The benefits of bamboo forest therapy on physiological responses were investigated in a sample of university students in a study conducted by Zeng and colleagues [12].

A total of 120 university volunteers (60 males and 60 females) aged 19 to 24 participated in this study and were randomly divided into four groups of equal size (n = 15 men, n = 15 women). None of the participants reported physiological or psychiatric disorders in their personal histories. Four sites were selected for the application of the therapy, including two natural bamboo forests (YA and YB), a bamboo forest park (DJY), and an urban environment (CS). During the trial period, all participants were asked to observe the landscape for 15 minutes in the morning and then walk the area for 15 minutes in the afternoon. Regarding the physiological indices, the following measurements were used: blood pressure (BP), heart rate (HR), and peripheral oxygen saturation (SpO2). Psychological reactions were assessed using the differential semantic method (SDM), a scientifically validated and practical method to evaluate the emotional reactions of subjects to the environmental space and consequently extrapolate the environmental factors that most influence mental well-being.

Overall, the participants’ assessment of the bamboo forest environment was significantly better than the urban site. Research results showed that compared to urban sites (CS), bamboo forest sites (YA, YB, DJY) may be more conducive to decreasing blood pressure and heart rate, and that viewing and walking activities have had a positive impact on the physiological and psychological indices of university students.

The results also showed that the preference for natural environments was influenced by temperature, radiant heat, wind speed, noise, the absolute intensity of illumination, and concentration of negative oxygen ions. Compared to the other three experimental sites, the environment of the YB site was more natural, comfortable, open, bright, and with a pleasant ambient noise; consequently, it was better for relaxation among university students. Both psychological measures, such as mental relaxation and positive emotional reactions, and physiological measures, such as blood pressure and heart rate, have therefore improved in university students thanks to the application of forest therapy.

Also, Kim and his research group carried out a prospective clinical study that aimed to investigate the efficacy of forest therapy for postmenopausal women with insomnia [13].

The study was conducted on women over the age of 40 who complained of insomnia caused by menopause. Menopause was diagnosed through an interview by monitoring the menstrual cycle, while insomnia was diagnosed by a doctor through the DSM-III classification. The sample was made up of 35 women who were active and capable of performing more or less strenuous physical exercises. Forest therapy was conducted at the Hoengseong National Forestry Center, a forest education and care center located 680m above the sea level. This site has 6 trails ranging from 450 to 2000m of trees, such as larch, birch, dogwood, and pine that grow naturally. Participants enjoyed various activities, including meditation, gymnastics, 30-minute morning walks in the forest before breakfast, and 1–2-hour afternoon walks after lunch. After the course, participants received leg massages and bathed alternately in hot and cold water. The “Five Senses Experience program” within this studio included varied activities to engage all the senses, such as walking barefoot.

Each participant's sleep and emotional state were assessed using the Epworth Sleepiness Scale (ESS), the Stanford Sleepiness Scale (SSS), the Korean version of the Pittsburgh Sleep Questionnaire Index (PSQI) sleep quality survey questionnaire, the STOP-Bang, to underestimate sleep apnea, the Hospital Anxiety and Depression Scale (HADS), and Polysomnography (PSG) recorded before and after the forest therapy program. Regarding physiological measures, blood samples were taken, before and after forest therapy, to evaluate common blood cells (CBC), glucose, aspartate transaminase (AST), alanine transaminase (ALT), nitrogen, urinary blood, creatinine, electrolytes including sodium, potassium, chloride, C-reactive protein (CRP), interleukin 6, and cortisol. As for cortisol, the blood was taken at 8:00 in the morning because it has circadian changes. The results presented in the study showed an improvement in all physiological and psychological measures, thus determining that forest therapy can be an effective alternative to non-drug treatment for insomnia, especially in postmenopausal women.

A year earlier, in 2019, Jiyune Yi and her research group focused on developing forest therapy programs (FTP) that could be applied in urban forest areas for the elderly population to prevent cognitive decline [14].

Within the study, two different FTP protocols were developed, and the effects of FTP on specific subgroups were evaluated following the approach of Sasang Constitutional Medicine (SC). SC is a branch of Korean traditional medicine that is well established as a standard modality for diagnosis and treatment. Two FTPs were developed with an average duration of two hours per daily session for a total of 11 sessions, with one session per week. To increase the effectiveness of FTP, some therapeutic modalities of Korean traditional medicine have been adopted. The first program is called the “Walking Program (WP)”, and its key design point is active forest walking to induce sweating and stimulate the Yongquan acupuncture point (K11) while walking. The second program is called the “Breathing Program (BP)”, and its key design point is meditation with guided breathing and simultaneous stimulation of the cervical spine.

The following psychological measures were used: the Mini Mental-Status Examination for Dementia Screening (MMSE-DS), the Mini Mental-Status Examination (MMSE) to evaluate the global cognitive status, and a neurocognitive test using the EEG at rest.

As for the physiological measures, on the other hand, Heart Rate Variability (HRV) was measured by photoplethysmography (PPG), and bioimpedance was measured to estimate general body composition factors.

The main objective of this study was to improve neurocognitive abilities, autonomic nervous activities, and metabolic functions through FTPs in order to prevent dementia. Both BP and WP had health benefits for the elderly population in terms of neurophysiology, HRV, and bioimpedance. From the results, it appears that forest therapy may be effective in preventing cognitive decline and that the beneficial effects can be maximized when the program treatment considers the participant's psychological and physiological traits. The beneficial effects, specifically, varied according to the types of FTP; for example, BP was effective in increasing the phase angle of the upper limbs, and WP was effective in increasing the phase angle of the lower limbs. This finding suggests that forest therapy can be effective for preventing dementia and that the beneficial effects can be maximized when the therapeutic program considers the participant's psychological and physiological traits.

The relationship between the experience of alcoholics participating in a forestry program and the effect of this program on their levels of depression was assessed in a study conducted by Shin and his research team [15].

The study sample consisted of 92 participants, i.e., all adult alcoholics enrolled in the Korean Alcohol Research Center in Chungbuk Province, South Korea. The participants were randomly assigned to a treatment group and a control group. This random allocation process resulted in 47 participants in the treatment group and 45 participants in the control group. The measure used in the study was the Korean version of the Beck Depression Inventory (BDI), administered pre-and post-treatment to determine possible differences in the participants’ levels of depression.

Regarding the results, compared to the control group, the test group showed a significant improvement in depression levels at the end of treatment, thus demonstrating that forest therapy can be a suitable treatment for people suffering from alcoholism and open the door to possible ideas of use also in the treatment of other forms of addiction to abuse substances.

The effectiveness of a forest therapy program specifically designed for patients with chronic widespread pain (CWP) was also assessed [16].

The program's overall goal was to provide psychoeducation in pain management and increase motivation to change one's lifestyle in the long term. The two-day program consisted, specifically, of various physical and educational activities, guided in the forest and based on cognitive behavioral therapy.

Among the various activities, there was, for example, mindfulness-based meditation and music therapy in forest environments. The hypothesis was to use nature as a safe and restorative environment in which an individual can autonomously experience cognitive change. The research sample consisted of 61 employees from a Seoul-based public organization, aged between 25 and 49 years and suffering from CWP.

Participants were assigned to an experimental group (n = 33) that participated in a forest therapy program or to a control group (n = 28) on a non-random basis.

The three forest therapy camps were held at Saneum Natural Recreational Forest in Yangpyeong County, Gyeonggi Province.

This forest was very lush, and the valley boasted one of the most beautiful views in the county. Each forest therapy camp lasted two days. During those two days, participants engaged in various indoor and outdoor activities aimed at providing relaxation, refreshment, and attention recovery. The activities were supervised by a team of professionals consisting of a psychiatrist, a rheumatologist, a forest guide, and a forest therapist.

Physiological measurements included cardiac measurements, such as heart rate and heart rate variability (HRV), measured using a supplied long-term electrocardiogram to detect measurements throughout the experimental period, and measurements of NK cell cytotoxic activity. As for the psychological measures, the following were administered: a visual analog scale for pain (VAS Pain), the Beck Depression Inventory (BDI), and the EuroQol visual analog scale (EQ-VAS), all administered pre-and post-treatment.

The results demonstrated that forest therapy program participants, compared to the control group, exhibited greater relaxation as indicated by an increase in HRV and greater immune competence as indicated by an increase in NK cell activity. Participants in the forest therapy group also reported stronger decreases in pain and depression than participants in the control group. Therefore, the present research findings add to the evidence base for the health benefits of forest therapy and confirm its relevance and clinical importance for CWP patients.

A study to investigate the effects of forest therapy on veterans with post-traumatic stress disorder (PTSD) was conducted by Poulsen and colleagues in 2016 [17]. Specifically, the study adopted a phenomenological approach that sought to understand participants' perceptions, perspectives, and understanding of the situation.

8 male veterans, aged between 26 and 47, were included in the project. Nature-based therapy (NBT) was carried out at the Nacadia Forest Therapy Garden of the University of Copenhagen, located in the Hørsholm Arboretum. The NBT program lasted 10 weeks, with 3 hours of therapy three times a week.

Each session began at the entrance to the arboretum, followed by a stroll through the arboretum to the therapeutic garden. Here, the veterans concentrated on different activities that stimulated the senses, and finally, in each session, there was an interval dedicated to “private time” in which the participants could choose for themselves where they wanted to be and what they wanted to do in nature. The program began and ended with a gathering around a campfire in the therapeutic garden. The NBT, specifically, was based on the following three elements: (1) awareness and applied awareness activities, (2) nature-based activities (NBA), and (3) individual therapy sessions. The NBA contained several types of assignments, some with a particular physical approach, such as splitting wood and planting trees, while others were moderately physically demanding and involved in performing routine tasks in the arboretum with the gardener. Through short narratives about, for example, special plants, bird breeding, and seasonal changes in the forest, the veterans have imparted knowledge of nature. Individual therapy sessions focused on guiding veterans on solving problems and conflicts in their daily lives through therapeutic conversations. Four semi-structured interviews were conducted with each participant: one at baseline, one after 5 weeks, one after 10 weeks, and one year after the treatment.

The whole study was based on a general consideration: the relationship of veterans with natural environments could be analyzed using the attention restoration theory (ART). According to this theory, the human mind has two types of attention: direct attention and spontaneous attention [18]. Direct attention is activated when the individual needs to focus on a specific stimulus. This process is mostly intentional and requires conscious effort. With prolonged use, direct attention can be depleted, which can cause direct attention fatigue [19, 20]. Veterans' feeling of low physical and mental energy could be seen as a consequence of their constant vigilance and can be compared to the condition of “direct attention fatigue” [17].

The experience of forest therapy in the light of the results of the interviews, therefore, showed that nature offered veterans relaxation and allowed them to regain physical and mental energy. Over the course of the therapy program, veterans' preferences for places changed from wanting to be alone and in a place that offered shelter in the beginning to looking for new places where they were together with other veterans or people in general in public spaces. It would be reasonable to see this change as a development in the mental and physical abilities of veterans [17].

NBT thus seemed to allow veterans to find their way of transferring the use of nature and the NBA into their daily lives. A year after the end of the NBT, most veterans were still using nature and the NBA to manage their mental health.

In the wake of the various studies, Bielinis and his colleagues [21] decided to investigate the possible benefits of forest therapy for patients in psychiatric hospitals.

The study sample consisted of 50 volunteer patients from the Olsztyn Provincial Psychiatric Care Unit. To determine whether forest therapy could induce positive effects on anxiety levels and mood quality, a pre- and post-test design was used that evaluated the curative effect in two groups of patients: those with affective disorders and those with psychotic disorders. Specifically, the forest therapy intervention consisted of a recreational walk in a suburban forest near the psychiatric hospital in the city of Olsztyn. To assess any health improvements, the following psychological questionnaires were administered: the Profile of Mood States (POMS) and the State-Trait Anxiety Inventory (STAI). Patients were encouraged to participate in forest walks with additional exercises in the forest environment (walking, stretching, and landscape observation). This intervention took place under the supervision of a qualified therapist; patients spent an hour and forty-five minutes in the forest. Before and after the interventions, patients were given psychological questionnaires, which allowed them to evaluate their perceived sensations before and after the forest therapy.

The observed changes in psychological indicators in psychiatric hospital patients indicated that forest therapy intervention could positively affect their mental health.

Different reactions were also observed according to the experimented group. In the case of people with psychotic disorders, the greatest effect of therapy was observed on vigor, while in the case of patients with affective disorders, the greatest effects were observed concerning the traits of confusion and depression. Changes in psychological indicators, therefore, depend on the characteristics of a given disorder.