Stress was defined as “the quality of experience, produced through a person–environment transaction that, through either overarousal or underarousal, results in psychological or physiological distress” ^[1][5]. Psychological stress (measured by self-report methods) is the result of the evaluation of a stimulus in terms of its harmful and threatening potential ^[2][6]. Physiological stress response is characterized by a normal general, non-specific rise in arousal levels or activation levels ^[3][7]. It mobilizes biological resources, prepares the organism for a prompt response, and provides, in the long run, either a healthy adaptation or the accumulation of allostatic loads ^[4][8]. For many years, it has been assumed that there is a correlation between the different stress outcome systems, namely cognitive-emotional, physiological, and behavioral ^[5][9]. A cognitive evaluation of the significance of a stimulus and the available coping strategies is provided by the prefrontal cortex through the integration of sensory information. This subsequently leads to the formation of emotional responses through limbic connections, eventually leading to the activation of physiological systems such as HPA ^[6][10]. Stress is believed to be a major contributor to the onset and progression of a wide range of psychological or physiological problems, such as high blood pressure, colitis, gastric ulcers, anxiety, and burnout ^[7][8][9][11,12,13]. According to statistics, stress-related diseases account for approximately 75% of all physician visits ^[10][14]. To tackle stress, millions of people worldwide turn to pharmacological treatment, which comes with countless contraindications and adverse effects ^{[11][12][13][14]}[15,16,17,18]. There is, therefore, a strong demand for examining and developing promising non-pharmacological stress reduction interventions.

Stress Reduction Theory (SRT) states that building-dominant environments increase the stress levels of their occupants, while nature, such as forests, has the potential to create positive emotions and feelings that invoke a restorative effect ^[15][19]. In ancient Rome, people used the forest environment as a setting to improve physical and mental health, and for relief by taking refuge occasionally in the forest to escape from urban congestion ^[16][20]. Several recent studies have also demonstrated that forest environments are beneficial to stress recovery both physically and mentally. With the assistance of healing factors (e.g., green foliage, blue sky and water, tree-derived phytoncide, or the sound of running water) contained in the forest environment ^[17][21], the forest environment may help one recover from stress by reducing heart rate ^[18][22], blood pressure ^[19][23], sympathetic nervous activity ^[20][24], and salivary cortisol level ^[21][25], and increasing parasympathetic nervous activity ^[22][26] and NK cell activity ^[23][27]. Attention Restoration Theory (ART) contends that prolonged use of directed attention fatigues neural mechanisms ^[24][28]. As suggested by ART, exposure to nature, such as forests, can reduce the feelings of fatigue or psychological stress. Certain key properties of settings enable the recovery of effective functioning, such as “being away”, “extent”, “fascination”, and “comparability”. These components refer to the key characteristics of forests that contribute to the experience of restorative wellness ^[25][29]. Being in the forest can provide a sense of distance from the daily routine or the urban environment ^[26][30], which is crucial for reducing negative emotions ^[27][31], depression ^[28][32], anxiety ^[29][33], stress ^[30][34], and increasing positive emotions ^[31][35].

To maximize these capacities, a concept known as “forest therapy program” has been proposed ^[32][36]. “Forest therapy program” refers to a series of structured activities and cognitive-behavioral therapy-based interventions that utilize the forest environment in various ways to promote health ^[33][34][37,38]. This distinguishes forest therapy programs from forest therapy, which refers to activities designed to improve human health through the use of various environmental factors in forests, either with or without structured activities and cognitive-behavioral therapy ^[33][35][36][37,39,40]. The body of research on forest therapy programs is much smaller compared to the amount of research on forest therapy. In most countries, forest therapy programs are still relatively new, and only a few research studies have been conducted on them ^[34][37][38,41]. Previous studies ^[10][38][14,42] examined the physio-psychological benefits of forest therapy, but did not address forest therapy programs specifically. Separately, previous research synthesized stress measures with other constructs and did not examine the direct association between forest therapy programs and stress reduction. The actual relationship between forest therapy programs and stress reduction is still not discerned.

In general, forest therapy programs have shown to be effective in relieving stress both physiologically and psychologically. The physiological studies found significant reductions in systolic blood pressure (SBP) ^{[37][39][40][41][42]}[41,61,64,65,71], mean arterial pressure (MAP)PA ^[43][63] and serum cortisol level ^[44][45][59,67], improvement in heart rate variability (HRV) ^[46][70], and an increase in biological antioxidant potential (BAP) ^[47][60]. Is there a reason why these physiological measurements yielded heterogeneous results? There is a possibility that some studies ^[39][42][61,71] did not take into account factors related to the environment such as temperature, humidity, negative air ions, and illumination. In previous studies, it has been demonstrated that many environmental factors affect physiological health ^[42][71]. The arrangement of the activity may also be a contributing factor. As stated in one study in which a pulse rate decrease was not observed, participants were measured for their pulse rate right after making essential oil body wash, which was part of structured activities designed to stimulate the participants’ senses. As participants made the essential oil body wash, they walked around the classroom, which could have affected their pulse rates ^[39][61]. Therefore, measuring physiological parameters immediately following an activity may result in inaccurate results. There is also the possibility that it could simply be the result of inappropriate use of the measurements, as demonstrated in one study ^[48][57], which measured salivary cortisol twice: first as a pre-measure and then as a follow-up measure eight weeks after the intervention ended.

The psychological studies found significant improvement in the total Profile of Mood States (POMS) ^{[39][40][43][46][49]}[61,63,64,69,70] and Positive and Negative Affect Schedule (PANAS)NAS ^[40][64], EuroQol Visual Analog Scale (EQ-VAS)-VAS ^[46][70], Restorative Outcomes Scale (ROS)S, and Subjective Vitality Scale (SVS) ^[43]VS [63]; reduction in State-Trait Anxiety Inventory (STAI) ^[39]AI [61], Stress Response Inventory (SRI) ^[34][50][51]I [38,62,68], non-standard stress self-assessment survey ^[37][41][41,65], Worker’s Stress Response Inventory Modified Form (WSRI-MF)RI-MF ^[46][70], Maslach Burnout Inventory-Student Survey (MBI-SS)I-SS, and the Job-Seeking Stress Survey ^[52][66]; and an increase in ROS and SVS ^[43][63]. However, one concerning matter is that psychological measures are dominated by self-rated subjects. Haga, Halin, Holmgreen, and Sörqvist ^[53][81] have scientifically demonstrated the “nature-positive” bias in psychological restoration through an experiment in which participants were exposed to the same soundtrack while performing cognitively demanding tasks. One group was told the sound came from a waterfall (nature sound-source condition), while the other was told it came from an industrial environment with active machinery (industrial sound-source condition). The findings showed that participants in the nature sound-source condition experienced more psychological restoration than those in the industrial sound-source condition. Due to the fact that the psychological measures relate to self-referred individuals, there is every reason to suspect that the results may be biased in a “nature-positive” direction.

Forest therapy programs lasting between two and six days appear to be physiologically more effective than interventions lasting less than one day in terms of stress reduction, when it comes to salivary cortisol levels. It has been proposed by Christup ^[54][82] that long-term interventions produce more significant changes than short-term interventions. The study confirmed the results of the previous study to some extent.

Forest therapy programs are still in their infancy, so there are only a few studies with similar designs that could be compared. Previous research into psychological stress recovery after forest therapy programs could support psychological effects. Results regarding physiological effects were more ambiguous. In conclusion, solid proof of forest therapy programs’ benefits for stress reduction has yet to be confirmed. There is a need for more robust empirical research studies in the future. Although the benefits of forest therapy programs are still being investigated, policymakers and health professionals should consider recommending forest therapy programs to their patients and the general public, since the intervention has low potential side effects and is likely to increase outdoor activity.