age=45.0 ± 9.7).
Total 155, M=69, F=86,
Age=44.0 ± 9.6,
Pre-post, no
comparator group.
2-hour forest
bathing programme
including breathing
and yoga.
Profile of Mood States
(POMS), measured on
the same day
Significant decrease in
depression, anxiety, and
anger. Participants with
depressive tendencies had
greater effects on those
subscales. Effect sizes r
total sample comparing
pre-post scores = - .46
(Depression), -.49
(Anxiety), -.42 (Anger).
2. Bielinis et
al., 2019
21 young healthy Polish
adults (Age=23.86 ± 2.67
years; F=9, M=12)
Pre-post, no
comparator group.
Forest recreation
programme focused
on auditory, visual
and tactile senses,
taken place at 3 sites
in a forest.
POMS, measured over
2 days
Significant decrease in
depression, anxiety and
anger. Effect sizes r
comparing pre-post scores
= -.35 (Depression-
Dejection), -.41 (Tension-
Anxiety), -.35 (Anger-
Hostility).
3. Song et al.,
2019
60 young women in Japan
(Age 21.0±1.3 years)
Pre-post RCT.
Divided into 15-
minute walk in
forest and city.
Crossover next day.
POMS, and StateTrait
Anxiety Inventory
(STAI; only State
Anxiety items),
measured on the same
day
Forest group received
psychological relaxation
effects, compared to city.
Effect sizes r comparing
post scores of forest-city
(based on Figs 5 & 6) r =
-.09 (Depression), -.36
(Anxiety), -.24 (Anger),
-.59 (State Anxiety).
4. Takayama
et al., 2019
46 Japanese male
undergraduate and
postgraduate students
(Age M=21.12 years).
Pre-post RCT.
Divided into forest
and urban groups:
15-minute walk and
15-minute viewing
of forest and urban
sites. Crossover next
day.
POMS, measured over
2 days
Forest group related to
higher restorative effects.
Effect sizes r comparing
pre-post scores for forest
group = -.40 (Depression),
-.49 (Anxiety), -.32
(Anger).
5. Bielinis et
al., 2018
62 Polish university
students (Age
21.45±0.18 years; F=26,
M=36)
Pre-post RCT.
Divided into forest
and urban groups.
15-minute walk and
15-minute viewing.
POMS, measured on
the same day
Significant decrease in
forest group in depression,
anxiety, and anger. Effect
sizes r comparing pre-post
in forest group = -.79
(Depression), -.89
(Anxiety), -.76 (Anger).
6. Chen et
al., 2018
16 Taiwanese women
(Age 46.88±7.83 years)
Pre-post. 2-day
forest therapy
programme. 2.5-h
guided walk, night
walk, and do-it-
yourself handcrafts
POMS and STAI (only
State Anxiety items),
measured over 2 days
Significant decrease in
depression, anxiety, and
anger. Effect sizes r
comparing pre-post= -.20
(Depression), -.23
(Anxiety), -.54 (Anger),
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-.33(State Anxiety).
7. Lee et al.,
2018
79 middle-aged (40-70,
Age M≈55) Korean
women with (n=35) and
without (n=44) metabolic
syndrome (MetS)
Pre-post RCT,
divided into wild
forest (18 with and
18 without MetS)
and tended forest
(17 with and 26
without MetS)
groups. 2-h healing
programme
including walk, 5-
sense feeling, and
meditation.
POMS, measured on
the same day
Women with MetS reduced
Anxiety more than without
in wild forest.No
significant difference
between wild and tended
forests in changes for
depression, anxiety and
anger. Effect sizes r (based
on Figure 5) comparing
pre-post scores in all
groups= -.14 (Depression),
-.50 (Anxiety), -.35
(Anger).
8. Song et al.,
2018
585 Japanese male
students (Age 21.7±1.6
years).
Pre-post RCT. Half
of them walked for
15 minutes in nature
first then city, and
the other half did
reversely.
POMS and STAI (only
Trait Anxiety items),
measured during 2
days
Forest walk decreased
depression, anxiety and
anger. Effect sizes r
comparing post scores of
nature-city= -.11
(Depression), -.34
(Anxiety), -.16 (Anger).
9. Chun,
Chang &
Lee, 2017
59 patients with chronic
stroke (Age 60.8±9.1
years; M=40, F=19) in
Korea
Pre-post RCT. 30 in
forest, 29 in urban.
Forest therapy
consisting of
meditation, use of
five senses, and
walk for both
groups. Urban group
stayed in a hotel,
while forest group
stayed in
recreational site.
Beck Depression
Inventory (BDI),
Hamilton Depression
Rating Scale (HAM)
andSTAI
Depression and anxiety
reduced in forest group,
while anxiety increased in
urban group. Effect sizes r
comparing pre-post in
forest group = -.62
(Depression; BDI), -.66
(Depression; HAM), -.51
(Anxiety; unclear which
anxiety, state or trait, was
measured).
10. Guan et
al., 2017
69 Chinese university
students (M=26, F=43)
Pre-post RCT. 2-h
forest walk in birch
(M=9, F=11), maple
(M=8, F=15), and
oak (M=9, F=17).
Anti-anxiety
questionnaire
(validation information
not reported)
Maple forest reduced study
anxiety. Birch forest
reduced employment
pressure the most. Oak
forest reduced lesson
anxiety.
11. Vujcic et
al., 2017
30 Serbian psychiatric
patients (Age
45.35±10.16; F=21,
M=9).
Pre-post RCT.
Divided into nature
therapy (n=16;
F=11, M=5) and art
therapy (n=14;
F=10, M=4). 12 1-h
forest therapy
sessions over 4
Depression, Anxiety,
and Stress Scale 21
Stress reduced significantly
in forest while it did not in
control. Effect sizes r
comparing pre-post in
forest group = -.50
(Depression), -.54
(Anxiety), -.67 (Stress)
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months
(intervention). 12 1-
h therapy sessions at
the institute
(control).
12. Yu et al.,
2017
128 Taiwanese elders
(Age 60.0±7.44 years;
M=43, F=85; 59 with
chronic diseases)
Pre-post. 2-h forest
walk focusing on
five senses.
PMOS and STAI (only
state anxiety),
measured on same day
Depression, anxiety, and
anger reduced. Effect sizes
r comparing pre-post = -.29
(Depression), -.43
(Anxiety), -.40 (Anger),
-.33 (State Anxiety).
13. Han et
al., 2016
61 Korean public workers
with chronic pain (Age
M=39.72 years).
Pre-post
experimental non-
random. Divided
into forest therapy
(n=33; M=16, F=17;
Age 41.6 ± 6.5) and
control (n=28;
M=10, F=18; Age
37.5 ± 8.4). 2 2-h
forest therapy
sessions for
intervention group,
and passive control.
BDI and the EuroQol
Visual Analog Scale
Depression decreased in
intervention (p < .001), and
in the control group
(p< .05). Effect sizes r
comparing pre-post in
forest group = -.38
(Depression).
14. Ochiai et
al., 2015
17 Japanese women
(62.2±9.4 years)
Pre-post. 5-h forest
therapy session
including breathing
and walking
POMS (Anxiety,
Fatigue and vigor),
measured on same day
Anxiety reduced. Effect
sizes r comparing pre-post
(based on Fig 5) = -.95
(Anxiety)
15.
Takayama et
al., 2014
45 Japanese university
male students (Age M=21
years).
Pre-post RCT.
Divided into forest
and city groups. 15-
minute walk and 15-
minute viewing.
Crossover next day.
POMS, measured
during 2 days
Depression, anxiety and
anger decreased. Effect
sizes r (based on Figures
3&4) comparing pre-post
scores in forest group =
-.18 (Depression), -.21
(Anxiety), -.08 (Anger).
16.
Horiguchi et
al., 2013
48 Japanese adults in
total:
a: 23 young people (Age
M=22 years; M=11,
F=12), and
b: 25 elders (Age <=59;
M=10, F=15).
Pre-post. 3-h forest
walk.
POMS, measured on
same day
Depression, anxiety and
anger decreased in both
young and old. Young had
larger effects. Effect sizes r
comparing pre-post scores
in total= -.53 (Depression),
-.84 (Anxiety), -.46 (Anger)
17. Shin,
Shin &
Yeoun, 2012
92 alcoholics (M=84,
F=8) in Korea.
Pre-post RCT.
Divided into 9-day
forest therapy
(n=47; Age
44.66±3.90) and
passive control
BDI
Depression decreased in
intervention, while it did
not in control. Alcoholics
in their 40s received
greatest effects. Effect sizes
r comparing pre-post in
15
Sensitivity: Internal
groups (n=45; Age
45.87±3.85). Forest
therapy included
walk and
meditation.
forest group = -.68
(Depression).
18. Lee et al.,
2011
12 Japanese male adults
(Age 21.2±0.9 years).
Pre-post. Divided
into forest and urban
groups, who walked
and viewed.
Crossover.
POMS, measured
during 3-day
intervention
Anxiety decreased but not
depression and anxiety in
forest group. Effect sizes r
(based on Fig 5., corrected
figure was referred; Lee, et
al., 2019) = .00
(Depression), -.69
(Anxiety), .00 (Anger)
19. Park et
al., 2011
168 Japanese male
university students (age,
20.4±4.1 years)
Pre-post RCT.
Divided into forest
and urban groups:
15-minute walk and
15-minute viewing.
Crossover next day.
POMS, measured on
same day
Significant differences
between forests and urban
in depression, anxiety and
anger. Effect sizes r
comparing pre-post (based
on Fig 6) =-.83
(Depression), -.98
(Anxiety), -.45 (Anger).
20. Morita et
al., 2007
498 volunteers (244
males, 254 females; Age
56.2±10.6 years) in Japan.
Pre-post after 1-2
hours of walking in
forest
Multiple Mood Scale-
Short Form
(depression) and STAI
A-State
Forest was advantageous
for mental health. Effect
sizes r comparing pre-post
= -.24 (Depression), -.09
(State Anxiety).
Characteristics of Included Studies
Twenty included studies were relatively recent, the oldest one being published in
2007 (Morita et al.), which is consistent with the recent subjection of shinrin-yoku to
empirical investigation. The majority of the studies were conducted in Asia (n=18; 86%): ten
in Japan (Furuyashiki et al., 2019; Horiuchi et al., 2013; Lee et al., 2011; Morita et al., 2007;
Ochiai et al., 2015; Park et al., 2011; Song et al., 2018; Song et al., 2019; Takayama et al.,
2014; Takayama et al., 2019), four in Korea (Chun, Chang & Lee, 2017; Lee et al., 2018;
Han et al., 2016; Shin, Shin & Yeoun, 2012), two in Taiwan (Chen et al., 2018; Yu et al.,
2017), and one in China (Guan et al., 2017). Three studies were conducted in Europe: two in
Poland (Bielinis et al., 2018; Bielinis et al., 2019), and one in Serbia (Vujcic et al., 2017)
16
Sensitivity: Internal
(Table 2). No studies were identified in Africa, Oceania, and South and North America. Ten
studies were non-randomised trials (Table 5), and twelve studies were randomised controlled
trials (RCT; Table 6). All the non-randomised studies employed a pre-post design; two
studies of ten had a comparator condition (Table 5). Twelve RCTs included six studies using
group-switching (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; Park et al., 2011 Song et al., 2018; Song et al., 2019;
Takayama et al., 2014; Takayama et al., 2019), and one study where groups were categorised
by different types of trees (birch, maple, and oak; Guan et al., 2017). Interventions included
walking and meditation, and time duration ranged from 15 minutes to nine days. While all
studies involved paying attention to the five senses (see Table 1 for the eligibility criteria), 18
(90%) studies involved walking (Bielinis et al., 2018; Chen et al., 2018; Chun, Chang & Lee,
2017; Furuyashiki et al., 2019; Guan et al., 2017; Han et al., 2016; Horiguchi et al., 2013; Lee
et al., 2011; Lee et al., 2018; Morita et al., 2007; Ochiai et al., 2015; Park et al., 2011; Shin,
Shin & Yeoun, 2012; Song et al., 2019; Takayama et al., 2014; Takayama et al., 2019; Vujcic
et al., 2017; Yu et al., 2017), four (20%) involved meditative activities (Furuyashiki et al.,
2019; Lee et al., 2018; Shin, Shin & Yeoun, 2012; Ochiai et al., 2015), and three (15%)
involved recreational activities (Bielinis et al., 2019; Chen et al., 2018; Han et al., 2016).
A total of 2257 participants (M=1478, F=779; age range 18 to 79 years old) were
involved in these included studies, indicating shinrin-yoku’s wide applicability. Six studies
involved clinical samples with: metabolic syndrome (Lee et al., 2018), chronic stroke (Chun,
Chang & Lee, 2017), psychiatric disorders (Vujcic et al., 2017), chronic diseases (Yu et al.,
2017), chronic pain (Han et al., 2016), and alcoholism (Shin, Shin & Yeoun, 2012).
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
17
Sensitivity: Internal
paper were the State-Trait Anxiety Inventory (STAI; n = 6), and the Beck Depression
Inventory (BDI; n = 3).
Table 3. Included studies organised by mental health measures.
POMS (14) STAI (6)
BDI (3) AAQ (1) DASS (1) EQVAS (1) HDR (1) MMS (1)
Measured outcome(s)
Depression
Anxiety
Anger
Anxiety
Depression
Anxiety
Depression
Anxiety,
Stress
Depression/A
nxiety
Depression
Depression
Furuyashiki et al.,
2019 *
Bielinis et al., 2019
*
Song et al., 2019
*
Takayama et al., 2019
*
Bielinis et al., 2018
*
Chen et al., 2018
*
Lee et al., 2018
*
Song et al., 2018
*
Chun, Chang & Lee,
2017 * *
Guan et al., 2017
Vujcic et al., 2017
*
Yu et al., 2017
*
Han et al., 2016
*
Ochiai et al., 2015
*
Takayama et al., 2014
*
18
Sensitivity: Internal
Horiguchi et al., 2013
*
Shin, Shin & Yeoun,
2012 *
Lee et al., 2011
*
Park et al., 2011
*
Morita et al., 2007
* *
POMS=Profile of Mood States; STAI=State-Trait Anxiety Inventory; BDI=Beck Depression Inventory; HDR=Hamilton
Depression Rating Scale; AAQ=Anti-Anxiety Questionnaire; EQVAS=EuroQol Visual Analog Scale; MMS=Multiple
Mood Scale. * = included in meta-analyses
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 (Table 3).
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 in the ‘depression/anxiety’ subscale. Chun, Chang and Lee’s study (2017) used HDR
and BDI, so HDR was removed because HDR emphasises physical symptoms of depression
(Hamilton, 1960). 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 4). The anxiety score in STAI from Chun et al.’s study (2017) was not
included as whether the 20 items used were related to state anxiety or trait anxiety was not
reported. Likewise, the anxiety score in AAQ (Guan et al., 2017) 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, Song et al., 2018, 2019, Yu et al.,
19
Sensitivity: Internal
2017), POMS was considered as it is more commonly used. Likewise, Chun et al. (2017)
employed two depression scales (BDI and HAM); only BDI was included in the meta-
analysis as BDI was used for both clinical and nonclinical samples, while HAM was only
used for a clinical sample. The random-effects model was used as the included studies
included diverse populations, thus heterogeneity was assumed.
First, data from six RCT studies were analysed (Bielinis et al., 2018; Chun, Chang &
Lee, 2017; Lee et al., 2011; Shin, Shin & Yeoun, 2012; Takayama et al., 2014 and 2019).
Song et al.’s RCT studies (2018 and 2019) were excluded as these studies only reported post-
intervention scores. Lee et al. (2018) was excluded as their RCT compared tended forest and
wild forest. Guan et al. (2017)’s RCT was excluded as this study employed a non-validated
scale. Lastly, Vujcic et al. (2017) 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., 2018, 2019, Chen et al., 2018, Chun, Chang & Lee, 2017,
Furuyashiki et al., 2019, Horiguchi et al., 2013, Lee et al., 2011, 2018, Morita et al., 2007,
Ochiai et al., 2015, Park et al., 2011, Shin, Shin & Yeoun, 2012, Takayama et al., 2014, 2019,
Yu et al., 2017, Vujcic et al., 2017).
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.
20
Sensitivity: Internal
Table 4. Effect sizes (g) and p-values for moderators and subgroups in each variable
(depression, anxiety and anger).
Moderators Subgroups
Effect Size 95%
CI
Crossov
er Asian Japanese
Intervention
Length Gender Age
Depr
essio
n
RCT
(n=6)
g = -2.54 (-3.56,
-1.52)p 0.27
0.26
0.27
0.002*
0.97
0.04
Pre-
Post
(n=16)
g = -1.04 (-
1.47,
-0.60)p NA
0.12
0.20
0.24*
 0.03*
0.22
Anxi
ety
RCT
(n=5)
g = -8.81 (-
21.91, 3.57) 0.003
0.007
0.003
0.19
< 0.001
0.20
Pre-
Post
(n=16)
g = -1.83 (-
3.07,
-0.58)p NA
< 0.001
< 0.001
0.02*
0.12
0.01*
Ange
r
RCT
(n=4)
g = -1.63 (-3.25,
-0.01)p 0.13
0.13
0.13
NA**
0.47
0.06
Pre-
Post
(n=12)
g = -0.81 (-
1.17,
-0.45)
p
NA
0.46
0.15
< 0.001
< 0.001
< 0.001
*After removing an extreme value, the significance of p-value changed. Bold scores are significant values p <
0.05 **All studies had the same length of intervention.
p
Publication bias identified.
Depression in RCT.
The total sample size for RCTs measuring depression was 417 (range 12 to 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 (I2 = 87.13%); in all studies the effect was negative - depression decreased more in
the forest setting compared to the urban setting.
Figure 2. Effect size for depression in RCT.
21
Sensitivity: Internal
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 of the effect size for depression. In
subgroup analyses, the length of the intervention (iv) was a significant predictor of effect size
for depression (slope = 0.04, SE = 0.01, Z = 3.06, p = 0.002). However, the length of the
intervention was not significant (p = 0.11) when one extreme value (Shin, Shin & Yeoun,
2012) was removed for depression. The female-male ratio (v) was not a significant (slope =
0.09, SE = 2.43, Z = 0.04, p = 0.97), whereas average age (vi) was a significant predictor for
depression (slope = 0.04, SE = 0.02, Z = 2.04, p = 0.04).
Publication bias was first examined by a funnel plot of effect size plotted against
standard error; the funnel plot for the effect sizes for depression appeared asymmetrical,
implying a possible bias (Appendix 3). The asymmetricity was further examined using the
Begg test (Begg & Mazumdar 1994) and the Egger test (Egger et al., 1997). Begg and
Mazumdar adjusted rank correlation (z = -1.32, pz = 0.19) and Egger’s coefficient (intercept
= -6.83, t = -2.48, p = 0.07) were both non-significant, suggesting possible evidence of
publication bias for this dataset.
Depression in Studies Reported Pre-Post Scores.
The total sample size for pre-post scores measuring depression was 1449 (range 12 to
498) from 16 studies. Figure 3 shows the forest plot for the meta-analysis with depression in
22
Sensitivity: Internal
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 (I2 = 95.48%); in all studies, apart from
Lee et al. (2011), the effect was negative - depression decreased from pre-shinrin-yoku to
post-shinrin-yoku.
Figure 3. Effect size for depression in studies reported pre-post scores.
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 of the
effect size for depression. In subgroup analyses, the length of intervention (iv) was not
significant (slope = -0.02, SE = 0.02, Z = -1.17, p = 0.24), however after removing an
extreme value (Park et al., 2011), it became significant and it was negative i.e., the longer the
intervention, the smaller effects (slope = -0.03, SE = 0.01, Z = -2.55, p = .01). The female-
male ratio (v) was significant (slope = 1.04, SE = 0.46, Z = 2.23, p = 0.03), however after
removing an extreme value (Park et al., 2011), it became non-significant (slope = 0.58, SE =
0.40, Z = 1.45, p = 0.15). Lastly, average age (vi) was non-significant (slope = 0.01, SE =
0.01, Z = 1.23, p = 0.22).
23
Sensitivity: Internal
Possible evidence of publication bias was identified by an asymmetrical funnel plot of
effect size against standard error (Appendix 4). Begg and Mazumdar adjusted rank
correlation (z = -0.99, pz = 0.32) and Egger’s coefficient (intercept = -7.27, t = -0.67, p =
0.51) were both non-significant, suggesting possible evidence of publication bias for this
dataset.
Anxiety in RCT.
The total sample size for studies measuring anxiety was 327 (range 12 to 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 (I2 = 96.80%).
Figure 4. Effect size for anxiety in RCT.
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 of effect size for anxiety (slope = 0.35, SE = 0.27, Z = 1.32, 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, SE = 8.73, Z = -3.53, p < 0.001),
whereas average age (vi) was not a significant predictor of effect size for anxiety (slope =
0.13, SE = 0.10, Z = 1.27, p = 0.20).
24
Sensitivity: Internal
Possible evidence of publication bias was not identified by a symmetrical funnel plot
of effect size against standard error (Appendix 5). Begg and Mazumdar adjusted rank
correlation (z = -2.45, pz = 0.01) and Egger’s coefficient (intercept = -8.75, t = -5.85, p =
0.01) were both significant, suggesting publication bias for this data.
Anxiety in Studies Reported Pre-Post Scores.
The total sample size for pre-post scores measuring anxiety was 1371 (range 12 to
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 (I2 = 97.55%); in all studies anxiety decreased
from pre-shinrin-yoku to post-shinrin-yoku.
Figure 5. Effect size for anxiety in studies reported pre-post scores.
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 of the
effect size for depression. In subgroup analyses, iv) the length of the intervention was
25
Sensitivity: Internal
significant i.e., the longer the intervention, the more effects observed (slope = 0.13, SE =
0.05, Z = 2.34, p = 0.02), however after removing one extreme value (Park et al., 2011), it
was not significant (slope = 0.05, SE = 0.04, Z = 1.17, p = 0.24). The female-male ratio (v)
was not significant (slope = 1.26, SE = 0.82, Z = 1.54, p = 0.12). Lastly, the average age (vi)
was a significant moderator (slope = 0.04, SE = 0.02, Z = 2.49, p = 0.01), however after
removing an extreme value (Park et al., 2011), it became non-significant (slope = 0.01, SE =
0.01, Z = 0.65, p = 0.52).
Possible evidence of publication bias was identified by an asymmetrical funnel plot of
effect size against standard error (Appendix 6). Begg and Mazumdar adjusted rank
correlation was not significant (z = -1.62, pz = 0.11) whilst Egger’s coefficient (intercept = -
18.28, t = -3.13, p = 0.01) was significant, suggesting the possibility of publication bias.
Anger in RCT.
The total sample size for studies measuring anger was 268 (range 12 to 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 (I2 = 88.25%).
Figure 6. Effect size for anger in RCT.
For anger, all the three moderators - crossover (p = 0.13), Asian (p = 0.13), and
Japanese (p = 0.13) - were not significant predictors of effect size. Likewise, the intervention
26
Sensitivity: Internal
length (data unidentified), female-male ratio (slope = -1.61, SE = 2.24, Z = -0.72 , p = 0.47),
and average age (slope -3.40, SE = 1.77, Z = -1.92, p = 0.06) were not significant.
Possible evidence of publication bias was identified by an asymmetrical funnel plot of
effect size against standard error (Appendix 7). This was confirmed by a significant Begg and
Mazumdar adjusted rank correlation (z = -2.04, pz = 0.04), however Egger’s coefficient
(intercept = -6.88, t = -2.57, p = 0.12) was not significant.
Anger in Studies Reported Pre-Post Scores.
The total sample size for pre-post scores measuring anger was 1365 (range 12 to 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 (I2 = 92.83%); in all studies anger decreased from pre-
shinrin-yoku to post-shinrin-yoku.
Figure 7. Effect size for anger in studies reported pre-post scores.
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 of the
effect size for anger. In subgroup analyses, all subgroups were significant: the length of the
27
Sensitivity: Internal
intervention (iv) slope = -0.13, SE = 0.02, Z = -5.47, p < 0.001; the female-male ratio (v)
slope = -0.65, SE = 0.09, Z = -6.85, p < 0.001; and the average age (vi) slope = -0.01, SE =
0.00, Z = -5.52, p < 0.001.
Possible evidence of publication bias was identified: while the funnel plot of effect
size against standard error appeared symmetrical (Appendix 8), Begg and Mazumdar adjusted
rank correlation (z = 0.27, pz = 0.78) and Egger’s coefficient (intercept = -4.63, t = -0.82, p =
0.43) were both non-significant.
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; Chen et al., 2018; Furuyashiki et al., 2019; Han et al., 2016;
Horiguchi et al., 2013; Morita et al., 2007; Ochiai et al., 2015; Yu et al., 2017). All of these
studies assessed the mental health outcomes before and after shinrin-yoku, and ascertainment
of the symptom exposure was confirmed (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).
Table 5. Assessment of quality of studies based on mental health outcome (non-randomised
trials)
Assessment of risk of bias for pre-post studies (The Newcastle-Ottawa Scale)
Selection
Comparability
Outcome
Numbe
r of
stars (0-
9)
Author,
year
Represent
ativeness
of
exposed
cohort
Selection of
non-
exposed
cohort
Ascertainm
ent of
exposure
Demonstra
te outcome
assessed
before
interventio
n
Comparability of
cohorts on basis of
design(*) or
analysis(*)
Assessmen
t of
outcome
Follow-up
length
Adequacy
of follow-
up
Furuyashik
i et al.,
2019
* * * 3
28
Sensitivity: Internal
Bielinis et
al., 2019 * *
* (site details
described) * 4
Chen et al.,
2018 * *
* (site details
described) * 4
Yu et al.,
2017 * *
* (site details
described) * 4
Han et al.,
2016 * * *
* (site details
described) * 5
Ochiai et
al., 2015 * *
* (site details
described) * 4
Horiguchi
et al., 2013 * * * 3
Morita et
al., 2007 * *
* (4 assessment
points) * 4
In the randomised controlled trials, the risk of bias was deemed high to medium: all
the studies scored from two (Bielinis et al., 2018) to six (Shin, Shin & Yeoun, 2012). For the
studies that employed crossover (Park et al., 2011; Song et al., 2019; Takayama et al., 2019;
Takayama et al., 2014), 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). The baseline
comparability of different groups was reported in seven studies (Chun, Chang & Lee, 2017;
Guan et al., 2017; Lee et al., 2011, 2018; Shin, Shin & Yeoun, 2012; Takayama et al., 2019;
Takayama et al., 2014), and achieved in seven studies (Chun, Chang & Lee, 2017; Guan et
al., 2017; Lee et al., 2011, 2018; Shin, Shin & Yeoun, 2012; Takayama et al., 2014; Vujcic et
al., 2017): Vujcic et al. (2017) 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 the assessors,
administration, and participants, hence the blinding procedure was poor and not further
assessed (Table 6).
29
Sensitivity: Internal
Table 6. Assessment of quality of RCT studies based on mental health outcomes
Quality Assessment Table of Randomised Controlled Trials (Brown et al., 2013)
Randomisation
Baseline
comparability
Inclusi
on
criteria
specifi
ed
Co-
interventi
ons
identified
Blinding Withdrawals
Inten
tion
to
treat
Other
outco
mes Score
Author
, year
Truly
rando
m
Allocation
concealment
Numbe
r stated
Prese
nted
Achiev
ed
Asses
sors
Ad
mini
strat
ion
Partic
ipants
Proced
ure
assesse
d
>80% in
final
analysis
Rea
sons
state
d
Song et
al.,
2019
* NA NA NA * * 3/12
Takaya
ma et
al.,
2019
* * * NA NA NA 3/12
Bielini
s et al.,
2018
* * NA 2/14
Lee et
al.,
2018
* * * * NA * 5/14
Song et
al.,
2018
* * NA * 3/14
Chun,
Chang
& Lee,
2017
* * * * * NA 5/14
Guan
et al.,
2017
* * * * NA 4/14
Vujcic
et al.,
2017
* * * NA 3/14
Takaya
ma et
al.,
2014
* * * * NA NA NA 4/12
Shin,
Shin &
Yeoun,
2012
* * * * NA * * 6/13
30
Sensitivity: Internal
Lee et
al.,
2011 * * * * NA * 5/14
Park et
al.,
2011
* * NA NA NA 2/12
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,
which should strengthen future shinrin-yoku research.
Shinrin-yoku was reported effective for depression, anxiety, stress and anger in both
clinical and non-clinical samples, especially for anxiety as our meta-analysis revealed. The
results reported in the selected studies were in line with relevant theories: spending time in
nature increased restoration (Bielinis et al., 2019) aligning with Kaplan’s Attention
Restoration Theory (Kaplan & Kaplan, 1989). Stress was reduced through shinrin-yoku
(Vujcic et al., 2017; Morita et al., 2007), supporting Stress Reduction Theory (Ulrich et al.,
1991). The role of nature in affect regulation is often overlooked (Korpela et al., 2018), and
although not explicitly explored, findings accord with the three emotion regulatory system
31
Sensitivity: Internal
model (threat, drive, and soothing systems) described by Richardson et al. (2016); being in
nature may activate our soothing system, endorsing compassion, safety, and connection,
which are conducive to 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 at al.
2017), none of the studies explored the psychological construct of nature connectedness -
oneness with nature (Nisbet, Zelenski & Murphy, 2009), which is associated with higher
levels of psychological well-being (Pritchard et al., 2019). 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, as perceptions of nature differ cross-culturally; Gierlach, Belsher & Beutler,
2010). Furthermore, none of the RCTs compared shinrin-yoku with other major therapeutic
approaches such as cognitive behavioural therapy (CBT) (while there was a study that
combined CBT and nature; Kim, Lim, Chung & Woo, 2009): shinrin-yoku was only
compared with spending time in urban settings (Vujcic et al. (2017) compared with art
therapy, not a major approach). Given that being in an urban setting has negative health
effects (Lederbogen et al., 2011; Marques & Lima, 2011), shinrin-yoku should be compared
with other major therapeutic approaches. Indeed, in our RCT meta-analyses targeting
depression, anxiety and anger, all control groups, apart from depression score in Chun, Chang
and Lee’s study (2017), reported increases in mean scores, indicating the negative effects of
being in an urban environment. These points suggest the need for shinrin-yoku research in
32
Sensitivity: Internal
Oceania, Africa, and North and South America, and the need for shinrin-yoku research to
compare it 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, amounting
to 4% of the world population (Ritchie & Roser, 2018) again suggesting more shinrin-yoku
research is needed.
Other limitations in shinrin-yoku research included a lack of follow-up assessments
and consideration for representativeness of the cohort in the pre-post studies. 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). 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, Barbieri & Donelli, 2019):
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 & Zipfel, 2019). Furthermore, failing to blind the researchers can
lead to placebo effects in participants; this 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), 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 (Miyazaki, 2018) to be recognised as shinrin-
33
Sensitivity: Internal
yoku (Table 1), shinrin-yoku practice included diverse forms: most commonly walking,
meditation, and recreational activities such as handcrafts (and mixture of these). 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
& Penman, 2011), but its flexibility may make practitioners feel that they were just sitting or
sleeping (Bojic & Becerra, 2017) and lead to biased reporting (Schumer, Lindsay & Creswell,
2018). 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; for example, the European Forest Therapy Institute (EFTI, 2019) uses the
pathways to nature connectedness (Lumber et al. 2017), which include sensory experience.
While this systematic review offers useful insights, several limitations need to be
noted. Firstly, unpublished studies, qualitative studies (e.g., Sonntag-Öström et al., 2015) or
studies not published in English language were excluded. For example, there were 2,880 hits
retrieved with a word 'shinrin-yoku' in Japanese and Chinese (‘森林浴’) on Google Scholar
(on 8th June 2019; see appendix 9 for a list of articles in Japanese satisfying the other
criteria). Also, some studies examined many variables (Bielinis et al., 2019; Bielinis et al.,
2018; Takayama et al., 2019; Takayama et al., 2014), 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 and reviewed.
Conclusion
The twenty studies included in this systematic review and meta-analysis reported that
shinrin-yoku is effective for mental health, particularly anxiety. Shinrin-yoku can be
practiced in 15 minutes to nine days, and reduce negative mental health symptoms (i.e.,
34
Sensitivity: Internal
depression, anxiety, stress and anger). Whilst promising results were reported, medium-high
risk of bias and publication bias were also 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
are needed for shinrin-yoku to be accepted as mainstream mental health intervention.
35
Sensitivity: Internal
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... The same review demonstrated psychological improvements including decreases in self-report stress, depression, and anxiety [41]. A recent (2020) review [42] of the impact of FT on mental illness assessed 20 international studies of healthy and unwell (depression, alcohol dependence, chronic pain, metabolic syndrome, stroke) adults completing short (15 min) to long (4 h) FT sessions and concluded FT improves symptoms of depression, anxiety, and anger [42] Despite this extensive evidence regarding the physical and psychological impacts of FT, there is very little extant evidence regarding the benefits of FT on psychosocial wellbeing (e.g., social capital, loneliness, etc.). A recent review of seven SP nature-based activities concluded that these can promote socially connected and physically active communities [43]; however, the results of this review are difficult to generalise to adults with mental illness as the majority of participants were children and carer-child dyads [43]. ...
... The same review demonstrated psychological improvements including decreases in self-report stress, depression, and anxiety [41]. A recent (2020) review [42] of the impact of FT on mental illness assessed 20 international studies of healthy and unwell (depression, alcohol dependence, chronic pain, metabolic syndrome, stroke) adults completing short (15 min) to long (4 h) FT sessions and concluded FT improves symptoms of depression, anxiety, and anger [42] Despite this extensive evidence regarding the physical and psychological impacts of FT, there is very little extant evidence regarding the benefits of FT on psychosocial wellbeing (e.g., social capital, loneliness, etc.). A recent review of seven SP nature-based activities concluded that these can promote socially connected and physically active communities [43]; however, the results of this review are difficult to generalise to adults with mental illness as the majority of participants were children and carer-child dyads [43]. ...
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Contact with, and psychological connectedness to the natural world are both associated with various health and sustainability-related outcomes. To date, though, the evidence base has been fragmented. Using a representative sample of the adult population of England (N = 4,960), we investigated the relationships between three types of nature contact, psychological connectedness, health, subjective wellbeing and pro-environmental behaviours within a single study. We found that specific types of nature contact, as well as individual differences in nature connectedness, were differentially associated with aspects of health, well-being and pro-environmental behaviours. Living in a greener neighbourhood was, unrelated to any wellbeing or sustainability outcomes. By contrast, visiting nature ≥ once a week was positively associated with general health and household pro-environmental behaviours. Moreover, people who watched/listened to nature documentaries reported higher levels of both pro-environmental behaviours. Nature connectedness was positively related to eudaimonic wellbeing and both types of pro-environmental behaviour. Moreover, connectedness moderated key relationships between nature contact, wellbeing and pro-environmental behaviours. The complexity of our findings suggests that interventions increasing both contact with, and connection to nature, are likely to be needed in order to achieve synergistic improvements to human and planetary health.