Forest|Health: a pilot study on feasibility and exploratory outcomes of a modular forest therapy intervention in Mecklenburg-Western Pomerania

Abstract

Introduction

Nature-based interventions aimed at promoting health, enhancing well-being and supporting medical treatment have long been a focus of public health research. In particular, forest therapy and forest bathing are well established in Asia as evidence-informed approaches to stress reduction. In Europe, research on forest therapy is expanding although the field remains comparatively young. Existing studies have mainly focused on the short-term effects of single forest visits while evidence on multiweek interventions and potential medium-term to long-term effects remains limited. The project Forest|Health (Wald|Gesund), embedded within the research consortium T!Raum-Initiative One-Health-Region Pomerania located in northern Germany, aims to address this gap by developing a structured forest therapy intervention tailored to three different target groups.

Methods and analysis

The study protocol describes a pilot study to evaluate a modular forest therapy intervention with regard to feasibility indicators as well as stress-related and symptom-related exploratory outcomes. A total of 120 participants will be recruited across three target groups: community-dwelling adults aged 65 years and older, adults with arterial hypertension and adults with depressive symptoms recruited from treatment waiting lists. Using a multimethod approach, two preliminary studies were conducted: a survey of community-dwelling individuals in the region and focus groups with regional stakeholders. Additionally, a 2-week pilot study of the intervention was conducted. Based on these findings, a 4-week modular forest therapy intervention will be implemented. Participants will be assigned to intervention modules with distinct content using a quasi-randomised procedure. The intervention will follow a pre–post design with follow-up assessments. Feasibility indicators will be assessed as key process outcomes. The primary exploratory outcome will be stress while secondary exploratory outcomes will include psychological and physiological health-related measures.

Ethics and dissemination

The study was approved by the Ethics Committee of the University Medicine Greifswald. Participants will provide informed consent, receive information about potential risks and support options. The results will be disseminated through scientific publications, conferences and public outreach.

Trial registration

This study protocol is preregistered on the Open Science Framework (OSF; Registration DOI: 10.17605/OSF.IO/C2D5P; last update 16 December 2025).

STRENGTHS AND LIMITATIONS OF THIS STUDY.

• Intervention design: the study includes a 4-week, multimodular forest therapy intervention focusing on mindfulness-based exercises, body-based experiences and creative engagement.

• Included sample: the study comprises a broad sample from three target groups: adults aged 65 years and older, adults with arterial hypertension and adults with depressive symptoms.

• Methodological variety: the study adopts a multimethod approach, including a range of subjective and objective quantitative measures (self-report questionnaires, salivary cortisol, accelerometry and heart rate variability) as well as qualitative interviews to gain in-depth insights.

• Study limitations and mitigation: while the pilot nature of the study and the absence of a control group may limit causal inferences, transparent reporting and a rigorous, multimethod design are used to strengthen the validity of the findings.

Introduction

Scientific background and study rationale

Health-related research in Europe increasingly recognises integrative, nature-based therapies as a valuable addition to established prevention and treatment approaches.¹ This is based on a growing understanding of multifactorial health conditions that are shaped not only by biological processes but also by societal, environmental and psychosocial factors characteristic of modern life. Currently, many countries, particularly developed industrialised nations, are experiencing considerable demographic changes, including declining birth rates and a rising proportion of older people.^2–4 As a result, prevention and health promotion are becoming increasingly central to ensuring health and well-being across all age groups.^{2 5} With regard to somatic diseases, arterial hypertension is one of the leading risk factors for and contributors to the global burden of disease.⁶ As disease prevalence increases across the lifespan, pharmacological interventions tend to become more prevalent with increasing age. However, non-pharmacological lifestyle interventions, such as physical activity or mindfulness-based stress reduction, are integral components of comprehensive hypertension management across all age groups.^{7 8} In addition, mental disorders represent a growing public health concern worldwide, with the number of diagnoses steadily increasing. Within this spectrum, depression and depressive symptoms are among the most prevalent conditions.^{9 10} Current evidence and clinical guidelines highlight the importance of lifestyle-based interventions, including physical activity and peer support, to reduce the burden of depressive symptoms.^11–13

To address the growing overall health burden, there is a need for additional, resource-efficient, non-pharmacological interventions that integrate both curative and preventive approaches. A growing body of research indicates positive short-term effects of nature-based interventions, such as forest bathing and forest therapy. Beneficial effects of nature-based interventions span a wide range of mental and physical health parameters,^{14 15} including reductions in depressive symptoms,^{16 17} loneliness¹⁸ and blood pressure¹⁹ as well as increases in physical activity²⁰ and overall well-being.²¹ Additionally, several studies have reported positive effects of forest therapy on stress-related symptoms.^22–25

Stress is considered a particularly relevant health outcome due to its intercorrelations with multiple bodily systems and a wide range of physical and mental disorders.^{26 27} Evidence links stress to adverse health outcomes, including anxiety and depression,^{28 29} pain³⁰, cancer³¹ and cardiovascular diseases.³² Accordingly, nature-based interventions are increasingly seen as a promising approach to promoting overall well-being and mitigating stress as a public health burden.³³

Despite growing research interest,^{34 35} there is no standardised definition of nature-based interventions, including forest bathing or forest therapy. Within their framework for nature-based trials, Gobster et al³⁶ distinguish several core components that shape intervention processes and outcomes: (a) characteristics of the individuals participating in forest activities, (b) features of the forest environment, (c) the nature of the interaction between people and the forest (eg, type and intensity of activities) and (d) the effects of the forest on physiological, psychological and nature-based outcomes. This framework, therefore, provides a structured lens for understanding how the design of nature-based interventions may influence human-forest interactions and the contextual mechanisms underlying observed health effects. Supporting this notion, several studies have shown that mindfulness-based exercises elicit stronger effects when practised in natural environments than in urban settings.³⁷ This aligns with the notion of nature acting as a ‘third medium’, complementing the participants–therapist interaction and potentially amplifying therapeutic effects.^{36 38}

Overall, existing studies on nature-based interventions have shown promising effects on both mental and physical health, while also highlighting important limitations.^{14 15 24 25 39 40} First, many studies examine only short-term effects following single or very brief intervention exposures providing limited insight into sustained or cumulative benefits. Second, study samples are often small and homogeneous, restricting conclusions regarding heterogeneous samples or clinical populations. Third, although intervention formats vary widely, few studies systematically compare different intervention components or modules. Finally, the majority of evidence originates from Asian settings, raising questions about the transferability of findings to European populations and to region-specific environmental and implementation contexts. Unlike other therapeutic approaches, nature-based interventions are more closely intertwined with the contextual characteristics of the natural setting. Consequently, further research in European contexts is required to elucidate interest in, acceptance of, feasibility, and the potential health-related effects of nature-based interventions, with particular attention to the forest as a therapeutic setting and to region-specific conditions relevant to implementation and long-term integration.

Aim

The present intervention study, conducted within the project Forest|Health (Wald|Gesund), aims to develop and evaluate a multimodal, multiweek nature-based intervention in the federal state of Mecklenburg-Western Pomerania. Compared with other federal states in Germany, Mecklenburg-Western Pomerania has a high proportion of older adults^{41 42} as well as high prevalence rates of arterial hypertension and depressive symptoms.^{43 44} Regarding its natural environment, the federal state is characterised by a wide variety of natural areas⁴⁵ and has established spa and healing forests as a natural health resource within its state forestry law.⁴⁶ Therefore, Mecklenburg-Western Pomerania provides an interesting framework in terms of its regional conditions and study populations. The described feasibility trial will examine the (1) recruitment processes for nature-based interventions in the region of Greifswald, (2) implementation of and adherence to the 4 week intervention, (3) the acceptability of data collection methods and (4) exploratory short-term and long-term associations between the intervention and stress as the primary outcome as well as secondary psychosocial, physical, health-related and nature-based outcomes. The feasibility findings will be used to identify barriers and potential adaptation needs in preparation for a planned future controlled efficacy trial. Following a multimethod approach, the study will integrate quantitative and qualitative data.

Roles and responsibilities

Forest|Health is a collaborative project, in a larger research consortium (T!Raum-Initiative One-Health-Region Pomerania)⁴⁷ that combines the expertise of the University Medicine Greifswald, which conducts the subproject Forest|Medicine (Wald|Medizin) and the University of Greifswald, which carries out the subproject Forest|Psyche (Wald|Psyche). The Department of Prevention Research and Social Medicine of the University Medicine Greifswald serves as the lead partner of this project. As part of the Institute of Community Medicine, the department examines prevention and health promotion with a special focus on social inequality and healthy ageing. In the subproject Forest|Medicine, the department works closely with the Clinic and Policlinic for Internal Medicine B—cardiology and constitutes one of two cooperating partners in the project. The focus of the cardiological partner in this project is on physiological biomarkers as well as population-based aspects of cardiovascular diseases. As part of the University of Greifswald, the Chair of Clinical Psychology and Psychotherapy at the Institute of Psychology serves as the project’s second cooperating partner in the subproject Forest|Psyche. Its affiliated Centre of Psychological Psychotherapy (Zentrum für Psychologische Psychotherapie, ZPP) provides evidence-based outpatient care with a focus on depressive disorders, thereby offering an essential clinical environment for recruitment and intervention research. All three project partners address prevalent health problems that encompass both preventive and curative perspectives, thus have high public health relevance. The project partners combine together different perspectives and target groups while intervention and data collection are conducted jointly.

Preliminary work

Following the ORBIT model ('Obesity-Related Behavioral Intervention Trials’),⁴⁸ the intervention has been developed iteratively. The study described in this protocol represents the main part (Pilots) of phase II of the ORBIT model (Proof of Concept and Pilots), in which feasibility indicators are tested in an initial intervention. The development process of this intervention (ie, phase I: Design and Refinement) focused on the definition of the targets and contents. It was mainly informed by three components: a certificate course in forest therapy⁴⁹ and findings from two preliminary studies including (a) a population-based postal survey of Greifswald’s population based on a random sample from the residents’ registration office to assess nature-based activities and interests and (b) focus group interviews with stakeholders from several areas including forestry and healthcare as well as representatives of the target groups.⁵⁰ Before conducting the main intervention, all components (recruitment, intervention content and assessments) were tested in a 2-week study representing the ‘Proof-of-Concept’ part of the ORBIT model (ie, phase IIa).

Methods and analysis

The study protocol was prepared according to the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) 2025 statement.

Trial design

The multigroup intervention feasibility study will represent the main part of the Forest|Health project. The 4-week, multimodal nature-based intervention will follow a multifactorial design (Module×Time) with a pre–post assessment structure including a total of nine measurement time points (figure 1). Follow-up assessments will be conducted up to 6 months after the intervention. Intervention sessions will be scheduled on a weekly basis. In the case of cancellations due to external factors (eg, adverse weather conditions), sessions will be rescheduled to ensure that all participants have the opportunity to attend a total of four intervention sessions. Further insights regarding feasibility and participant impressions will be obtained through qualitative interviews conducted with a subsample.

Figure 1. Study process: measurement time points and assessments (created with BioRender (https://www.biorender.com/)).

Participants

Eligibility criteria

The study focuses on three target groups: (a) community-dwelling adults aged 65 and older; (b) adults aged at least 18 years with arterial hypertension and (c) adults aged at least 18 years with depressive symptoms. Eligibility criteria are divided into two components: all participants are required to meet general inclusion criteria and additional criteria specific to each target group. To meet the general inclusion criteria, individuals must be able to reach the meeting point independently of the study team, with or without assistance from third parties and must be able to take part in a forest therapy session lasting up to two hours. Detailed information on the specific inclusion and exclusion criteria for each target group is provided in table 1.

Table 1. Inclusion and exclusion criteria of study participants.

Inclusion criteria	Exclusion criteria
Adults aged 65 years and older
Community-dwelling adults aged 65 and older Adults independently living in a private household	Adults living in a care facility Adults diagnosed with dementia or significant cognitive impairment Adults who receive care due to at least considerable impairment, ie, medical care level of II to V according to the German social security code (Sozialversicherungsgesetzbuch XI, SGB XI)
Adults with arterial hypertension
Adults diagnosed with arterial hypertension and undergoing treatment (medication or lifestyle changes)	Adults with uncontrolled arterial hypertension (ie, blood pressure ≥180/110 mm Hg) or unstable cardiovascular conditions
Adults with depressive symptoms
Adults on the waiting list for therapy at the psychotherapeutic outpatient clinic of the University Greifswald or on a waiting list of any regional outpatient clinic or individual psychotherapist Confirmed or suspected diagnosis of mild to moderate depressive disorder, documented after a psychotherapeutic consultation and prior to placement on a treatment waiting list, corresponding to ICD-10 codes: F32.0, F32.1, F33.0, F33.1 and F34.1	Confirmed or suspected diagnosis of severe depressive disorder, documented after a psychotherapeutic consultation and prior to placement on a treatment waiting list, corresponding to ICD-10 code F33.2, or with a confirmed or suspected diagnosis of bipolar disorder (ICD-10 F31), regardless of the current episode Acute suicidality, irrespective of the primary psychiatric diagnosis Active substance abuse (defined as less than 6 months of abstinence) Confirmed or suspected diagnosis of severe personality disorders, psychosis or posttraumatic stress disorder, documented after a psychotherapeutic consultation and prior to placement on a treatment waiting list

Sample size

Although an a priori power analysis was undertaken during the planning phase, it indicated limited statistical power to detect small to medium size potential effects. Therefore, it served an orienting purpose rather than ensuring sufficient power for confirmatory hypothesis testing. As is customary for feasibility studies in the context of psychosocial research, this study was not designed to formally test intervention effects and does not include a control group. Instead, the sample size reflects pragmatic considerations, including the seasonal timeframe, recruitment capacity and available resources, while still maintaining a sufficient number of participants to generate descriptive statistics, and to estimate effect sizes and calculate corresponding CIs with reasonable precision.⁵¹ In addition, the planned sample size will enable the estimation of key feasibility parameters, including recruitment and enrolment rates, participant retention, intervention adherence and data completeness, to be estimated for the total sample and descriptively within each of the three target groups. The planned sample size of 40 participants per target group is intended to support descriptive feasibility estimates and the identification of major procedural challenges, rather than enabling confirmatory subgroup comparisons. Preliminary outcome analyses will, therefore, be interpreted as exploratory and hypothesis-generating and will primarily serve to estimate variability, effect sizes and CIs for the planning of a future, fully powered, randomised controlled trial. Consequently, the target sample size will be 120 participants, with 40 participants from each of the three target groups. Regarding the qualitative interviews, a total sample of 12 participants will be targeted, drawn from the total sample. The participants will be selected in order to ensure maximum heterogeneity in terms of sociodemographic parameters and the subjective experiences of the forest therapy instructors. Additionally, a purposive subsample of participants who discontinued the intervention will be interviewed to gain further insights into the reasons for dropout and to identify potential limitations of the study design.

Recruitment

Recruitment approaches will be adapted for each target group and will be conducted concurrently. Community-dwelling adults aged 65 years and over, as well as adults with arterial hypertension, will be recruited from the general population using flyers, posters, local media and press releases. Interested individuals will undergo a telephone screening, after which those meeting the eligibility criteria will be invited to an in-person meeting.

Participants with depressive symptoms will be recruited primarily from the waiting list of the Centre for Psychological Psychotherapy and cooperating outpatient clinics. Individuals will have attended an initial psychotherapeutic consultation, after which they will have been placed on the clinic’s internal waiting list and will have received a suspected diagnosis of mild to moderate depression according to ICD-10 criteria. If the suspected diagnosis meets the inclusion criteria and none of the exclusion criteria apply, the study team will contact individuals proactively via telephone. To increase recruitment reach, the strategy will additionally include patients on waiting lists of psychotherapeutic outpatient clinics and of licensed psychotherapists in the extended urban area of Greifswald. In addition, press releases will be disseminated in regional newspapers, and informational materials will be distributed to private–practice psychotherapists for their own reference and for distribution to patients on their waiting lists. Interested individuals will be invited to contact the study team. They will be included in the study if they are already registered on a waiting list, or if they register for one prior to enrolment. They will then be invited to an in-person meeting, during which inclusion and exclusion criteria will be assessed.

The preassessment will primarily be conducted to formally enrol participants (ie, informed consent). For participants in the target group with depressive symptoms, the preassessment will additionally include a semistructured preassessment protocol based on established clinical screening instruments, to ensure systematic coverage of safety-relevant domains. These instruments will be used as orientation aids rather than standardised diagnostic tools and will include the following: the Patient Health Questionnaire (PHQ-D) modules for major depression and other depressive syndromes,^{52 53} additional screening questions for other mental disorders derived from the PHQ-D as well as clinical reference literature. These include the Trauma Screening Questionnaire for post-traumatic stress symptoms,^{54 55} and a brief clinical evaluation of schizophrenia spectrum or psychotic disorders as well as antisocial personality disorder. This evaluation is informed by selected elements adapted from the Basel Screening Instrument for Psychosis (BSIP).⁵⁶ These tools will be used to structure the clinical assessment and to inform clinical judgement regarding eligibility, but will not be administered as fully standardised diagnostic interviews.

Intervention content and procedure

The intervention will comprise three modules, developed based on the forest therapy certificate course and preliminary studies. Nature-associated methods, as well as the forest as a symbol and metaphor, formed the basis of all sessions, while the content of the modules was designed to remain as distinctive as possible. All modules are structurally equivalent in terms of number of sessions, session length and delivery format. Each session will be led by a certified forest therapy instructor (‘forest therapist’) with a psychology background and will be accompanied by a second project team member (eg, student assistant; ‘cotherapist’). All sessions follow a standardised manual specifying session objectives, therapeutic components and exercises to promote consistent intervention delivery. Intervention fidelity will be further supported and documented by structured session reports completed by the forest therapy instructors after each session. These reports will include information on intervention delivery, group dynamics, any deviations from the planned procedures and any adverse events. The first meeting will serve as an introductory session focusing on group formation, familiarisation with the intervention procedures and natural environments and introduction to the core therapeutic principles (see online supplemental table S1). Each of the three subsequent sessions will be structured according to the assigned content modules and combine experiential exercises, therapist-guided group discussion, reflection on individual experiences, peer exchange and psychoeducation, while maintaining a comparable duration and structure across modules. They will take place in the nature reserve Eldena in Greifswald and are planned to last a maximum of two hours. The complete intervention manual will be published separately. A detailed overview of the intervention modules, including session objectives, key therapeutic components and examples of standardised exercises, is provided in online supplemental table S2. The three modules focus on the following main content:

1. Present-moment awareness and mindfulness.

2. Physical activity and body-based experience.

3. Creative engagement and expression in nature.

The intervention groups will consist of at least three and a maximum of nine participants from any of the three target groups. This small group format is chosen to facilitate active participation, individual guidance during exercises and peer interaction. To ensure a structured procedure (eg, content assignment, etc), participants will be encouraged to remain within their initial intervention group. However, if individuals attend only the initial appointment and are unable to continue participating, they will be offered the opportunity to transfer to another intervention group for the module-specific part of the intervention. Each forest session will be accompanied by brief preassessments and postassessments to capture immediate responses in the empirical setting. No formal home practice will be given during the intervention. However, participants will be encouraged to integrate exercises into their everyday lives.

Trial settings and nature areas

The study will be conducted in Greifswald, a medium-sized university city on the Baltic Sea and characterised by a variety of natural areas, including nature reserves. Preassessments and a basic physiological measurement will be conducted at the outpatient study centre of the Cardiology Department of the University Medicine Greifswald. The intervention will take place in two natural areas. As part of the Botanical Institute of the University of Greifswald, the Arboretum is a park-like area which features a global variety of trees and plants. The Eldena Nature Reserve is located at the northern outskirts of Greifswald and offers a mixture of old trees (mainly beeches and oaks) and natural regeneration areas, especially in its southern natural forest plots, where the intervention will mainly take place. Both nature reserves are accessible by public transport and have a network of trails and pathways. Sessions will be conducted under typical outdoor conditions. However, sessions will be postponed in the event of severe or persistent rainfall, thunderstorms or following severe storms and rescheduled whenever possible within the subsequent week. During the sessions, instructors adapt the pace and timing of breaks according to weather conditions and participants’ needs.

Participant allocation

Participants will first be assigned to an intervention group based on their appointment availability. Once recruitment for an intervention group has been completed, the group will be allocated to one of the three intervention modules by a project coordinator using a quasi-randomised allocation procedure. The first intervention group will be assigned to the mindfulness module, after which subsequent groups will be allocated sequentially to one of the three modules according to the quasi-randomisation algorithm. Participants eligible for two target groups, such as individuals aged 65 and over with arterial hypertension, will be distributed equally between these two target groups. The resulting allocation schedule will be shared among the project partners via a secure online platform hosted by the University of Greifswald. Due to the study design and the content of the modules, no explicit blinding will be performed. As allocation will only be performed once each intervention group has been recruited, and as the study is not designed as a randomised controlled trial, allocation concealment will not be implemented. A quasi-randomised rather than a fully randomised allocation procedure was chosen to achieve a balanced distribution of target groups and modules and because it aligns with resource-efficient implementation, while also allowing for the concurrent assessment of feasibility indicators. However, this approach limits internal validity, as non-random group assignment cannot rule out selection bias or unobserved confounding.

To achieve the most balanced allocation of heterogeneous intervention groups across the three intervention modules, an iterative allocation algorithm will be applied using a programmed quasi-randomisation tool developed by one of the authors. Before each new intervention group starts the intervention, all three possible module assignments will be evaluated, and the assignment resulting in the smallest imbalance across target groups and intervention modules will be selected. Closeness to equal distribution will be assessed using the χ² statistic. Although perfect balance cannot be guaranteed because of the sequential recruitment process, this procedure optimises module allocation based on the information available at each allocation step. Elements of randomness could be integrated into the procedure in a variety of ways. They are opted against as (a) the unpredictable inclusion of new participants effectively provides pseudo-randomness and (b) no objective way of tuning the influence of randomness exists. Subsequent groups will be assigned based on the updated sample assignment, overseen by the project coordinators.

Outcomes

The outcomes will comprise feasibility indicators as well as exploratory psychosocial, health-related and nature-based constructs, which will be measured using a multimethod approach. Data will be collected through standardised procedures (ie, questionnaires, physiological assessments) and semistructured interviews. To minimise participant burden and to optimise the assessment of key outcomes, the measures will vary across time points (see table 2). Depending on the measurement time points, parameters are considered as either state or trait. To further put results into context, potential confounding factors, such as outdoor information (eg, rain, temperature, mosquitoes) and adverse events during the intervention sessions, will be monitored and documented.

Table 2. Outcomes and measurement time points.

	Baseline	Intervention	Posttest	Follow-up
Construct (abbreviation of instrument)	T0	T1 – T6	T7	T8
Sample characteristics
Sociodemographic variables	X
Medication	X
Primary exploratory outcome
Stress (PSQ-20)	X*		X*	X*
Secondary exploratory outcomes
Affect (PANAVA-KS)	X†	X‡	X†	X†
Stress single item (visual analogue scale 0–100)	X†	X‡	X†	X†
Inclusion of nature in self (INS)	X		X	X
Time spent in nature	X		X	X
Experience of nature (ENS)	X		X	X
Subjective age	X		X	X
Future selves	X		X	X
Self-rated Health (SF-8)	X		X	X
Depressiveness (PHQ-9)	X		X	X
Physical activity (EHIS-PAQ)	X		X	X
Loneliness (UCLA-LS)	X		X	X
Self-efficacy (GSE)	X		X	X
Personality (BFI-10)	X		X	X
Physical measurements
Height, weight, waist circumference	X
Blood pressure (mm Hg)	X
12-lead ECG	X
6-min walk test	X
Salivary cortisol		X
Heart rate variability§		X
Accelerometry¶	X	X**
Qualitative Interview††	X‡‡			X§§

Outcomes: PSQ-20: Perceived Stress Questionnaire; PANAVA-KS: Short scales for measuring positive activation, negative activation and valence in experience sampling studies; INS: Inclusion of Nature in Self ; ENS: Experience of Nature, Subjective Age, Future Selves (short form); SF-8: Self-rated Health; PHQ-9: Patient Health-Questionnaire; EHIS-PAQ: European Health Interview Survey Physical Activity Questionnaire ; UCLA-LS: UCLA-Loneliness Scale ; GSE: Generalised Self-Efficacy Scale ; BFI-10: Big-Five-Inventory.

^*Reference time point: past 4 weeks.

^†Reference time point: past weeks.

^‡Reference time point: current time point.

^§Polar Verity Sense (Polar Elektro, Kempele, Finnland).

^¶ActiGraph GT3X, ActiGraph, Pensacola, FL, USA, worn for 7 consecutive days 24 hours.

^**Only provided at T6.

^††Qualitative questions were adapted.

^‡‡Prior positive and negative experiences and personal connection to nature.

^§§Experience in relation to the intervention.

Feasibility indicators

In line with established concepts of feasibility in psychosocial and intervention research and following the feasibility framework proposed by Gadke et al,⁵⁷ feasibility will be assessed across several complementary domains, including recruitment capability, data collection procedures, social validity, practicality, implementation, safety and preliminary effectiveness. Together, these domains provide information on the feasibility of the intervention, study procedures and the potential utility of the intervention for future controlled evaluations (see table 3). Participant-related and process-related indicators will be assessed quantitatively and qualitatively.

Table 3. Feasibility domains and corresponding indicators in the Forest|Health pilot study.

Feasibility domain	Indicators in the present study
Recruitment capability	Recruitment rate, enrollment rate, recruitment duration, reasons for non-participation
Data collection procedures	Questionnaire completion rates, physiological assessment completion, accelerometer compliance, salivary cortisol sample completeness
Social validity	Participant experiences, acceptability, perceived relevance, individual fit (qualitative interviews)
Practicality	Feasibility of forest-based assessments, environmental and logistical challenges, assessment burden
Implementation	Session attendance, retention and dropout rates, postponed sessions
Safety	Adverse events and unintended negative effects
Preliminary effectiveness	Exploratory changes in stress and secondary psychosocial, health-related and nature-related outcomes

Recruitment capability will be evaluated through recruitment-related indicators, including recruitment rates, enrollment rates, recruitment duration and reasons for non-participation. Data collection procedures will be evaluated by examining the feasibility and completeness of questionnaire assessments, physiological assessments, accelerometer measurements and salivary cortisol sampling. Social validity will be assessed through participants’ perceptions of the intervention and study procedures, including acceptability, perceived relevance, individual fit and overall experiences. These aspects will be explored through semistructured qualitative interviews conducted after the intervention. Practicality will focus on the feasibility of conducting intervention sessions and assessment procedures in a forest setting. Particular attention will be given to logistical challenges, environmental conditions, assessment burden and the feasibility of data collection under natural outdoor conditions. Implementation will be assessed through indicators reflecting intervention delivery and adherence, including retention and dropout rates and their timing, the number of intervention sessions attended and the number of postponed intervention sessions. Safety will be assessed through the documentation of adverse events and unintended negative effects occurring during the intervention period. Additionally, factors facilitating or hindering intervention delivery will be explored qualitatively. Additionally, the recruitment materials and the process of initial contact, the total duration of the intervention and the time required for each session, as well as the feasibility of assessments in the forest, will be evaluated in the subsequent qualitative interviews.

Given the exploratory nature of this pilot study, no formal go-/no-go progression criteria were prespecified. Instead, feasibility findings will be used to identify procedural challenges, barriers to participation and potential needs for adaptation and to inform the design, implementation and methodological refinement of future randomised controlled trials. The decision regarding further development towards a controlled efficacy trial will be informed by an integrated assessment of the feasibility domains described above, with particular attention to recruitment capability, participant retention, intervention adherence, acceptability, individual fit, participant burden, practical implementation and the completeness and feasibility of outcome assessments. Quantitative feasibility indicators and qualitative accounts of participants’ experiences, including those of participants who discontinue the intervention prematurely, will be considered jointly to identify barriers and specific procedural or intervention-related adaptations required before a future controlled trial.

Exploratory outcomes

Primary exploratory outcome

Stress will be the primary exploratory outcome because it constitutes a prevalent risk factor across diverse groups in the general population and relates to each of the three target groups, thereby enabling comparability across the heterogeneous samples. The Perceived Stress Questionnaire (PSQ-20), a comprehensive economic instrument comprising four subscales (worries, tension, joy and demands), has been defined as the primary indicator. The potential short-term effects will be analysed intraindividually by comparing T0 and T7, while the potential long-term effects will be analysed in comparison to the 6-month follow-up at T8. Due to its holistic nature, stress will additionally be measured in several ways, including self-reports (eg, PSQ-20), biomarkers²⁹ and physiological or behavioural changes^{58 59} to compare results with the primary indicator.

Secondary exploratory outcomes

Secondary exploratory outcomes will be selected based on their associations with stress and include psychosocial constructs (eg, positive and negative affect, views on ageing, self-efficacy) and health-related parameters (eg, depressive symptoms, physical activity). Further secondary exploratory outcomes will capture nature-associated constructs (eg, nature connectedness). Detailed information is structured in table 2.

Physiological measures

To identify any major health concerns prior to the intervention, and to provide baseline information, participants will undergo a series of brief physiological tests, including measurements of height, weight and waist circumference, a 6 min walk test and a 12-lead ECG. Before and after the 4-week intervention, participants will be asked to wear accelerometers (GT3X) for 7 days in order to assess physical activity. During the intervention phase, salivary cortisol and heart rate variability will be assessed to obtain physiological information on the primary exploratory outcome of stress. Based on findings from preliminary testing, blood pressure will not be assessed during the intervention phase as this procedure was shown to be not feasible in the forest setting.

Qualitative questions

In addition to quantitative assessments, a range of qualitative variables will be assessed through open questions. In the preassessment prior to the intervention, the questions asked will focus on individual positive and negative experiences with nature and forest environments as well as personal nature connectedness (eg, living area, childhood experiences) to gain practical implications for the intervention period. To gain further insights regarding the feasibility and acceptance of the intervention as well as to better contextualise quantitative results, semistructured interviews will be conducted with a subsample after the final assessment timepoint. Two main topics will be addressed:

1. Experience and acceptance of intervention content, potential effects and individual fit.

2. Experience of technical structure of recruitment, assessment processes, communication

Data collection process

The participants will pass through different study phases, as shown in figure 1. Following verification of the inclusion criteria during the telephone screening process, eligible participants will be invited to a preassessment, in which informed consent will be obtained and questions will be clarified. For individuals from the target group with depressive symptoms, an additional brief eligibility assessment will be conducted. As a part of the inclusion criteria, all participants will undergo a basic physical assessment at the preassessment (table 2) and will be given an accelerometer and a questionnaire (T0), which should be completed before their first appointment at the Arboretum. For their first session, the group will meet in the Arboretum in Greifswald and in the following 3 weeks the sessions will take place in the nature reserve Eldena. Before and after each session in the forest, a minor assessment will take place (T1–T6), comprising a short stress questionnaire, salivary cortisol sampling and the application of a wrist-worn pulse sensor (table 2). To provide further context for each session, the forest therapy instructors will issue a report describing, among other aspects, meteorological data (eg, temperature and rainfall), details of the session, adherence to the planned intervention procedures (eg, any unexpected or adverse events) and an overall evaluation of the group dynamics. At the last appointment of the 4-week forest therapy intervention, participants will again receive an accelerometer for the postassessment. In the subsequent week, the participants will receive a questionnaire by mail (T7) and finally, individuals will receive a questionnaire for the follow-up assessment by mail (T8) 6 months after the postassessment. Participants will be able to alternatively complete the questionnaire online if they prefer. To minimise dropout, participants who do not complete the questionnaire will be reminded by telephone 2 weeks after receiving the questionnaire by mail at T7. To ensure consistency, the same procedure will be followed for T8. Qualitative interviews will be conducted after the individual T8 assessment has been completed.

Public and participant involvement

Public involvement was included during the preliminary studies and targeted the specification of intervention content. During the intervention, public involvement will focus on supporting participant recruitment (eg, dissemination of posters and flyers) and additionally on consultation with the managers of the nature areas where intervention sessions will take place.

Data management

Data will be collected in two ways. Participants will either complete paper-based questionnaires or will access the questionnaires independently via the REDCap online software platform (Research Electronic Data Capture)^{60 61} using a mobile device. The REDCap system is hosted by the University Medicine Greifswald.⁶² Data security and quality management will be ensured by defining user roles and by including individually generated access codes for each participant and point in time. Responses collected via paper questionnaires will be entered into the REDCap system by a research assistant and will be verified by a second team member to ensure accuracy. Data backup will be performed weekly on an internal server of the University Medicine Greifswald. A standard operating procedure (SOP) will be adapted for all implemented procedures (eg, recruiting, screening, informed consent, collection of salivary cortisol, application of sensors, data entry, questionnaire-based data collection) and any upcoming difficulties will be discussed in the project team.

Planned statistical analysis

Following the pilot character of the study, all analyses will be considered hypothesis-generating and will be interpreted with caution. The primary aim of the quantitative analyses is to assess feasibility, estimate potential intervention effects and generate information for the planning of future confirmatory trials. However, the factorial longitudinal design will allow for evaluation using repeated-measures analyses, including variance-analytic approaches and, where appropriate, mixed-effects models and regression models including both within-subject and between-subject effects. The statistical analyses will primarily focus on changes within groups in the primary exploratory outcome (stress) and the secondary exploratory outcomes. Additionally, between-group differences focusing on intervention content and target groups will be explored. The intervention module, target group, time and seasonal factors may be examined as explanatory factors, including exploratory interaction effects where appropriate. Additionally, confounding factors, such as sociodemographic information, time spent in nature or psychological parameters will be considered in the analyses. Potential clustering effects arising from the delivery of the intervention in small groups will be considered during model selection where appropriate. Missing data and dropout rates will be reported descriptively. Comparisons between participants who complete the study and those who discontinue participation will be conducted to assess potential selective attrition. Where statistically appropriate, missing data will be handled using state-of-the-art estimation procedures, including Full Information Maximum Likelihood approaches. Where applicable, effect sizes and corresponding 95% CIs will be reported. Given the exploratory nature of the pilot study and the large number of outcomes assessed, no formal adjustment for multiple testing is planned. Consequently, the findings will be interpreted as exploratory and hypothesis-generating, with emphasis placed on estimation and effect sizes rather than formal hypothesis testing. Analysis will be conducted using established programmes, such as SPSS,⁶³ Stata⁶⁴ or R.⁶⁵ Qualitative interviews will be analysed according to Kuckartz⁶⁶ using MAXQDA software.⁶⁷

Ethics and dissemination

This study has been approved by the ethics committee of the University Medicine Greifswald (Ref. BB 013/25) and has been preregistered on OSF⁶⁸ including ongoing adaptations (last updated 16 December 2025). The preregistration is maintained as a living document, and all protocol amendments are documented within the OSF project, along with the corresponding dates and rationale for the changes. All participants will be informed about the study contents, the potential risks of spending time in nature, such as tripping, falling branches and tick bites, and the data protection regulations. A project-specific insurance will be provided to cover these risks. Prior to the intervention, participants will also receive written information on typical forest-related hazards and recommended safety precautions. Participants with depressive symptoms will be monitored throughout the intervention for adverse psychological events. In the event of clinically relevant psychological deterioration or suicidality, a predefined safety protocol will be implemented, including clinical assessment by the intervention staff, a structured suicide risk assessment, consultation with a licensed psychotherapist and referral to appropriate mental health services if required. Should participants exhibit signs of psychological distress or clinical deterioration, or if concerns arise during these informal check-ins or observations, individual follow-up discussions will be conducted immediately. The predefined safety protocol will be initiated where appropriate. Additionally, participants will be informed that they might experience negative feelings while completing the questionnaires and during the intervention phase. Participants may pause or discontinue study procedures at any time. If psychological distress occurs during assessments or intervention sessions, immediate support will be provided by trained study staff and, if necessary, consultation with a licensed psychotherapist will be arranged. For participants with arterial hypertension, blood pressure is measured at the initial appointment before the intervention begins. Participants with recurrent hypertensive derailments or very high blood pressure on screening are excluded. If the results indicate clinically relevant findings, participants are asked to consult their primary care physician. In the case of a suspected hypertensive crisis during the intervention, forest therapy instructors are trained to follow an SOP to address the situation. All adverse events and corresponding actions will be documented. The well-being of all participants will be routinely monitored through direct contact with the forest therapy instructors and cotherapists before, during and after each intervention session. Individuals who will be excluded during the preassessment, for example, due to the severity of their depressive or cardiological symptoms, will be informed about alternative treatment options (eg, ‘eHelp’ app or consultation of a cardiologist). A standardised procedure will be in place for any emergency. All participants will provide written consent in accordance with the Declaration of Helsinki.⁶⁹ Dissemination will be conducted in a multimodal approach including research communities (eg, international peer-reviewed open access journals, presentations at conferences^70–73), the broad public (eg, newspapers and public presentations) and study participants (eg, flyer with descriptive overview of results). The results will also be disseminated among experts and stakeholder groups in order to provide practical implications in the field of public health and to initiate further developments in the field of nature-based interventions.

Discussion

This paper describes the protocol of a feasibility study investigating a 4-week forest therapy intervention within the Forest|Health project, targeting three groups of individuals: adults aged 65 years and over, adults with arterial hypertension and adults with depressive symptoms. Nature-based interventions represent a promising source for health promotion as well as for the treatment and prevention of health conditions. A particular advantage of these interventions is their ability to address a wide range of health conditions and to target a broad spectrum of people. The present study aims to address current gaps in forest therapy research by conducting a multiweek intervention designed to explore potential medium-term to long-term effects, while meeting high methodological standards and following a multimethod approach in a classic pre–post design with subjective and objective parameters.^{14–21 25 39 40} In addition, the study is designed to support regional capacity building and interdisciplinary transfer, with a particular strength being the provision of forest therapy courses free of charge to a broad population. The three target groups addressed in this study are of high public health relevance,^{5 6 10} as nature-based interventions are a promising source for promoting general well-being and combating stress, which represents a common burden in the general population.²⁴

Several potential limitations of the study should be considered. Recruitment strategies will differ across target groups. Adults aged 65 years and older and individuals with arterial hypertension will be primarily recruited through media, advertisements, notices or flyers, which may result in a positively selected sample with greater interest in nature, physical activity or health. In contrast, individuals with depressive symptoms will be recruited through clinical waiting lists, involving stricter access criteria (eg, prior diagnostic consultation and documented suspected diagnosis). However, similar self-selection mechanisms may still occur in the target group of adults aged 65 years and older and individuals with arterial hypertension, as participation requires individuals to actively express interest in a nature-based intervention. Thus, despite differing recruitment pathways, selection bias favouring individuals open to outdoor or nature-related activities cannot be ruled out across target groups and may additionally be distorted by expectation bias. Moreover, the group-based format of the intervention may entail both positive (eg, social support) and negative (eg, social comparison) group dynamics, which limits the ability to analyse potential group and intervention effects separately and should be considered when interpreting results.⁷⁴ Additionally, the possible influence of cointerventions cannot be ruled out. However, in particular among the target group of adults with depressive symptoms, the exclusion criteria comprised those currently undergoing psychotherapy. Furthermore, as a feasibility study, the intervention will not include a control group, consequently no causal interpretations are possible and the account of methodological biases (eg, regression to the mean) is limited. The present study focuses on three exemplary target groups; module allocation follows a quasi-randomised allocation procedure. Blinding of the researchers was not feasible due to the overlap in individuals involved in the design and implementation of the intervention process. Regarding the participants, no proactive announcement of the specific intervention module was provided. However, participants may have recognised module content, for example, based on the information provided prior to the intervention phase, so that no formal blinding of the participants was aimed for. Confounding factors related to seasonal and environmental conditions could not be eliminated, so individual protocols were assessed for each session to account for these. The study is conducted as a single-site study, which may limit its generalisability to other regions or populations (eg, age groups). The feasibility of conducting assessments directly before and after each intervention module may be challenging in forest environments such as the Eldena nature reserve, where infrastructure (eg, shelters or seating) is limited. This contextual factor may influence the participant response behaviour and will need to be considered when interpreting results.

Finally, although the present study aims to advance the field beyond many existing studies on nature-based interventions, a longer intervention period may be required to fully explore intervention content and to foster sustained behavioural changes, such as habitual engagement with nature. However, the strong dependence of nature-based interventions on weather conditions and seasonal variability poses practical constraints on the implementation of longer intervention durations. Future research should, therefore, build on these findings by employing fully powered randomised controlled trial designs (ie, ORBIT model phase III: Efficacy).

Trial status

According to the ORBIT framework, two preparatory studies were conducted in 2024, including questionnaire surveys and focus groups with stakeholders (ie, ORBIT model phase I). These studies informed the contextual relevance, target group selection and intervention development. Following this preparatory work, a 2-week proof-of-concept study was conducted from May to June 2025 (ie, ORBIT model phase IIa). A total of 19 participants from all three target groups across four intervention groups were included in this trial. Recruitment procedures, intervention components and assessment logistics were tested and refined. The present pilot study (ie, ORBIT model phase IIb) builds on both the preparatory studies (ie, ORBIT model phase I) and the 2-week proof-of-concept study (ie, ORBIT model phase IIa). Recruitment for the pilot study was initiated in June 2025. The intervention phase was conducted between June and November 2025. Starting in January 2026, follow-up assessments were carried out 6 months after completion of the intervention lasting until May 2026. The qualitative interviews are conducted between May and June 2026. This study protocol describes the study as it was finally implemented. The original study protocol was published on OSF⁶⁸ as part of the preregistration process prior to the start of the study and transparently includes all modifications (last update 16 December 2025). At the time of the initial submission, data collection (follow-up assessment and qualitative interviews) was ongoing. Data preparation and analysis are conducted concurrently.

Ethics approval and consent to participate

The study has been approved by the ethics committee of the University Medicine Greifswald (Ref. BB 013/25) and was preregistered on OSF⁶⁸ including ongoing adaptations. All participants were informed about the study contents and data protection regulations and provided written consent in accordance with the Declaration of Helsinki.⁶⁹

Consent for publication

All participants agreed on the publication of the collected data as part of their informed consent.

Availability of data and materials

The datasets generated during the current study will not be publicly available due to potentially reidentifiable health-related information. Deidentified data, statistical analysis code and additional study materials may be made available from the corresponding author upon reasonable request, subject to approval by the responsible ethics committee and in accordance with applicable data protection regulations.