Participants allocated to the noticing-nature active control group will be instructed to walk in nature for 30 minutes per day and pay attention to how the natural objects/scenes they encounter in their daily surroundings make them feel over the 14-day period.

Measured using the Spiritual Attitude and Involvement List-Short Form (SAIL-SF; Bohlmeijer et al., 2023). The SAIL-SF measures seven dimensions of spiritual well-being: meaningfulness, trust, acceptance, caring for others, connectedness with nature, transcendent experiences, and spiritual activities. Each dimension is assessed by one item (e.g., "I experience the things I do as meaningful", "I try to take life as it comes"). Items are rated on a 6-point Likert scale ranging from 1 (not at all) to 6 (to a very high degree), with higher scores indicating greater spiritual well-being. The total score is calculated as the mean of all items.

Measured using the 22-item Traumatic Grief Inventory-Self Report Plus (TGI-SR+; Lenferink et al., 2022). Participants rated the extent to which they experienced each grief symptom during the past two weeks on 5-point Likert scale (1 = never to 5 = always). Total scores are calculated by summing all items (range: 22-110), with higher scores indicating greater grief intensity. A total score ≥71 indicates probable clinical DSM-5-TR PGD.

Measured using the 22-item Traumatic Grief Inventory-Self Report Plus (TGI-SR+; Lenferink et al., 2022). Participants rated the extent to which they experienced each grief symptom during the past two weeks on 5-point Likert scale (1 = never to 5 = always). Total scores are calculated by summing all items (range: 22-110), with higher scores indicating greater grief intensity. A total score ≥71 indicates probable clinical DSM-5-TR PGD.

Measured using the 22-item Traumatic Grief Inventory-Self Report Plus (TGI-SR+; Lenferink et al., 2022). Participants rated the extent to which they experienced each grief symptom during the past two weeks on 5-point Likert scale (1 = never to 5 = always). Total scores are calculated by summing all items (range: 22-110), with higher scores indicating greater grief intensity. A total score ≥71 indicates probable clinical DSM-5-TR PGD.

Measured using 1-item Inclusion of Nature in Self scale (INS; Schultz, 2001). The INS is a single-item pictorial measure that assesses the extent to which individuals include nature within their cognitive representation of self. The measure consists of seven pairs of circles labeled "self" and "nature" with varying degrees of overlap, from completely separate (1) to almost completely overlapping (7). Participants select the picture that best describes their relationship with the natural environment.

Measured using 1-item Inclusion of Nature in Self scale (INS; Schultz, 2001). The INS is a single-item pictorial measure that assesses the extent to which individuals include nature within their cognitive representation of self. The measure consists of seven pairs of circles labeled "self" and "nature" with varying degrees of overlap, from completely separate (1) to almost completely overlapping (7). Participants select the picture that best describes their relationship with the natural environment.

Measured using 1-item Inclusion of Nature in Self scale (INS; Schultz, 2001). The INS is a single-item pictorial measure that assesses the extent to which individuals include nature within their cognitive representation of self. The measure consists of seven pairs of circles labeled "self" and "nature" with varying degrees of overlap, from completely separate (1) to almost completely overlapping (7). Participants select the picture that best describes their relationship with the natural environment.

Measured using 1-item Inclusion of Nature in Self scale (INS; Schultz, 2001). The INS is a single-item pictorial measure that assesses the extent to which individuals include nature within their cognitive representation of self. The measure consists of seven pairs of circles labeled "self" and "nature" with varying degrees of overlap, from completely separate (1) to almost completely overlapping (7). Participants select the picture that best describes their relationship with the natural environment.

Measured using 14-item Connectedness to Nature Scale (CNS; Mayer & Frantz, 2004). The CNS measures individuals' trait levels of feeling emotionally and experientially connected to the natural world. Participants rated items such as "I often feel a sense of oneness with the natural world around me" and "I think of the natural world as a community to which I belong" on a 5-point Likert scale (1 = strongly disagree to 5 = strongly agree). The total score is calculated as the mean of all items after reverse-scoring items 4, 12, and 14, with higher scores indicating greater connectedness to nature.

Measured using 14-item Connectedness to Nature Scale (CNS; Mayer & Frantz, 2004). The CNS measures individuals' trait levels of feeling emotionally and experientially connected to the natural world. Participants rated items such as "I often feel a sense of oneness with the natural world around me" and "I think of the natural world as a community to which I belong" on a 5-point Likert scale (1 = strongly disagree to 5 = strongly agree). The total score is calculated as the mean of all items after reverse-scoring items 4, 12, and 14, with higher scores indicating greater connectedness to nature.

Measured using 14-item Connectedness to Nature Scale (CNS; Mayer & Frantz, 2004). The CNS measures individuals' trait levels of feeling emotionally and experientially connected to the natural world. Participants rated items such as "I often feel a sense of oneness with the natural world around me" and "I think of the natural world as a community to which I belong" on a 5-point Likert scale (1 = strongly disagree to 5 = strongly agree). The total score is calculated as the mean of all items after reverse-scoring items 4, 12, and 14, with higher scores indicating greater connectedness to nature.

Measured using 14-item Connectedness to Nature Scale (CNS; Mayer & Frantz, 2004). The CNS measures individuals' trait levels of feeling emotionally and experientially connected to the natural world. Participants rated items such as "I often feel a sense of oneness with the natural world around me" and "I think of the natural world as a community to which I belong" on a 5-point Likert scale (1 = strongly disagree to 5 = strongly agree). The total score is calculated as the mean of all items after reverse-scoring items 4, 12, and 14, with higher scores indicating greater connectedness to nature.

Measured using the Mental Health Continuum-Short Form (MHC-SF; Lamers et al., 2011), a 14-item scale measuring three dimensions of well-being: emotional (3 items), psychological (6 items), and social well-being (5 items). Participants rated the frequency of experiencing various feelings of well-being during the past month on a 6-point Likert scale (0 = never to 5 = every day). Scores are calculated as the mean of all items, with higher scores indicating greater well-being.

Measured using the Mental Health Continuum-Short Form (MHC-SF; Lamers et al., 2011), a 14-item scale measuring three dimensions of well-being: emotional (3 items), psychological (6 items), and social well-being (5 items). Participants rated the frequency of experiencing various feelings of well-being during the past month on a 6-point Likert scale (0 = never to 5 = every day). Scores are calculated as the mean of all items, with higher scores indicating greater well-being.

Measured using the Mental Health Continuum-Short Form (MHC-SF; Lamers et al., 2011), a 14-item scale measuring three dimensions of well-being: emotional (3 items), psychological (6 items), and social well-being (5 items). Participants rated the frequency of experiencing various feelings of well-being during the past month on a 6-point Likert scale (0 = never to 5 = every day). Scores are calculated as the mean of all items, with higher scores indicating greater well-being.

Measured using the Mental Health Continuum-Short Form (MHC-SF; Lamers et al., 2011), a 14-item scale measuring three dimensions of well-being: emotional (3 items), psychological (6 items), and social well-being (5 items). Participants rated the frequency of experiencing various feelings of well-being during the past month on a 6-point Likert scale (0 = never to 5 = every day). Scores are calculated as the mean of all items, with higher scores indicating greater well-being.

Perceived Body Boundaries Scale (PBBS; Dambrun, 2016). This scale comprises a single item designed to assess the strength of participants' perceived body boundaries.

A series of images of human forms with increasingly transparent boundaries to indicate gradations of body boundary salience. "Which images best represent your perception of your body's boundaries right now?" Responses range from 1 (no boundary dissolution) to 7 (a lot of boundary dissolution).

Perceived Body Boundaries Scale (PBBS; Dambrun, 2016). This scale comprises a single item designed to assess the strength of participants' perceived body boundaries.

A series of images of human forms with increasingly transparent boundaries to indicate gradations of body boundary salience. "Which images best represent your perception of your body's boundaries right now?" Responses range from 1 (no boundary dissolution) to 7 (a lot of boundary dissolution).

Perceived Body Boundaries Scale (PBBS; Dambrun, 2016). This scale comprises a single item designed to assess the strength of participants' perceived body boundaries.

A series of images of human forms with increasingly transparent boundaries to indicate gradations of body boundary salience. "Which images best represent your perception of your body's boundaries right now?" Responses range from 1 (no boundary dissolution) to 7 (a lot of boundary dissolution).

Perceived Body Boundaries Scale (PBBS; Dambrun, 2016). This scale comprises a single item designed to assess the strength of participants' perceived body boundaries.

A series of images of human forms with increasingly transparent boundaries to indicate gradations of body boundary salience. "Which images best represent your perception of your body's boundaries right now?" Responses range from 1 (no boundary dissolution) to 7 (a lot of boundary dissolution).

Spatial Frame of Reference Continuum (SFoRC; Hanley & Garland, 2019). This scale comprises a single item used to assess the extent to which the field of awareness is perceived to extend beyond the physical body.

How far does your self extend into the world? Using the letters and image below, please indicate how far you feel that your self extends beyond your physical body. "A" represents your self stopping at your physical body. "F" represents your self extending into everything (e.g., the entire Universe). The letters inbetween "A" and "F" represent different levels of self-extension. The rings are symbolic, and do not represent actual distances.

Spatial Frame of Reference Continuum (SFoRC; Hanley & Garland, 2019). This scale comprises a single item used to assess the extent to which the field of awareness is perceived to extend beyond the physical body.

How far does your self extend into the world? Using the letters and image below, please indicate how far you feel that your self extends beyond your physical body. "A" represents your self stopping at your physical body. "F" represents your self extending into everything (e.g., the entire Universe). The letters inbetween "A" and "F" represent different levels of self-extension. The rings are symbolic, and do not represent actual distances.

Spatial Frame of Reference Continuum (SFoRC; Hanley & Garland, 2019). This scale comprises a single item used to assess the extent to which the field of awareness is perceived to extend beyond the physical body.

How far does your self extend into the world? Using the letters and image below, please indicate how far you feel that your self extends beyond your physical body. "A" represents your self stopping at your physical body. "F" represents your self extending into everything (e.g., the entire Universe). The letters inbetween "A" and "F" represent different levels of self-extension. The rings are symbolic, and do not represent actual distances.

Spatial Frame of Reference Continuum (SFoRC; Hanley & Garland, 2019). This scale comprises a single item used to assess the extent to which the field of awareness is perceived to extend beyond the physical body.

How far does your self extend into the world? Using the letters and image below, please indicate how far you feel that your self extends beyond your physical body. "A" represents your self stopping at your physical body. "F" represents your self extending into everything (e.g., the entire Universe). The letters inbetween "A" and "F" represent different levels of self-extension. The rings are symbolic, and do not represent actual distances.

Measured using the Elevating Experiences Scale (EES; Huta & Ryan, 2010), a 13-item measure that captures feelings of transcendence, moral inspiration, and connection to something greater than oneself. Participants rated their experiences over the past two weeks using a 7-point Likert-type scale ranging from 1 (not at all) to 7 (extremely). Scores are calculated as the mean of all items, with higher scores indicating greater elevation.

Measured using the Elevating Experiences Scale (EES; Huta & Ryan, 2010), a 13-item measure that captures feelings of transcendence, moral inspiration, and connection to something greater than oneself. Participants rated their experiences over the past two weeks using a 7-point Likert-type scale ranging from 1 (not at all) to 7 (extremely). Scores are calculated as the mean of all items, with higher scores indicating greater elevation.

Measured using the Elevating Experiences Scale (EES; Huta & Ryan, 2010), a 13-item measure that captures feelings of transcendence, moral inspiration, and connection to something greater than oneself. Participants rated their experiences over the past two weeks using a 7-point Likert-type scale ranging from 1 (not at all) to 7 (extremely). Scores are calculated as the mean of all items, with higher scores indicating greater elevation.

Measured using the Elevating Experiences Scale (EES; Huta & Ryan, 2010), a 13-item measure that captures feelings of transcendence, moral inspiration, and connection to something greater than oneself. Participants rated their experiences over the past two weeks using a 7-point Likert-type scale ranging from 1 (not at all) to 7 (extremely). Scores are calculated as the mean of all items, with higher scores indicating greater elevation.

Measured using the Generic Sense of Ability to Adapt Scale (GSAAS; Franken et al., 2023). The GSAAS is a 10-item scale measuring perceived ability to readjust and actively deal with the psychosocial consequences of challenging events. Items (e.g., "If something unexpected happens, I can easily adapt") are rated on a 5-point Likert scale from "not at all" (0) to "always" (4). The total score is calculated as the mean of all items, with higher scores indicating greater ability to adapt.

Measured using the Generic Sense of Ability to Adapt Scale (GSAAS; Franken et al., 2023). The GSAAS is a 10-item scale measuring perceived ability to readjust and actively deal with the psychosocial consequences of challenging events. Items (e.g., "If something unexpected happens, I can easily adapt") are rated on a 5-point Likert scale from "not at all" (0) to "always" (4). The total score is calculated as the mean of all items, with higher scores indicating greater ability to adapt.

Measured using the Generic Sense of Ability to Adapt Scale (GSAAS; Franken et al., 2023). The GSAAS is a 10-item scale measuring perceived ability to readjust and actively deal with the psychosocial consequences of challenging events. Items (e.g., "If something unexpected happens, I can easily adapt") are rated on a 5-point Likert scale from "not at all" (0) to "always" (4). The total score is calculated as the mean of all items, with higher scores indicating greater ability to adapt.

Measured using the Generic Sense of Ability to Adapt Scale (GSAAS; Franken et al., 2023). The GSAAS is a 10-item scale measuring perceived ability to readjust and actively deal with the psychosocial consequences of challenging events. Items (e.g., "If something unexpected happens, I can easily adapt") are rated on a 5-point Likert scale from "not at all" (0) to "always" (4). The total score is calculated as the mean of all items, with higher scores indicating greater ability to adapt.

Measured using the 5-item Brief INSPIRE-O, modified from the Brief INSPIRE support measure (Williams et al., 2015). Items are rated on a 5-point Likert scale from 0 (not at all) to 4 (very much). Following scoring guidelines (Moeller et al., 2023), raw scores were summed and multiplied by 5 to give a total score ranging from 0 to 100, with higher scores indicating better personal functioning.

Measured using the 5-item Brief INSPIRE-O, modified from the Brief INSPIRE support measure (Williams et al., 2015). Items are rated on a 5-point Likert scale from 0 (not at all) to 4 (very much). Following scoring guidelines (Moeller et al., 2023), raw scores were summed and multiplied by 5 to give a total score ranging from 0 to 100, with higher scores indicating better personal functioning.

Measured using the 5-item Brief INSPIRE-O, modified from the Brief INSPIRE support measure (Williams et al., 2015). Items are rated on a 5-point Likert scale from 0 (not at all) to 4 (very much). Following scoring guidelines (Moeller et al., 2023), raw scores were summed and multiplied by 5 to give a total score ranging from 0 to 100, with higher scores indicating better personal functioning.

Measured using the 5-item Brief INSPIRE-O, modified from the Brief INSPIRE support measure (Williams et al., 2015). Items are rated on a 5-point Likert scale from 0 (not at all) to 4 (very much). Following scoring guidelines (Moeller et al., 2023), raw scores were summed and multiplied by 5 to give a total score ranging from 0 to 100, with higher scores indicating better personal functioning.

"Please respond to the following items based on your experiences and feelings today, reflecting specifically on the past 24 hours." Responses range from 1 (not at all) to 7 (very much).

1. Grief reactions (2 items)

2. Positive affect (2 items)

3. Negative affect (2 items)

4. Nature connectedness (1 item; Schultz, 2001)

5. Selflessness (1 item; Dambrun, 2016)

6. Spiritual well-being (1 item; Underwood, 2011)

7. Ability to adapt (1 item; Franken et al., 2023)

   • Analysis metric: Daily fluctuations and sudden gains

Using two complementary items. First, participants will respond to the question "Do you have experience with meditation?" with three response options: "Yes, I would call myself an expert and have a lot of experience with meditation," "Yes, I have a little experience with meditation," or "No, I have no experience with meditation (yet)." Second, current practice frequency will be assessed with the question "How many times have you meditated in the past month?" with four response options: "Daily," "Weekly," "Occasionally," or "Never."

Participants will respond to the question "Are you regularly in nature (such as the forest, a park or the beach)?" with five response options: "Yes, every day," "Yes, one or more times a week," "Yes, once a month," "No, rarely," or "No, never."

A post-intervention adherence item measuring self-reported frequency of practice completion and duration of practice sessions. Adherence will be operationalized as the percentage of assigned sessions completed (ranging from 0-100%) and reported as means, standard deviations, and frequency distributions.

Participants will respond to the question: "During the past 2 weeks, did you receive additional psychological professional support from a psychologist, therapist, or psychiatrist for dealing with your emotional problems?" Results will be reported as frequencies and percentages. Co-intervention use will be examined as a potential confounding variable in the analysis to determine whether concurrent psychological support interacts with the intervention effects.

Measured using 8-item Client Satisfaction Questionnaire (CSQ-8; Attkisson & Zwick, 1982). Items will be scored on a 4-point scale, and a sum score (8-32) will be calculated with higher scores being indicative of a larger satisfaction with the intervention.