Effects of Forest Therapy on Participant-Reported Outcomes: A Randomized Controlled Dose-Comparison Trial Among the...

Background: Evidence regarding the optimal dosage of forest therapy is limited. The aim of this study was to compare the effects of two dosages of standardized forest therapy sessions on participant-reported outcomes in the general population. This study also explored whether these effects depended on a specific forest location. Methods: In this randomized controlled multisite trial taking place in three different German forests, participants of the general population were assigned to either a two-session group (2SG) of forest therapy of 2 h each with a break-day in between or three consecutive sessions (3SG) of forest therapy of 2 h each. The primary outcome was the between-group difference of 2SG versus 3SG of forest therapy on Total Mood Disturbance (TMD) using the Profile of Mood States (POMS) at day 3, immediately postintervention. Secondary outcomes were vitality (SVS-G), self-efficacy (GSE), physical/mental health (PH/MH), anxiety (STAI), physical symptoms (BLR/BLR'), perceived stress (PSQ), and perceived benefits of nature (PBNQ). A subset of questionnaires was used before and after each session (POMS, STAI, SVS-G, and PSQ). Pre- and postintervention differences were computed for comparisons within each group and between them using t test statistics, while analysis of variance (ANOVA) tested for differences between forests. Results: One hundred and seventy-one participants (91 in 2SG and 80 in 3SG; 56.1 ± 14.5 years) were included in the intention-to-treat analysis. No significant group differences were found for the TMD (p = 0.99), although there was an overall improvement in both groups (TMD: within-group p < 0.001 for both, 2SG: d = 0.95, 3SG: d = 0.81). No significant group differences were found for any secondary outcomes, although significant improvements were seen within groups for most outcomes. The ANOVA revealed neither statistically significant interactions between the three forests nor statistically significant interactions between the factors "group" and "location" for TMD. Conclusions: Participation in two versus three sessions of standardized forest therapy sessions with 2 h each may have similar beneficial effects on physical/mental health parameters in the general population. A low-dose approach may already achieve beneficial effects on mental health. These findings can provide evidence for the possible implementation of forest therapy as a therapy form in Germany.