There were no missing data. We analyzed the outliers using a box plot. It was found that there were some outliers for the pre-test and post-test values of the indicators. However, through comparison observation, it was found that the pre-test and post-test had certain correspondence. We judged that these outliers were not errors in measurement, recording, or experimental design. In order to maintain the sample size and authenticity of the data, we chose to keep these outliers.
The data were analyzed, including the age, gender, baseline SBP/DBP, and heart rate. No significant differences were found between the types of individuals. From a statistical point of view, it might be considered that there was no difference between the individuals.
3.1. Physiological Index
Due to funding constraints, the basic measurements only collected data on blood pressure and heart rate. The baseline data were analyzed. We found that the DBP and heart rate values between the groups were in a normal distribution, satisfying the homogeneity of the variance, so the ANOVA test could be performed. The results were: DBP, F (6, 89) = 1.724, p > 0.05; Heart Rate, F (6, 89) = 0.543, p > 0.05. SBP did not satisfy the homogeneity of variance, so a nonparametric test was performed. The results of the Kruskal–Wallis test were p > 0.05. The above results indicated that there was no significant difference between the baseline between the groups, i.e., the physiological index of the subjects in each group could be considered to be consistent.
The baseline and the pre-test data were compared by a related sample t-test. The results were: SBP, t (95) = −2.242, p < 0.05; DBP, t (95) = −2.115, p < 0.05; Heart Rate (basic) − Heart Rate (pre-), t (95) = −2.077, p < 0.05. The results showed that the pre-measurement values of blood pressure and heart rate had increased relative to the baseline values. This showed that the TSST test had played a role in increasing stress.
3.1.1. Comparison of Pre-Test and Post-Test
The data of the pre-test and post-test of each group were in accordance with the normal distribution by Q-Q diagram analysis, and the homogeneity of variance was satisfied. The relevant sample t-test could be performed.
Statistical differences were found in four types of forest resting environments between the pre-test and post-test data. They were: Type 3, DBP, t(11) = 3.36, p < 0.05; Type 4, DBP, t(13) = 2.84, p < 0.05 and salivary amylase concentration, t(13) = 3.51, p < 0.01; Type 5, SBP, t(11) = 2.21, p = 0.05 and DBP, t(11) = 2.51, p < 0.05; and Type 6, SBP, t(14) = 3.64, p < 0.01 and DBP, t(14) = 2.23, p < 0.05. The others did not have statistically significant differences.
Regarding trends, only the blood pressure and heart rate of Type 1 increased, while all the other types decreased. The post-test value of the salivary amylase concentration of Type 4 was significantly increased (Figure 3
and Figure 4
3.1.2. Comparison of ΔD
The study found that the overall variance of the difference between systolic and diastolic blood pressure was not uniform. A two-two group analysis of the data was performed. Nonparametric tests were performed on data that did not satisfy the homogeneity of the variance, and an ANOVA was performed on the data satisfying the homogeneity of the variance. The results showed that except for environments Type 1 and Type 6, the ΔD among the other environments was not significant. The data of the ΔD between Type 1 and Type 6 of the SBP and DBP satisfied the homogeneity of variance, and an ANOVA was performed: SBP F (1, 28) = 5.607, p < 0.05, DBP F (1, 28) = 4.854, p < 0.05.
The ΔD of SBP and DBP in different types of forest resting environments was as follows: Type 6 had the greatest impact, where the SBP and DBP decreased the most (Figure 5
The ΔD of the heart rate satisfied the homogeneity of the variance, and ANOVA and the multiple comparisons of the LSD were performed. The significant differences were as follows: Type 1 and Type 3—p
< 0.05, Type 1 and Type 6—p
< 0.05, and Type 1 and Type 7—p
< 0.05 (Table 3
The heart rate mean value of the ΔD in different forest resting environments was as follows: Type 3 reduced the most, followed by Type 6 and Type 7, and Type 1 (the pure artificial landscaping environment) was the only type with an increased heart rate (Figure 6
The ΔD of the salivary amylase satisfied the homogeneity of the variance, and ANOVA and multiple comparisons of the LSD were performed. The significant differences were as follows: Type 4 and Type 3—p
< 0.05, Type 4 and Type 6—p
< 0.05, and Type 4 and Type 7—p
= 0.01 (Table 4
The mean value of the ΔD of salivary amylase concentration in different forest resting environments was as follows: Type 4 was the largest and the change was increasing, which might indicate that the environment increased the stress, followed by Type 1 that also showed an upward trend. Furthermore, the biggest declining trend was found in Type 7 (Figure 7
3.2. Psychological Index
3.2.1. Comparison of Pre-Test and Post-Test
The TMD values of all types of forest resting environments were reduced. All but Type 3 reached a significant level, with Type 1 and Type 6 reaching a significant level of 0.01 (Figure 8
From the changes of the five dimensions of the pre-test and post-test of the BPOMS, the dimension values of Type 1 were significantly different; the dimension values of T and C + D of Type 2, Type 4, and Type 5 were significantly different; the V dimension of Type 3 was significantly different; except for the A dimension value, the other dimension values reached a 0.01 significant level; and the A and C + D dimension values were significantly different.
Regarding the trend of the pre-test and postmeasurement values, except for the V dimension value, the dimension values reduced. For the V values, Type 1, Type 3, and Type 6 increased significantly, while Type 4, Type 5, and Type 7 decreased (Figure 9
3.2.2. Comparison of ΔD
The ΔD of the TMD and every dimension of the BPOMS were analyzed by Q-Q diagram, which conformed to the normal distribution and satisfied the homogeneity of variance. ANOVA and multiple comparisons of LSD after the event of the ΔD found that: for TMD, Types 1 and 3 had significant differences; for A, Types 3 and 7 had significant differences; for V, Types 1 and 4/7, Types 3 and 4/5/7, and Types 6 and 4/5/7 had significant differences. This was important for discovering the rules and developing the discussion of different types. Taking TMD as an example, the LSD multiple comparison found that there was a significant difference between Type 1 and Type 3, p < 0.05.
For the mean value of the ΔD, different forest resting environments had different effects on TMD and different dimensions. For TMD, Type 1 and Type 6 had the greatest impact; for A, Type 7 had the greatest impact and Type 3 had the smallest; and for V, Type 6 had the greatest impact, followed by Type 3, and then Type 1, while Types 4, 5, and 7 decreased, with the trend of Type 7 being the most obvious (Figure 10
4.1. All the Seven Types of Forest Resting Environments Could Relieve Stress to Some Extent
The correlation sample t-test on the pre-test and post-test values of each indicator showed that all the seven types of forest resting environments could have certain effects on relieving stress. For Type 1 (structure), all the indicators of the BPOMS were significant and showed a positive effect on the improvement of mood. For Type 2 (wood), significant relief effects were found in two dimensions (tension and confusion–depression) of the BPOMS. For Type 3 (wood with bench), the diastolic blood pressure decreased significantly, and the vigor dimension of the BPOMS increased significantly. For Type 4 (wood with platform and bench), although the concentration of the salivary amylase increased, the diastolic blood pressure, tension, and confusion–depression decreased significantly. For Type 5 (platform with trees), the diastolic blood pressure, systolic blood pressure, tension, and confusion–depression decreased significantly. For Type 6 (waterfall with trees), the diastolic blood pressure and systolic blood pressure decreased significantly and the BPOMS index reached a significant level of 0.01, except for the anxiety dimension. For Type 7 (pool with plants), the anxiety and confusion–depression dimensions decreased significantly.
These results might be due to the fact that there were many plants and other natural factors in the VR videos of the seven types of forest resting environments. This is consistent with the conclusions of a large number of studies related to the natural environment, including stress reduction, adjustment of emotional state, the positive relationship between the natural environment and health parameters, the positive effect of the forest on central nervous system activity (including autonomic activity), endocrine activity, and immune function [2
4.2. Different Forest Resting Environments have Different Effects on Stress Relief
The ANOVA or nonparametric tests of the ΔD showed that different forest resting environments had different effects on stress relief:
For the diastolic and systolic blood pressures, the comparison of the mean of the ΔD revealed that there was a significant difference between Type 1 (structure) and Type 6 (waterfall with trees).
For the heart rate, the analysis of the ΔD showed that there was a significant difference between Type 1 (structure) and Types 3 (wood with bench)/6 (waterfall with trees)/7 (pool with plants).
For the salivary amylase, the analysis of the ΔD showed that there was a significant difference between Type 4 (wood with platform and bench) and Types 3 (wood with bench)/6 (waterfall with trees)/7 (pool with plants).
In summary, the stress relief effects of Type 1 (structure) and Type 4 (wood with platform and bench) were very different from those of Type 3 (wood with bench), Type 6 (waterfall with trees), and Type 7 (pool with plants). The former were far less effective than the latter.
Among the seven different types of forest environments, Type 1 was dominated by buildings and structures and Type 4 had podiums/wood platforms and benches. These two types of environments had many artificial features, especially Type 4 with its performance and podium functions, which might lead to tension for Chinese subjects. Type 3 was a forest and had a more natural wooden bench environment. Type 6 was a plant, rock, and waterfall environment. Type 7 was a plant, stone barge, and pool environment. These three types of environments were more natural than Types 1 and 4.
These results were consistent with previous research that, for stress relief, the forest environment is better than the urban environment [21
], which indicates that the natural environment is more active than the artificial environment.
For TMD, the comparison of the mean of the ΔD showed that there was a significant difference between Type 1 (structure) and Types 3 (wood with bench)/4 (wood with platform and bench).
For the anxiety dimension of the BPOMS, the difference between Type 3 (wood with bench) and Type 7 (pool with plants) was significant.
For the vigor dimension, there were significant differences between Types 1/3/6 and Types 4/5/7 and between Type 2 and Types 4/7.
Type 1 (wooden pavilion, building, wooden platform, gravel paving, plants, tables and chairs, etc.), which had the highest degree of artificialization, decreased the total mood disturbance much more than Type 3 (forest with wooden chairs) and Type 4 (forest with podiums/wood platforms, and benches).
Type 7 with still water decreased the anxiety dimension much more than Type 3.
In terms of the vigor dimension, Types 1, 3, and 6, which played a similar role, caused a significant increase. This was in stark contrast to Types 4, 5, and 7, which caused a reduction.
4.3. Not the Most Natural Forest Resting Environment Gives the Most Effective Stress Relief
It is worth noting that among the seven types of forest resting environments, the most natural environment was Type 2 (wood) without any artificial facilities—only plants, land, and sky. However, the results showed that, for both physiology and psychology, Type 2 was not outstanding. In addition, it only had a significant effect on the tension and confusion–depression dimensions of the BPOMS.
For physiology, Type 3 (wood with bench), Type 5 (platform with trees), and Type 6 (waterfall with trees) showed significance. These three types of environments all had some manual intervention. There were wooden benches in Type 3 and a large wooden platform in Type 5. Type 6 was relatively natural, but it was artificially natural with artificially planted plants, stacked rockeries, created waterfalls, etc.
For psychology, Type 1 (structure) and Type 6 (waterfall with trees) were outstanding. Indeed, Type 1 was the most artificial environment, full of buildings and structures. However, Type 1 had a very significant effect on the regulation of emotions.
These results are in contrast to some of the conclusions that stress relief in the natural environment is best [38
]. However, some studies have suggested that it is not the case that the more natural the environment is, the better the recovery effect. Historical blocks, monasteries, art galleries, shopping centers, and coffee houses might all have a recovery effect [73
According to Kaplan’s research, the environment for better recovery is rich in content [17
]. It is speculated that the environment with rich content might have a better effect on relieving stress. For Type 3, besides the forest, there are wooden seats to enrich the content of the pure natural environment. For Type 1, the content elements are abundant and refuge characteristics could be found. Perhaps these are the reasons that explain the results.
4.4. Water Landscape Has a Positive Effect on Stress Relief
The water landscape has a positive effect on the relief of stress, but this effect cannot be generalized. Among the seven environments, Type 6 (waterfall with trees) contained moving water and Type 7 (pool with plants) was mainly composed of still water. Type 6 was the most prominent type of environment for stress relief among all environments, both physically and psychologically. For Type 6, physiologically, it caused a significant decrease in diastolic and systolic blood pressure and, psychologically, the effect of relieving psychological stress was obvious—all except the anxiety dimension reached a significant level of 0.01. For Type 7, physiologically, although they did not reach a significant level, blood pressure, heart rate, and salivary amylase concentrations all showed a downward trend, with the salivary amylase concentration declining the most among the seven types of environments. Psychologically, Type 7 was somewhat different from Type 6 and caused a significant drop in the dimensions of anxiety and confusion–depression; additionally, the vigor dimension decreased but did not reach a significant level.
These results are evidenced by the conclusions of some previous studies. For physical and psychological health, a “blue” gradient has been found. In self-perception, the closer to the ocean, the more the effect of water exceeds the green environment effect [78
]. Compared to exercising in urban green spaces, on farmland, and in woodland environments, exercise in the environment near water has been shown to better improve the mood [80
]. Water environments, such as beaches and rivers, have been shown to produce better mental health and low negative emotions [81
]. There has also been a study inconsistent with the conclusions of the present study. It found that there was no difference in happiness, stress, anger, depression or tension, with or without water, in a park-like forest environment [29
4.5. Limitations of This Study and Future Research Direction
This study has some limitations, and should be improved in future research:
First, this study only explored the stress relief effects of seven types of forest resting environments. These seven types were only a summary of the survey of forest parks in Beijing. In an actual situation, the types of environments that could be used for forest therapy may be more diverse and should be expanded much more in future research.
Second, this study found that different types of forest resting environments did have different stress relieving effects, but the essential reasons for the differences were not found. That is, the internal mechanism responsible for producing the different stress relieving effects was not established.
The reasons for the stress relief effect may be naturalness, comfort, beauty, familiarity, peace of mind, spatial characteristics, etc. An in-depth study of these reasons will make the theory of environmental health promotion clearer and more conducive to guiding actual planning and design, which should be the focus of future research.
Third, the theoretical background for selecting the seven environments was not sufficient. Although we quoted PSD theory, the theory was not very compatible with the seven environments.
This study analyzed the characteristics of the seven environments in various PSDs. Type 1 (structure) had “refuge, social, and space” characteristics, Types 2 (wood)/3 (wood with bench)/6 (waterfall with trees) were “rich in species and nature”, Type 4 (wood with platform and bench) had “social and culture” characteristics, Type 5 (platform with tree) had “refuge”, and Type 7 (pool with plants) was “serene and prospect”. In this study, with regards to the stress relief effects of various environments, the pre-test and post-test differences of the physiological indicators of Types 2 (wood), 3 (wood with bench), and 5 (platform with trees) were significant. In addition, the pre-test and post-test differences of the psychological indicators of Types 1 (structure) and 6 (waterfall with trees) were significant. The stress relief effects did not establish a direct connection with the PSDs. However, the results of the study suggested that “refuge” (as Type 1 had) might have an important role in the psychological relief of stress.
Future research should pay more attention to the background of the theoretical framework, and the independent variables should be better designed and based on certain theories:
Fourth, the water landscape in this study showed some complexity for the relief of stress. The stress relief effect of water seems to be affected by many factors. Different types of waterscapes have different effects on relieving stress, which is worthy of further exploration in future research.
Fifth, the indicators need to be further improved. In this study, salivary amylase concentration increased to a certain extent, which was inconsistent with previous studies. This might be related to factors such as the small number of subjects and the distortion of watching VR video. In the future, the sensitivity index of heart rate variability (HRV) should be added, which will make the research results more objective.
Sixth, the visual angle of the VR video was fixed, so aesthetic fatigue might occur in the later stages of viewing. At the same time, wearing VR glasses was not the same as the real environment experience. In the future, field experiments should be implemented as much as possible to increase the ecological validity of the study.
Seventh, a forest environment is a place full of the five senses—hearing, smell, taste, touch, and sight. Additionally, air (especially phytoncide and negative oxygen ions), temperature, humidity, etc. could also have an effect on stress in the real field. However, this study only focused on visual factors. Other factors should be incorporated in future research.
Eighth, the specific activities for “forest rests” were ignored in this study. There are different kinds of activities for forest rests, such as meditation and body scanning, and more activities for forest therapy, such as walking and painting. These activities could also affect stress relief. Studies focused on the health benefit of different kinds of activities of forest therapy are critically needed.
Ninth, most of the participants were undergraduate students and could not be representative of the general public who participate in forest therapy. The number of participants also needs to be increased.