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Review

Indicators of the Psychosocial and Physiological Effects of Forest Therapy: A Systematic Review

by
Hae-ryoung Chun
1,
Yoon-Young Choi
1,
Inhyung Cho
1,
Hee-kyoung Nam
1,
Geonwoo Kim
2,
Sujin Park
2 and
Sung-il Cho
1,*
1
Graduate School of Public Health, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
2
National Institute of Forest Science (NIFoS), 57, Hoegi-ro, Dongdaemun-gu, Seoul 02455, Republic of Korea
*
Author to whom correspondence should be addressed.
Forests 2023, 14(7), 1407; https://doi.org/10.3390/f14071407
Submission received: 15 June 2023 / Revised: 30 June 2023 / Accepted: 30 June 2023 / Published: 10 July 2023
(This article belongs to the Special Issue The Healing Power of Forests)
Browse Figure
Review Reports Versions Notes

Abstract

:
Research on forest therapy and health outcomes is insufficient and involves the limited unification of various health indicators. Information on the natural therapeutic elements of forests and forest-therapy programs is insufficient. We investigated forest-therapy programs, the natural therapeutic elements of the forest environment, and health-outcome measures to standardize the health indicators of forest therapy. A systematic review was conducted on the health outcomes of forest therapy from 2010 to 2021. We summarized natural therapeutic elements and forest-therapy sites, characteristics, and health outcomes. Twenty-nine articles were analyzed. Among them, forest therapies of walking and physical activity were conducted most frequently. Additionally, a positive improvement in general sentiment condition was substantial in all vegetation environments, and a reduction in depression, psychological enhancement, stress alleviation; and an enhancement in the cardiovascular system and the endocrine system occurred in both narrow- and broad-leaved forests. Walking, five-senses stimulation, physical activities, and meditation benefited psychological and physiological outcomes. We evaluated natural therapeutic elements of the forest-environment program and health outcomes. This systematic review can be used as a reference for the planning and operation of forest-therapy programs.

1. Introduction

Mental and physiological health issues burden not only the individual but also society. Mental health is more than the absence of mental disorders [1]. Furthermore, poor mental health is a risk factor for various diseases [2]. Poor health affects more than merely the quality of life. Mind–body theory indicates that the body and mind are not independent. One’s mental state can affect physiology, and physical health can affect mental health [3,4].
Forest therapy is a nature-based intervention that uses various natural elements of forest environments [5]. Forest therapy is based on participant characteristics as well as the forest’s natural therapeutic elements [6]. Scenery, phytoncides, anions, sounds, sunlight, oxygen levels, plants, water, nutrition, psychological therapy, climate, and exercise therapy are examples of the natural therapeutic elements of forests [7]. These forest elements exert a variety of health-promoting effects via exposure to natural features and interaction with the forest environment [8]. Forest therapy can enhance both psychological and physical health [9,10]. Forest therapy benefits physical health, such as the cardiovascular system [11,12], blood pressure [13,14,15], sleep [16], and immunity [17,18]. Also, depressive mood [9,19], anxiety, and stress have been ameliorated by forest therapy [11,13].
Forest therapy is more cost-effective [9,20] and less aggressive [21] than other medical interventions. Therefore, there is a low risk of side effects [16]. Forest therapy reportedly exerts positive health effects on healthy individuals and patients [13,14,15].
The health-promoting effects of forest environment factors have been described [6]. Research to date has focused on relatively limited aspects of the health outcomes of forest therapy [22,23]. The studies included in prior systematic reviews were not standardized. Also, most did not assess the medium-to-long-term effects of forest therapy using standardized indicators. Few studies considered components of forest therapy, such as program composition [24]. Additionally, prior studies did not include a follow-up, or unified indicators or protocols [22,23].
We reviewed the natural therapeutic elements, forest-therapy program content, and health-promoting effects of forest therapy. Also, we cross-tabulated the content of forest-therapy programs and their psychological and physiological outcomes, the natural therapeutic elements of forest therapy and the content of forest-therapy programs, and the natural therapeutic elements of forest therapy and the psychological and physiological outcomes.

2. Methods

2.1. Literature Search

A systematic review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. We conducted literature searches in five bibliographic databases (PubMed, Science Direct, Embase, Cochrane Library, and Scopus) using the following search terms: “forest bathing (Shinrin-Yoku), forest therapy, or forest medicine of any kind” in the title or abstract. The search terms are listed in Supplementary Table S1. We explored the indicators used in forest-therapy programs. Therefore, we searched the literature over the last decade for “forest therapy program” without limiting the study participants, control, or outcome.

2.2. Selection Criteria

Figure 1 shows a flowchart of the database searches, followed by the exclusion/inclusion strategy. In the first phase (Figure 1), certain types of articles were excluded, including comments, letters, editorials, viewpoints, correspondence, grey literature, and articles without full text. In addition, duplicate works and those not written in English, as well as studies that did not include humans, were excluded. In the next phase (Figure 1), studies not related to forest therapy were further excluded. For example, we excluded observational studies of the health effects associated with exposure to forest characteristics without purposeful intervention. Two papers were added, based on a snowballing approach, from the most-cited forest therapy program systematic review [25]. The characteristics of 31 articles are listed in Supplementary Table S2 [11,12,15,16,21,22,23,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. Only 29 of the 31 articles describe the content of forest therapy programs in detail.

2.3. Data Extraction

Four independent reviewers (HR, YY, IH, and HK) assessed the studies by reading the titles, abstracts, and full texts. Four authors independently extracted the studies that fulfilled the inclusion criteria. The authors evaluated the study characteristics (author, journal, publication date, country, study design, and study period), total number of participants, type of forest treatment program, types of forest therapy elements, and outcomes with measures.

2.4. Data Cleaning

2.4.1. Study Characteristics

Risk of bias (ROB), study design, year of publication, country, season, and characteristics of participants are listed in Supplementary Table S3. Four researchers evaluated the quality of studies independently using Cochrane’s ROB tool. The results showed moderate reliability, promising feasibility, and validity. After cross-checking the ROB results, if the opinions of the researchers differed, the quality evaluation was completed using discussion with the corresponding author (Supplementary Figure S1). ROB has seven domains: generating a random sequence, assigning concealment, blinding of participants and staff, blinding of outcome evaluation, and incomplete outcome, selective reporting, and other bias. The ROB was categorized as high, unclear, or low.
Participant age was categorized in 10-year intervals from 10 to 60 years. The subjects were divided into the exposed and control groups. Subjects were classified as healthy or unhealthy. Seasonality was categorized by season in Korea, China, and Japan, where most of the included studies were conducted. All studies were performed in the Northern Hemisphere (January to March, winter to spring; May, spring; June to mid-September, summer; mid-September to November, autumn to winter). If there was no information on season, researchers deduced the season from the month, temperature, and humidity.

2.4.2. Natural Therapeutic Elements

Natural therapeutic elements are environmental factors that affect health. Natural therapeutic elements of forest therapy were categorized as particulate matter, atmospheric pressure, phytoncide, light, negative ion, wind speed, sound, weather, temperature, humidity, and vegetation (Table 1). Weather was classified as sunny, cloudy, or fair. Fair included fair and sunny or cloudy.
The vegetation factor is the type of vegetation described in the article and organized by frequency. All vegetation in the articles consisted of trees. Identical trees were grouped, and the scientific name was searched and classified to the order level.
The theory of forest healing consists of attention restoration theory (ART) [50], stress recovery theory (SRT) [51], and prospect–refuge theory [52]. ART states that a forest environment promotes calmness and brings attention back to oneself; SRT emphasizes the healing aspect of forest environments. PRT posits that a person feels secure and psychologically relaxed. When a study described the health effects of natural therapeutic elements, the effects were explained based on the natural therapeutic elements of forest therapy, and we defined the preliminary theory as forest-therapy theory. The frequency of forest-therapy theory was determined.

2.4.3. Characteristics of Forest-Therapy Programs

A forest therapy program is a series of planned activities that deliver a nature-based intervention using various natural elements of forest environments to reduce stress and improve health [41]. The forest-therapy programs described in the 29 studies were analyzed by session content and duration, and the duration of the forest-therapy intervention (Table 2). Two articles were excluded due to a lack of information on the forest-therapy intervention and the use of different forest-therapy programs for multiple groups.
A session is the smallest block of a program. Forest-therapy programs contain at least one forest-therapy session. Depending on the program, subjects experienced one or more forest-therapy sessions during the program. Session content features were categorized as walking, five-senses stimulation, physical activities, meditation, viewing, olfactory stimuli, and others. According to the program classification method used in earlier studies, different categories of physical and sensory activities were separated [6].
Session length was categorized in 10 min intervals: 10–20 min, 21–30 min, and 31–40 min. Longer durations were categorized as 41–60 min, 61–90 min, 91–120 min, and 121 min or more. Articles missing information on session duration were excluded.
We also evaluated the duration of the forest-therapy interventions as stay-over-programs and day programs. Stay-over programs were divided according to duration in days.

2.4.4. Psychological and Physiological Health Outcomes

The tools used to evaluate both psychological and physical health are listed in Table 3 and Table 4 in duplicate. HRV is categorized into psychological (Table 3) and physical health (Table 4) based on the purpose of the original articles. Each study determined HRV as either a psychological or physical condition, which determined the inclusion of HRV metrics in each table. If an article stated that HRV was utilized to evaluate mental health, it is presented in Table 3. On the other hand, if the objective of using HRV were to assess physical health, it is categorized and summarized in Table 4. For example, if the article stated that HRV was used to evaluate mental health, which is in terms of stress reduction, it is presented in Table 3.

Psychological Health Outcome and Measure

Table 3 lists the types and number of mental health tools used in the studies that evaluated mental health improvement. The number of measurement tools used to assess psychological outcomes in each article is presented. Furthermore, we recorded the number of questions and the methods of the psychological measurement tools to examine whether different measurement methods were applied using the same measurement tool. In addition, the number of significant changes made using mental health assessment tools was assessed.
Psychological health outcomes (Table 3) were categorized as attention deficit hyperactivity disorder (ADHD), anxiety, depression, psychological status, self-reported health status, sentiment, sleep, social relationships, and stress. The DSM-5 classification standard was used. Other psychological measures that could not be categorized based on the DSM-5 criteria, such as self-esteem, were grouped as psychological status. The mood was categorized as sentiment, defined as the immediate emotional status, such as confusion or vigor. Subjective health status was sorted by self-reported health status. We categorized psychological health outcomes as follows: (1) psychological health status: positive and negative effects, restorative outcome, subjective vitality, feeling comfortable, soothed, and refreshed, and perception of environment; (2) self-reported health: overall health condition, feeling about health status, health-promoting behaviors, and symptoms; (3) sentiment: mood, confusion, and vigor; and (4) stress: general and job stress.
Most of the measurement tools were subjective self-reported tools. Stress and sleep were measured by subjective self-reported and objective tools. For example, sleep was assessed by multiple self-reported questionnaires and by polysomnography, a physiological measure.

Physiological Health Outcome and Measure

Twenty-five articles reported physiological outcomes. Health-outcome domains, measurement targets, measures, number of measures, and significant effects are listed in Table 4. The health-outcome domains were the cardiovascular system, immune system, endocrine system, behavioral system, nervous system, musculoskeletal system, and others. Behavioral change was evaluated separately because it involves multiple physiological systems. Physiological outcomes involving two or more systems were categorized as other. A significant improvement in physiological health outcomes in the original article was taken as a significant health promotion effect.

2.5. Data Analysis

We performed frequency analysis and cross-tabulation analysis using SAS software (ver. 9.4) (SAS Institute Inc., Seoul, Republic of Korea). Supplementary Figure S2 shows the six main outcomes. The cross-tabulation of the forest-therapy session content and length is presented in Table 2. The first row in Table 2 represents the number of articles that described the program and included information about the duration of each session. For example, a 10–20 min walking program was described in five articles. The number of session lengths or stay over the program in Table 2 is the sum of the number of sessions conducted. Each study recorded the count of conducted forest-therapy sessions and the duration of stay-over programs, whether they lasted for several days or a single-day program. Specifically, for stay-over programs, if the session involved an overnight stay of one or more nights, it was counted. In other words, the 10–20 min walking program was operated several times in the five articles, so the count of the program was described 11 times. We presented the frequencies of mental and physical health measures used in the reviewed original papers, as well as the frequencies of significant effects, in Table 3 and Table 4.
Table 5 shows the cross-tabulation of health outcomes and forest-therapy programs. With regard to forest-therapy-program sessions, we present the number of articles that examined physical and mental health changes and the number reporting significant physical and mental health changes. The first row of Table 5 is the number of articles describing session content, and the second row is the number of articles with significant results. The first column is the number of articles reporting a significant health promotion effect compared to the number of articles that evaluated health. The number of articles about mental health (effect ÷ total) refers to the number of articles reporting a mental-health-promoting effect. For example, in the study of walking, seven articles examined depression changes, all of which indicated relief of depression.

3. Results

3.1. Characteristics of the Included Studies

Supplementary Table S3 summarizes the characteristics of the 31 articles published between 2010 and 2021, with 19 (61.28%) published in the last 5 years. Furthermore, 25 out of the 31 studies (80.65%) were conducted in Asian countries (Korea, Japan, and China). The most frequent seasons observed were summer (19.35%), followed by autumn to winter (12.9%) and summer to autumn (12.9%).
ROB ratings were high (87.1%), unclear (9.68%), or low (3.23%). The study design distribution included 11 pre–post studies (35.48%), 11 RCTs (35.48%), 5 quasi-experimental studies (16.13%), and 3 within-group comparisons (9.68%). Additionally, there was one crossover study and three between-group pre–post-test studies (12.91%).
Only two studies involved healthy subjects (6.45%). Among the other studies (n = 29), the health status of subjects was not provided (41.94%), and the majority of participants (51.61%) had one or more health problems, such as metabolic syndrome, cancer, or mental disorders. In terms of age distribution, subjects in their 20s accounted for 29.03% of the total, and the age range was 10–60 years.

3.2. Natural Therapeutic Elements

Table 1 presents nine natural therapeutic elements, including vegetation, phytoncide, sound, light, wind speed, anion, particulate matter, and atmospheric pressure. Information on vegetation was provided by 26 studies (83.87%), on phytoncide by 1 study (3.23%), on sound by 5 studies (16.13%), and on weather by 8 studies (25.81%). Sunny and cloudy weather was reported in four (12.9%) and three articles (9.68%), respectively, and fair weather was reported in two (6.45%). Narrow-leaved forests were the focus of 13 articles (41.94%), and broad-leaved forests were the focus of 23 articles (74.19%).
The cross-tabulation analysis between vegetation and forest-therapy-program content (Supplementary Table S4) revealed that broad-leaved forests were frequently utilized in therapy sessions, with walking and physical activities being conducted at high frequencies in these forest environments.
The cross-tabulation of vegetation and health outcomes (Supplementary Table S5) indicates a significant reduction in stress in forest environments with broad-leaved (five of five articles) and narrow-leaved (five of five articles) forests. Broad-leaved forests had a positive effect on psychological health status (three of four articles). Among the studies conducted in narrow-leaved forests, five out of six studies reported a reduction in depression, and four out of four reported an enhancement in endocrine markers. Five and three articles reported improved sentiment in broad-leaved and narrow-leaved forests, respectively.

3.3. Types of Forest-Therapy Programs

Table 2 summarizes the types of forest-therapy programs in the 29 articles. Session content features were categorized into seven groups. The most frequent session content features were walking (n = 22) and physical activities (n = 15). Meditation and viewing were used in ten and nine forest-therapy programs, respectively. Seven forest-therapy programs used five-senses stimuli, and two used olfactory stimuli. Four forest-therapy programs involved self-expression activities with natural materials, a forest day camp, and an introduction to forest therapy.
Regarding walking session durations, the most common duration was 61–90 min (n = 12), followed by 10–20 min (n = 11). Nine sessions were of 91–120 min duration, and there were two each of 21–30 min and 31–40 min. One program involved sessions of 41–60 min duration. Also, 16 programs were day programs, and 4 were stay-over-programs.
According to the cross-tabulation of session duration and content, physical activities were of 61–90 min duration (n = 3). Sessions using five-senses stimuli were of 10–20 min (n = 1) and 61–90 min (n = 1) durations. Meditation sessions lasted for 10–20 min (n = 1) and 31–40 min (n = 2).

3.4. Effect of Forest Therapy on Mental Health

Of the 31 articles, Table 3 lists 29 articles that assessed mental health and provides information on the number of studies utilizing specific measurement tools and the reported effectiveness of those tools. These articles encompass domestic and international research on forests. In 29 articles, 9 measurement targets and 108 measurement tools related to mental health have been used in domestic and foreign studies of forest-therapy programs. Sixty-four tools showed significant effects. Most tools were used to measure sleep (n = 25), stress (n = 23), and sentiment (n = 18).
Uniquely, sleep and stress were measured using subjective and objective evaluation tools. Sleep efficiency (n = 3) and total duration of sleep (n = 3) are objective measurements of sleep. The most common indicators of stress are cortisol (n = 7) and HRV (n = 3). The sentiment was evaluated in 18 articles, most of which used POMS (n = 15). Psychological health status (n = 11) was measured most widely, but the frequency of the tools used varied, usually with the use of one or two. Depression was evaluated in eleven articles, with the Beck Depression Inventory (BDI) being the most frequently utilized instrument (n = 5). Anxiety was assessed in eight articles, predominantly employing the State–Trait Anxiety Inventory (STAI) (n = 3). Four studies examined social relationships, each utilizing a different measurement tool. One study investigated ADHD using a single tool, but no significant effects were observed. The psychological factors demonstrating the most substantial health-promoting effects were stress (17 out of 23), sentiment (13 out of 18), and depression (9 out of 11).

3.5. Indicators of Physical Health

Table 4 shows the frequencies and effects of physiological measures and health outcomes. Of the 31 studies, 18 analyzed cardiovascular outcomes. Under the cardiovascular domain, blood pressure, sympathetic-to-parasympathetic system, and cardiovascular risk factors were targeted to evaluate the effect of forest therapy. The fourth column in Table 4 displays the article count utilizing each measure. Specifically, out of the 25 articles that used physiological health indicators, 18 focused on the cardiovascular system, with 11 articles measuring blood pressure. Additionally, the fifth column in Table 4 indicates the number of articles that reported significant changes.
Regarding other health domains, the immune system (n = 6), endocrine system (n = 1), behavioral (n = 3), nervous system (n = 2), musculoskeletal system (n = 4), and other (n = 1) systems were analyzed. The most frequently used cardiovascular measures were blood pressure (n = 11), heart rate (n = 7), and blood markers (n = 12).
One study evaluated the effectiveness of forest therapy for hypertension [15].
The second most frequently used measures were heart rate (n = 7) and HRV (n = 7). Heart rate was used in seven studies, four of which showed a positive effect. Most articles used heart rate and HRV as indicators of the sympathetic-to-parasympathetic system. HRV was analyzed in seven articles, four of which reported a positive effect.
Blood markers and NK cells were assessed to evaluate immunity. One study evaluated changes in endocrine system status based on blood and urine markers. Three articles evaluated physical activity, which is a behavioral status, as physical activity, energy impact, and pedometer values.
EEG was used twice to examine changes in the nervous system. Four studies examined changes in chronic pain and body composition as indicators of musculoskeletal status. Changes in chronic pain were evaluated by assessing changes in self-reported symptoms or using surveys of physical pain and activity.
One study measured peripheral oxygen saturation.

3.6. Cross-Tabulation of Psychological and Physiological Health Outcomes

Supplementary Table S6 shows the cross-tabulation of positive psychological and physiological outcomes. Significant health outcomes in the cardiovascular system and sentiment were simultaneously observed in seven studies. Five studies reported positive outcomes for the endocrine system and stress. Stress and depression (n = 4) and cardiovascular and sentiment (n = 8) were both observed in one study. Also, the cardiovascular system and stress (n = 5) positively affected both health indicators.

3.7. Cross-Tabulation of Psychological and Physiological Outcomes of Forest-Therapy Session Content

The number of studies investigating changes in physical and mental health according to forest-therapy-program sessions is listed in Table 5, which also presents the number of papers that showed a significant health-promoting effect. For example, eight studies examined changes in depression during walking sessions, seven of which reported significant changes in depression. One intervention was executed per study, but several health outcomes were typically assessed concurrently. However, we intended to depict not only counts of the health evaluations performed in a study but also how the programs and health outcomes combined.
Seven session elements (walking, physical activities, viewing, olfactory, five-senses stimulation, meditation, and other) and nine psychological outcomes were cross-tabulated. A total of 22 studies involved walking sessions, 15 involved physical activities, 9 involved viewing, 2 involved olfactory factors, 7 involved five-senses stimulation, 10 involved meditation, and 5 involved others. The number of papers evaluating mental health in each session was 20, 13, 8, 2, 7, 10, and 4. The number of studies that evaluated physical health was 20, 12, 6, 2, 6, 9, and 3. Regarding mental health, the number of studies that evaluated ADHD, anxiety, depression, psychological improvement, self-report, sentimental, sleep, social relationship, and stress was 1, 8, 10, 7, 5, 2, 14, 3, 4, and 13, respectively. Among them, the number of papers that reported significant changes was 0, 4, 8, 6, 2, 9, 2, 2, and 10. Regarding physical health, the number of studies that evaluated changes in the cardiovascular system, immune system, endocrine system, behavior, nervous system, respiratory system, and musculoskeletal system was 18, 6, 9, 6, 2, 1, and 4, respectively. Among them, the number of papers that reported significant changes was 17, 3, 6, 3, 0, 0, and 3.
We conducted a cross-tabulation analysis of depression, emotion, stress, walking, physical activity, and meditation sessions. Eight studies showed significant mitigation of depression in seven after walking sessions, five after five-senses stimulation, five after physical activities, and four after meditation sessions. In total, 22 studies involved walking sessions and reported positive effects on depression (7 of 8), sentiment (7 of 11), stress (8 of 10), cardiovascular function (14 of 15), and the endocrine system (5 of 7). In total, 15 studies involved physical activity sessions and reported positive effects on depression (5 of 6), sentiment (2 of 4), stress (7 of 8), cardiovascular function (6 of 6), and the endocrine system (4 of 6). In total, 10 studies involved meditation and reported positive effects on depression (4 of 6), sentiment (3 of 3), stress (6 of 7), cardiovascular function (4 of 4), and the endocrine system (4 of 5).
Nine studies involved viewing sessions as part of forest therapy. Psychological status (three of four) and cardiovascular health (four of four) outcomes were enhanced by viewing sessions. Seven studies involved five-senses stimulation as forest therapy. Five of five studies reported positive effects of five-senses stimulation on depression and stress.

4. Discussion

We systematically reviewed studies on the effects of forest-therapy programs on health from 2010 to 2021. The natural therapeutic elements of forest therapy, base theory of natural therapeutic elements, forest-therapy content, and health outcomes and measures were analyzed. Unlike previous systematic reviews, we cross-tabulated health outcomes that showed a significant positive effect with the content of forest-therapy programs, as well as natural therapeutic elements, health indicators, and health measures. In narrow- and broad-leaved forests, walking and physical activities were frequently conducted. In narrow-leaved forests, meditation and viewing sessions, respectively, were used most frequently. In environments with both broad- and narrow-leaved forests, stress, psychological status, depression, and the endocrine system showed the greatest effects. Positive cardiovascular results were reported in broad- and narrow-leaved forests but also in environments with other forms of vegetation. Walking, five-senses stimulation, physical activities, and meditation improved stress, depression, cardiovascular function, and endocrine function. Viewing sessions promoted cardiovascular health, whereas walking and meditation were used to enhance sentiment. Concurrent positive effects on cardiovascular function and sentiment, and endocrine function and stress were reported. The highest number of studies examined sentiment, stress, and cardiovascular changes, with walking activity being commonly used, particularly in studies conducted in broad-leaved forests. Considering the potential variations in health effects depending on different therapeutic elements and program types, These frequently utilized indicators can serve as references for future studies, guiding their application in research.

4.1. Natural Therapeutic Elements

Vegetation, temperature, weather, and humidity are reported to be elements of forest therapy. However, more than half of the studies did not evaluate natural therapeutic elements. For vegetation, the most frequent were broad-leaved and narrow-leaved forests. In narrow-leaved forests, walking, physical activities, and meditation were frequently performed. In broad-leaved forests, viewing sessions were frequently used in addition to walking and physical activities. Future research on forest therapy should determine whether various forest types and vegetation have a significant impact on the health outcomes of forest therapy programs. In addition, the direct impact and mechanisms of walking in broad-leaved forests on health promotion must be investigated.

4.2. Characteristics of Forest-Therapy Programs

Forest therapy is reported to modulate health [53]. We evaluated forest-therapy program components such as duration, accommodation, session content, and session duration. The ideal forest-therapy program should involve 61–90 min of physical activities and 10–20 min of walking, meditation, and viewing. Most forest therapies were day programs, but some were stay-over programs (1, 2, 3, and 5 nights). Previous systematic reviews reported the total program duration but not the session duration [54,55]. In a previous study, a day program took 15 min to 2 h [54], which is similar to our findings. It is thought that 1–2 h is the optimum program duration in terms of participant satisfaction and preference [56]. Future studies can refer to the session types, durations, and possibilities for accommodation in the forest-therapy programs described in this paper. Additionally, it would be necessary to investigate whether the health-promoting effects varied based on the program’s session types and durations.

4.3. Health Outcomes

4.3.1. Health Outcomes and Measures

Sleep, stress, sentiment, psychological status, and depression were the most frequently analyzed factors. The most frequently used physiological measures were blood pressure; heart rate; and blood, urine, and saliva markers. Psychological outcomes were assessed by subjective measures and physiological outcomes by objective measures.
Sleep and stress were assessed by both subjective and objective measures, the latter being more frequent. Assessing the quality of sleep is often subjective because the sleeping process happens unconsciously. More objective measures including EEG, ENG, and electromyogram are more accurate [57].
Objective measures to evaluate sleep and stress are needed. Polysomnography is an objective measure of sleep; to assess stress, cortisol level and HRV are used. To assess sentiment and depression, POMS and BDI are the most frequently used measures. In previous studies, polysomnography [58], cortisol level [59], HRV [60], POMS [61], and BDI [62] were the most commonly used measures.
A single indicator can be interpreted by various measurement targets; therefore, multi-dimensional data collection is feasible. Cardiovascular function and sentiment, endocrine function, and stress showed concurrent improvement. This implies that psychological and physiological health are not independent. Therefore, instead of considering only one dimension, it is important to identify and collect indicators as a multidimensional mechanism that can account for their interactions.

4.3.2. Health Outcomes and Natural Therapeutic Elements

Broad-leaved and narrow-leaved forests had beneficial effects on stress, psychological status, depression, and endocrine function. Also, forest environments with other forms of vegetation benefitted cardiovascular health. It is important to identify the natural therapeutic elements of forest therapy [63], but there is little information on the effects of these elements on health. A standardized measurement of natural therapeutic elements should be utilized to elucidate the health effects of each factor in future studies.

4.3.3. Health Outcomes and Forest-Therapy Sessions

Walking was the most frequent session content, followed by physical activities and meditation. Sentiment, stress, and depression were the most frequently used psychological outcomes, and blood pressure was the most frequently used physiological outcome.
Walking, five-senses stimulation, physical activities, and meditation exerted positive effects on stress, depression, and cardiovascular and endocrine function. Walking was the most frequently used, followed by physical activities. Physical activities can prevent mental disorders [64,65,66,67] and improve mental health [68]. During forest therapy, natural therapeutic elements activate all sensory systems. This induces relaxation of the central nervous system, thereby improving mental health [69]. Therefore, walking, physical activities, and five-senses stimulation sessions affect both psychological and physiological health.
According to our cross-tabulation, sentiment and cardiovascular function, and stress and endocrine function, showed positive outcomes concurrently. Viewing sessions exerted positive effects on sentiment and cardiovascular function, and meditation exerted positive effects on stress and endocrine function. Forest therapy reportedly promotes cardiovascular relaxation by suppressing the sympathetic nervous system and facilitating the parasympathetic nervous system. It also mitigates negative mood [11], but this may involve factors other than viewing sessions. Meditation decreases stress and depression [70]; however, few studies have been conducted on this particular topic. As a result, more research is required to investigate the underlying mechanisms through which meditation modulates stress and depression in forest therapy conditions [71]. To enhance positive sentiment, alleviate stress, and improve cardiovascular function, incorporating walking sessions as a significant component in the program can be considered.

4.4. Limitations

This study had limitations. First, we could not determine the direct links between natural therapeutic elements and health outcomes because the included studies evaluated the effects of forest-therapy programs, not of natural therapeutic elements. For example, no study has conducted a comparative analysis of the health-promoting effects of forest-therapy programs between broad-leaved and narrow-leaved forests. Future studies should distinguish the effects of natural therapeutic elements from the effects of the program components. Second, most of the included studies had a high ROB. This was likely because of the difficulty in blinding the subjects to the measures employed. We evaluated the frequencies of health indicators but did not assess their suitability for forest-therapy programs. Therefore, a detailed investigation of tools for assessing mental and physical health is needed.

4.5. Strengths and Message

Various forms of forest therapy have been developed and most studies focused on their effects on health [63]. The medium-to-long-term effects of forest therapy have not been evaluated using standardized measures. Most studies did not consider natural therapeutic elements or the content of forest-therapy programs. Also, standardized indicators are required for forest-therapy research. The application of tools to forest-therapy programs will enable the identification of the health effects of forest-therapy elements. Forest-therapy programs require different disease indicators, depending on the disease. We present the disease indicators used in the articles analyzed in this study in Supplementary Table S7.
We used systematic research to cross-tabulate the frequencies of indicators in the existing literature. The cross-tabulation of the forest-therapy-program content and the measures that had the greatest effect on health outcomes were presented. The frequencies of natural therapeutic elements excluded from prior systematic reviews were measured.

5. Conclusions

This is the first systematic review of the health outcomes of forest-therapy programs, health measures, natural therapeutic elements of forest therapy, and the content of forest-therapy programs. The cross-tabulation of health outcomes allows a multi-dimensional interpretation. Frequent pairs of health outcomes could be used as a reference for program/study planning. We discovered that psychological and physiological health are interdependent and can be improved in multiple dimensions. This suggests that the indicators proposed in this study can be utilized in future research examining multidimensional mechanisms. In addition, our findings can be used to analyze the medium-to-long-term effects of forest-therapy programs and to facilitate the development of standardized health outcomes.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/f14071407/s1: Figure S1: Summary of risk of bias based on the Cochrane risk of bias tool; Figure S2: Key results. Table S1: Literature search query; Table S2: Characteristics of the studies; Table S3: Literature characteristic; Table S4: Cross-tabulation of vegetation and program contents; Table S5: Cross-tabulation of effective health outcome and vegetation; Table S6: Cross-tabulation of psy-chological health outcome and physiological health outcome; Table S7: Indicators for the health promotion of patients with diseases in the forest therapy program.

Author Contributions

Conceptualization, H.-r.C.; methodology, H.-r.C.; validation, H.-r.C., S.-i.C., S.P. and G.K.; formal analysis, H.-r.C.; investigation, H.-r.C., I.C., Y.-Y.C. and H.-k.N.; writing—original draft preparation, H.-r.C. and Y.-Y.C.; writing—review and editing, H.-r.C., I.C., Y.-Y.C. and H.-k.N.; visualization, H.-r.C., I.C. and Y.-Y.C.; supervision, S.-i.C.; project administration, S.-i.C. All the authors have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the R&D Program for Forest Science Technology (project no.: 2021388A00-2123-0102) of the Korea Forest Service (Korea Forestry Promotion Institute).

Data Availability Statement

The published articles included in our analysis are publicly available.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

AbbreviationExplanation
MBI-GSMaslach Burnout Inventory–General Survey
REQRecovery Experience Questionnaire
SMBQShirom–Melamed Burnout Questionnaire
korean_CASS-S Conners–Wells’ Adolescent Self-Report Scale (Short Form)
HADSHospital anxiety and depression scale
POMSProfile of Mood States
STAIThe State–Trait Anxiety Inventory
STAI-SThe State–Trait Anxiety Inventory to measure state
POMSProfile of Mood States
PSQPerceived Stress Questionnaire
BDIBeck Depression Inventory
CDIKorean version of the Children’s Depression Inventory
GeriatricDSThe Geriatric Depression Scale
HAM-D17Hamilton Depression Rating Scale
KPDS-K6 Kessler Psychological Distress Scale
Perception_env Perception environment
CIS Checklist Individual Strength questionnaire
VAS-FVisual analogue scale to evaluate fatigue severity
BRUMSThe Brunel Mood State Inventory
PANASThe Positive and Negative Affect Schedule
ROSThe Restorative Outcome Scale
SemanticDTSemantic differential techniques to assess subjective feelings of “comfortable” and “soothed”
SVSSubjective Vitality Scale
ZIPERSZuckerman Inventory of Personal Reaction Scale
sf-36The Short Form Health Survey 36
SF-8Short version of Health-Related Quality of Life
RSERosenberg Self-Esteem Scale
SCQSocial Communication Questionnaire
HPLP2Health-Promoting Lifestyle Profile II
q_healthconSingle-sentence question: “How do you feel about your overall health condition?”
Q_SymptomSelf-reported symptoms and medicines used by the group
ESSEpworth sleepiness scale
insomniaSIInsomnia severity index
PSQIPittsburgh Sleep Questionnaire Index (PSQI)
SSSStanford sleepiness scale
STOP-BangAssessment tool used to help diagnose obstructive sleep apnea (STOP-BANG—snoring, tired, observed, pressure, body mass index, age, neck size, gender)
PLMiPeriodic limb movement index
ODIOxygen desaturation index
AHIApnea Hypopnea Index
Peer_relationThe peer relationship instrument
RCSThe Relationship Change Scale
SASSSocial Adaptation Self-evaluation Scale
KPDS-K6Kessler Psychological Distress Scale K6
PSS-KoreanPerceived Stress Scale, Korean
SRI-MFStress Response Inventory-Modified Form
Stress-CTStress-coping techniques
Tool_hemodialysis Tool developed for hemodialysis patients by Kim (physiological stress, psychological stress)
WSRIWorker’s Stress Response Inventory

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Forests 14 01407 g001 550
Figure 1. Flow diagram of the study selection process.
Figure 1. Flow diagram of the study selection process.
Forests 14 01407 g001
Table
Table 1. Number of articles and rates of natural therapeutic elements (n = 31).
Table 1. Number of articles and rates of natural therapeutic elements (n = 31).
Natural Therapeutic ElementNumber of ArticlesPercentage
PM10 * (mg/m3)13.23
Atmospheric pressure13.23
Phytoncide (ng/m3)13.23
Light (lx)39.68
Negative ion (m−3)26.45
Wind speed (m/s)516.13
Sound (dB)516.13
Weather825.81
Weather—sunny412.9
Weather—cloudy39.68
Weather—fair26.45
Vegetation2683.87
Mixed13.23
Narrow-leaved forests1341.94
Broad-leaved forests2374.19
* PM10—particulate matter < 10 μm in aerodynamic diameter.
Table
Table 2. Cross-tabulation of session duration and content (n = 29).
Table 2. Cross-tabulation of session duration and content (n = 29).
Characteristic of ProgramCompositionNumber of Articles *Number of Session Lengths or Stay-Over Program
Session contentWalk22-
Session length10–20 min511
21–30 min12
31–40 min12
41–60 min01
61–90 min512
91–120 min19
Stay-over programDay program416
Stay-over program—1 day64
Session contentPhysical activities15
Session length61–90 min311
Session contentMeditation10
Session length10–20 min111
31–40 min23
Session contentViewing9
Stay-over programA day program116
Stay-over program—2 days11
Stay-over program—3 days12
Session contentFive senses stimuli7
Session length10–20 min111
61–90 min110
Stay-over programa-day program22
Stay-over program—5 days12
Session contentOlfactory stimuli2
Session contentOthers4
Session length41–60 min11
61–90 min211
121–min12
Stay-over programDay program116
Stay-over program—2 days22
* Some studies did not specify the session duration or program type with regard to day vs. stay-over and are not included in this column.
Table
Table 3. Frequencies and effects of psychological measures and health outcomes (n = 29).
Table 3. Frequencies and effects of psychological measures and health outcomes (n = 29).
Mental Health TargetMental ToolNo. of Tools UsedNo. of Significant Effects
ADHD 10
Korean CASS-S10
Anxiety 84
HADS21
POMS (Anxiety)11
STAI32
STAI-S20
Depression 119
BDI55
CDI11
GeriatricDS11
HADS21
HAM-D1711
KPDS-K610
Psychological status 117
PANAS20
Perception_env11
ROS21
RSE11
SemanticDT22
SVS21
ZIPERS11
Self-reported health status 74
CIS11
HPLP210
Q_healthcon10
Q_Symptom11
sf-3611
SF-810
VAS-F11
Sentiment 1813
BRUMS11
POMS1510
POMS-JAPAN11
PSQ11
Sleep 258
Subjective assessment toolESS21
InsomniaSI20
PSQI20
SSS21
STOP-Bang20
Objective assessment toolOxygen desaturation index (ODI)10
PLMi11
REM sleep latency11
REM sleep latency 10
Sleep architecture (N1–N3)10
Sleep efficiency32
Total time of sleep 30
Wake after sleep onset22
Apnea hypopnea index (AHI)20
Social relationships 42
Peer_relation10
RCS11
SASS11
SCQ10
Stress 2317
Subjective assessment toolKPDS-K610
PSS-Korean11
SRI-MF11
Stress-CT11
Tool_hemodialysis10
WSRI11
Objective assessment toolBiological antioxidant potentials (BAPs)11
Cortisol75
Heart rate variability (HRV)33
Malondialdehyde11
Reactive oxygen metabolite (d-ROM)10
Salivary amylase10
Job stressMBI-GS11
REQ11
SMBQ11
Total 10864
MBI-GS: Maslach Burnout Inventory–General Survey; REQ: Recovery Experience Questionnaire, SMBQ: Shirom–Melamed Burnout Questionnaire, Korean; CASS-S: Conners–Wells’ Adolescent Self-Report Scale (Short Form); HADS: hospital anxiety and depression scale; POMS: Profile of Mood States; STAI: The State–Trait Anxiety Inventory; STAI-S: The State–Trait Anxiety Inventory to measure state; PSQ: Perceived Stress Questionnaire; BDI: Beck Depression Inventory; CDI: Korean version of the Children’s Depression Inventory; GeriatricDS: The Geriatric Depression Scale; HAM-D17: Hamilton Depression Rating Scale; KPDS-K6: Kessler Psychological Distress Scale; Perception_env: perception environment; CIS: Checklist Individual Strength questionnaire; VAS-F: visual analogue scale to evaluate fatigue severity; BRUMS: the Brunel Mood State Inventory; PANAS: the Positive and Negative Affect Schedule; ROS: Restorative Outcome Scale; SemanticDT: subjective experiences, attitudes, or emotions associated with a particular concept; SVS: Subjective Vitality Scale; ZIPERS: Zuckerman Inventory of Personal Reaction Scale; sf-36: Short Form Health Survey 36; SF-8: short version of Health-Related Quality of Life; RSE: Rosenberg Self-Esteem Scale; SCQ: Social Communication Questionnaire; HPLP2: Health-Promoting Lifestyle Profile II; Q_healthcon: single-sentence question: “How do you feel about your overall health condition?”; Q_Symptom: self-reported symptoms and medicines used by the group; ESS: Epworth sleepiness scale; insomniaSI: insomnia severity index; PSQI: Pittsburgh Sleep Questionnaire Index (PSQI); SSS: Stanford sleepiness scale; STOP-Bang: assessment tool used to diagnose obstructive sleep apnea (STOP-BANG: snoring, tired, observed, pressure, body mass index, age, neck size, gender); Peer_relation: the peer relationship instrument; RCS: Relationship Change Scale; SASS: Social Adaptation Self-evaluation Scale, KPDS-K6: Kessler Psychological Distress Scale K6, PSS-Korean: Perceived Stress Scale, Korean; SRI-MF: Stress Response Inventory-Modified Form; Stress-CT: Stress-coping techniques; Tool_hemodialysis: tool developed for hemodialysis patients by Kim (physiological and psychological stress); WSRI: Worker’s Stress Response Inventory.
Table
Table 4. Frequencies and effects of physiological measures and health outcomes. (n = 25).
Table 4. Frequencies and effects of physiological measures and health outcomes. (n = 25).
Health Outcome Domain (No. of Articles)Measure TargetMeasureNo. of MeasuresNo. of Significant Effects
Cardiovascular system (18)Blood pressure Blood pressure 116
Sympathetic-to-parasympathetic system Heart rate 74
Heart rate variability 74
Cardiovascular disease risk factorBlood Marker
Endothelin-1 (ET-1)11
Homocysteine (Hcy)11
Renin11
Angiotensinogen (AGT)11
Triglyceride10
Cholesterol30
Plasma ET-1(vasoconstrictor)10
Platelet activation10
T-SOD (superoxide dismutase)10
MDA(lipid peroxidation in the serum)11
Immune system
(6)		Cytokine production Blood Marker
Interleukin 632
C-reactive protein (CRP)10
Tumor necrosis factor (TNF-a)21
ImmunityNK cell42
Total T cell10
Total B cell11
T helper lymphocyte10
T Suppressor lymphocyte10
CD4/CD810
Endocrine system
(1)		Hormonal changeBlood marker—adiponectin11
Urine Marker
Noradrenaline11
Dopamine10
Adrenaline10
Behavioral (3)Physical Activity Activity Level
Physical activity 10
Energy expenditure11
Pedometer11
Nervous system (2)Cognitive healthBrain waves (EEG)22
Musculoskeletal (4)Chronic painHealth Status (self-reported)
Self-reported symptoms11
Visual analog scale (VAS)21
EuroQol 5D-3L VAS (EQ VAS)10
Physical Pain and Activity
Neck disability index (NDI)10
EQ index21
McGill pain questionnaire (MPQ)10
Trigger points (TRPs) in the posterior neck region11
Cervical range of motion (C-ROM)10
Body Composition Body composition
BMI10
Percent of body fat 11
Skeletal muscle mass10
Amount of muscle10
Other (1)OtherPeripheral oxygen saturation *11
* Peripheral oxygen saturation is classified separately because it could be included in the cardiovascular and respiratory systems.
Table
Table 5. Cross-tabulation of the psychological and physiological effects of forest-therapy session contents (n = 31).
Table 5. Cross-tabulation of the psychological and physiological effects of forest-therapy session contents (n = 31).
No. of Articles about Session ContentPhysical ActivitiesSense ActivitiesMeditationOthers
WalkingPhysical
Activities		ViewingOlfactoryFive
Senses
2215927105
No. of articles about mental health (effect/total)2013827104
ADHD0/10/10/10/00/00/10/00/1
Anxiety4/83/52/51/20/12/22/30/1
Depression8/107/85/61/10/15/54/62/2
Psychological improvement6/73/42/23/40/02/21/11/1
Self-report2/51/41/21/10/01/21/20/1
Sentiment9/147/112/43/70/01/13/30/0
Sleep2/31/22/30/11/11/12/30/0
Social2/41/31/31/20/00/10/11/2
Stress10/138/107/82/41/15/56/71/1
No. of articles about physical health
(effect/total)		201262693
Cardiovascular
system		17/1814/156/64/40/04/44/42/2
Immune system3/63/51/30/00/00/21/31/1
Endocrine system6/95/74/61/31/13/34/51/1
Behavioral3/62/52/50/21/11/22/40/0
Nervous system0/20/20/10/00/00/10/00/0
Respiratory0/10/10/00/10/00/00/00/0
Musculoskeletal3/43/42/30/10/00/01/21/1
Note: other physical activities included stretching and pilates mat exercises.
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Chun, H.-r.; Choi, Y.-Y.; Cho, I.; Nam, H.-k.; Kim, G.; Park, S.; Cho, S.-i. Indicators of the Psychosocial and Physiological Effects of Forest Therapy: A Systematic Review. Forests 2023, 14, 1407. https://doi.org/10.3390/f14071407

AMA Style

Chun H-r, Choi Y-Y, Cho I, Nam H-k, Kim G, Park S, Cho S-i. Indicators of the Psychosocial and Physiological Effects of Forest Therapy: A Systematic Review. Forests. 2023; 14(7):1407. https://doi.org/10.3390/f14071407

Chicago/Turabian Style

Chun, Hae-ryoung, Yoon-Young Choi, Inhyung Cho, Hee-kyoung Nam, Geonwoo Kim, Sujin Park, and Sung-il Cho. 2023. "Indicators of the Psychosocial and Physiological Effects of Forest Therapy: A Systematic Review" Forests 14, no. 7: 1407. https://doi.org/10.3390/f14071407

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