1. Introduction
Urbanization has promoted social and economic development while causing negative effects on nature’s ecosystem. Urban living represents convenient transportation, better medical and education services, more opportunities for employment and higher incomes [
1]. Meanwhile, rapid urbanization is usually associated with environmental pollution, high population density, and the loss of chances to experience nature [
2] as well as exposure to chronic, noncommunicable and mental health-exacerbated conditions [
3,
4]. A natural environment is likely to provide a distinctive intervention to help people with solving many of these health problems. Furthermore, an increasing number of studies have confirmed the restorative effects between nature and physiological and psychological well-being [
5,
6], including the prevention of cardiovascular and respiratory diseases [
7,
8], reduction of stress levels [
9], lifting of mood condition [
10,
11,
12] as well as the restoration and sustainment of vitality [
13,
14,
15]. Restorative effects refer to the restoration and replenishment, or updating effects of restorative environments on people’s depleted physical, psychological or social resources and abilities that are constantly consumed under stress conditions; that is, the effect of the reduction in stress and the decrease of various negative emotions as well as the promotion of psychological and physical health [
16,
17].
Forests are, as an important part of nature, considered as a basic health resource that may play a key role in human health improvement [
18]. Forests restore the physical and psychological health of the human body through a “five senses experience” (vision (landscape), hearing, smell, touch and taste) when people are exposed to a forest environment [
19]. Public preferences for the structural attributes of forests are an important factor that are responsible for the perceived response in forests. The degree of public preference for tree species is affected by public culture, the geographic area and subjective expectations [
20]. Moreover, factors such as visibility distance, light, number of canopy layers in the forest stand and public viewing habits [
21] largely determine people’s preference for tree species. Mixed forests are the most popular type of forest followed by broad-leaved forests and coniferous forests [
22]. Trees with a large size generally dominate the visual aesthetics of forests and exert their positive influence through their diameter at breast height, forest density and height [
20]. In addition, the density of the ground vegetation in a forest is beneficial for increasing public preference, but too high a density of shrubs and grass may make people feel that “the landscape looks too desolate” and there is “poor accessibility in the forest” [
23]. When the visibility distance of the forest is about 40–50 m, the landscape reaches the highest value [
24]. However, in addition to all of that, factors in the forest environment that may provide beneficial health effects [
25] include a special interior forest climate with reduced air temperature, high air purity and humidity, special light conditions and negative oxygen ions concentrations. The influence of a forest on these factors mainly depends on the plant species and size, stand density, growth status and canopy closure [
26]. In addition, it has been identified that the effect of biogenic volatile organic compounds (BVOCs) emitted by trees and plants, such as terpenes (usually called phytoncides), would be potential factors for positive health effects in terms of their anti-inflammatory, antioxidant, or neuroprotective activities [
27,
28]. Studies have found that the composition of trees can significantly influence the concentration of phytoncides, and that coniferous plants such as pine and cypress have higher emission values [
29].
Widespread attention has focused on people’s physiological and psychological relaxation with forest exposure, and forests are bound up with great performance in terms of positive health restoration outcomes [
30,
31,
32,
33]. Moreover, short-term forest exposure could significantly improve the cardiovascular and psychological health of people, including lowering blood pressure and lifting mood [
34,
35]. Although the use of forests has been considered as a novel approach to promoting human health and well-being, the restorative effects of forests for human health are not the same [
36,
37]. This is because the promotion effects of forests on subjects’ health might depend on the different characteristics of forests that have shaped the forest landscapes and environment [
38]. It was found that a forest stand’s age affected its restorative qualities, an old-growth forest was more restorative compared to a mature forest or a young forest [
39]. In addition, the forest’s structure, tree species and forest cover are considered to be influential factors for forest relaxation and restoration [
21,
40]. There is evidence that forest vegetation density is related to stress recovery, and high vegetation density in a forest leads to better attentional functioning than medium density [
41]. The beneficial physiological effects relating to decreased levels of blood pressure and saliva cortisol have been associated with different types of forest management; managed forests lead to a better response than unmanaged forests after a stress stimulus [
42]. Researchers have also explored the psychological responses to different forest environments through virtual reality experiments, and the results have shown that different types of forest led to uneven responses to stress, reaching a consistent conclusion that coniferous forests and evergreen trees are more conducive to stress relief [
43,
44]. In particular, the results of studies indicate that a large body of papers have focused on the topic of forests’ restoration effects, but there still needs to be a focus on integrating descriptions of forests’ characteristics and how they are related to human health.
Furthermore, recent studies have paid less attention to the effects of the experiential characteristics of nature exposure, termed nature experience, on individuals’ mental and physical well-being. Nature experience has been proposed as a method to classify nature exposure, which is an important component of the health outcomes of subjects [
45]. The health benefits of nature experience varied according to the ways of interacting with nature and the response of different sensory forms [
46,
47]. A study of human’s nature experience [
48] confirmed that specific types of experience in which people interact with nature may throw light on differential impacts of exposure on health. The types of activity were consistent in most studies; for example, walking acted as an active exposure, while sitting or viewing was taken as a passive exposure [
48]. Research also showed that specific types of nature experience activity in a green environment have different impacts on human health restoration [
49]. For example, one study confirmed that vigor is more enhanced by walking than viewing in a forest environment [
50], while another concluded that viewing had better physiological restoration than walking [
51].
To our knowledge, studies on the relationship between and the importance of forests and human health restoration have increased quickly, but fewer studies have investigated how restorative effects vary in different types of forest. In this respect, research on forest types and nature experience associated with health restoration is scarcer and fragmented. Therefore, it is not entirely clear whether different types of forest have different restorative effects or which patterns of nature experience activity stimulate a restoration effect the most. The main objective of this study was to investigate the restorative effects of different types of forest (mixed forest, deciduous forest and coniferous forest) and nature experience activities (sitting and walking). Based on the previous studies, we assumed that different types of forest and experience activity would have uneven impacts on people’s health promotion. The hypotheses were that: (I) all three types of forest would have impacts on subjects’ physiological and psychological restoration and relaxation; (II) there would be differences in the restorative effects among the mixed forest, deciduous forest and coniferous forest; (III) both sitting and walking activities would have unequal restorative effects.
2. Materials and Methods
2.1. Participants
In the highly competitive atmosphere of modern society, university students are faced with various pressures from study, social interaction and employment. These pressures have a profound impact on university students’ physiology, psychology and behavior [
52]. In most countries, more than 50% of colleges and university students live with different degrees of stress, anxiety or depression [
53]. In addition, university students are commonly used as test groups in the study of the relationship between humans and the natural environment. Young adults at university, who at a group level can be assumed not yet to be overly affected by various diseases, are similar in age, have a higher level of knowledge and are homogeneous research subjects.
The study enrolled self-referred participants after dissemination of recruitment information via group messages using the WeChat mobile application. All the subjects were healthy adult students (at least 18 years old) and were Chinese speakers. Applicants meeting any of the following criteria were excluded from participation in this study: physical or mental disorders; taking insomnia drugs, stimulants or other psychoactive drugs; heavy smoking and/or drinking habits. Students willingly participated in the research and were informed about all the necessary information regarding the study (aims and experiment procedure as well as precautions) and the study did not lead to any harm to the participants. All participants were contacted by phone call for a brief interview to check their eligibility for the study 3 days before beginning the experiment. The final participants consisted of 30 young adult students aged 22–28 of Beijing Forestry University. All participants were from different majors such as Law, Management and Philosophy, etc. All procedures involved in this study were in accordance with the ethical standards of the Ethics Committee of Beijing Forestry University (Z161100001116084) and the Declaration of Helsinki from 1964. All participants gave written consent for their voluntary participation.
2.2. Study Sites
All experimental sites in this study were in Beijing, China (
Figure 1). Three types of forest sites were in Mangshan National Forest Park of Changping District, 40 km away from downtown area. The total area of Mangshan National Forest Park is 8622 ha, and it is the largest national forest park in Beijing. The forest coverage rate is 86%.
The urban site (CK), Wudaokou, was a city square in the center of downtown area with a large amount of people and vehicles. The surrounding area is characterized by urban elements such as subway station, shopping mall, roads, supermarkets and community houses, which is a typical urban site (
Figure 2A). The first forest site was the mixed forest. The species composition was dominated by
Quercus mongolica and
Pinus tabuliformis, which are typical coniferous and broad-leaved mixed forest trees. Mixed forest formed a stand structure with multiple layers and thick canopy, and it had the most abundant understory vegetation (
Figure 2B). The second forest site was deciduous forest. The dominant tree species was
Cotinus coggygria, which is winter deciduous (also known as summer green forest). There were many shrubs, herbs, and other plants under the forest (
Figure 2C). The third forest site was coniferous forest. The dominant tree species was
Platycladus orientalis, which is an evergreen coniferous forest tree. The composition of the community structure was simple, and the understory shrubs were sparse (
Figure 2D). All three types of forest were planted about 40 years ago, and the specific forest characteristics in forest sites are showned in
Table 1.
Meteorological research data were collected in the forest sites and urban site. The level of noise was measured by digital noise equipment (TES-1350R). The lighting was measured by a digital photometer (Victor 1010d). DustMate (Turnkey) handheld environmental dust detector was used to measure total suspended particulate (TSP). Aero-anion was measured by air ions counter. Air temperature, relative humidity and wind velocity were measured by hot wire anemometer (TES-1341). All the data were measured multiple times in randomly chosen points and recorded every 30 min at each experimental site from 9:00 a.m. to 5:00 p.m. (
Table 2).
2.3. Procedure
The physiological and psychological parameters of participants were measured at the gathering point before the nature experience activities, and then were measured after the end of the experimental fields. Measurements of physiological and psychological indicators of all participants in the study were performed at the same time. All participants undertook separate sitting and walking activities at the experimental sites. The participants were not allowed to talk to each other, drink or eat any energy drinks or food during the experiment period.
The experiment was conducted in June 2021. Participants visited the three forest sites and the urban site at the same time of different days. The same procedure was conducted for each experiment in the study. One experiment consisted of six time periods with specific measurements (T0–T6). All participants arrived at the gathering point (a classroom at the University) at 8:00 a.m. Then, participants blood pressure and heart rate were measured and all questionnaires were completed at the gathering point (baseline measurements, T0). Next, all participants took school bus (nearly 40 min) to the experimental areas (T1). Participants were randomly, but evenly, divided into three groups on the bus (group A, group B and group C). Afterwards, participants were asked to sit for 30 min in the respective forest site (T2). After sitting, researchers measured participants’ blood pressure and heart rate while participants finished all questionnaires at sitting fields (T3). After a 10 min break (T4), participants were invited to walk 30 min in the respective forest study site (T5). Walking speed was moderate pace throughout the whole walking activity. After walking, researchers measured participants’ blood pressure and heart rate while participants completed all questionnaires (T6). The whole sitting and walking periods were guided with three researchers in each group. The groups were formed by the same people during all visits to forest sites and urban site.
2.4. Measurements
Blood pressure (systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (HR) were measured to record participants’ physiological responses. Blood pressure indicates were measured by Omron IntelliSense blood pressure monitor (HEM-7207). Heart rate was measured by Polar monitor (TEAM PRO).
Four psychological questionnaires, all in Chinese, were used to measure participants psychological responses. The Profile of Mood States (POMS) measured mood states. POMS can measure six kinds of psychological states (tension–anxiety (T–A), anger–hostility (A–H), fatigue (F), depression (D), confusion (C) and vigor (V)) value, and its reliability and validity have been fully documented [
54]. POMS had been previously used for the evaluation of the forest exposure effects on subjects’ mood. In the study, participants were asked to rate 30 items with a Likert five-point scale ranging from 0 (=not at all) to 4 (=extremely). The Restorative Outcome Scale (ROS) evaluated the restorative effects, which includes relaxation and calmness, attention restoration and thought clearness; it is composed of six items and each item had a Likert seven-point scale ranging from 0 (=totally disagree) to 6 (=totally agree). In the study, we used the scale modified for forest-related experience by Takayama et al. [
55]. The Subjective Vitality Scale (SVS) was used to measure level of vitality. It is composed of four items with a Likert seven-point scale ranging from 0 (=totally disagree) to 6 (=totally agree). The reliability and validity of SVS have also been confirmed in previous research [
56]. The Warwick-Edinburgh Mental Well-being Scale (WEMWBS) was used to evaluate positive mental health. This scale can assess the positive emotions of mental health and satisfaction of interpersonal relationship [
57]. The Chinese version has 14 items and its reliability and validity have been verified before [
58]. Each item was evaluated with a Likert five-point scale ranging from 0 (=not at all) to 4 (=extremely).
All four questionnaires in this study that we used, were the same as in previous research [
59,
60]; the time frame “at this moment” was used for timely measurement of participant responses.
2.5. Data Analysis
In this study, we used Excel 2010 (Microsoft Corporation) to record raw data from physiological and psychological measurements. All statistical analysis was processed by SPSS23.0 (IBM, Armonk, NY, USA). The paired t-test was used for investigating the changes in variables of the subjects before and after the nature experience activities in different experiment sites. We set urban site and forest sites as factors and each measurement point (before experiment (TO), after sitting (T3) and after walking (T6)) as levels. The significant difference values of the indicators change among four experiment sites and two activities were analyzed with repeated-measures ANOVAs. Significance was considered at the p < 0.05 level for both paired t-test and repeated-measures ANOVAs analysis.
6. Conclusions
The study demonstrated the results from a field experiment of three types of forest on physiological and psychological health parameters. Results showed that all forests, compared to the urban site, were beneficial to the lowering of blood pressure and heart rate as well as for reducing negative emotions and boosting positive emotions. There were significant changes in analyzed physiological and psychological parameters in different forests after both sitting and walking compared to the baseline. In terms of blood pressure and heart rate, the mixed forest was more effective in reducing them. Moreover, vitality had a higher-level score in the mixed forest. The levels of restoration and positive mental health increased significantly, while all subscales of the POMS (except vigor) decreased greatly in the coniferous forest. Relative to the sitting activity, obvious decreases in blood pressure and negative emotions but significant increases in restoration, vitality and positive mental health were observed after the walking activity. Therefore, it can be summarized that physiological and psychological changes were closely related to the forest types and experience and activity patterns. In conclusion, the impact on subjects’ physiological and psychological restoration varied according to the forest characteristics, and the experiential characteristics of forest exposure may be helpful from the perspective of creating supportive forest interventions and lifting the benefits of forest therapy as people interact with a forest environment.
In light of the study results, walking activity is beneficial to health restoration; thus, it is necessary to choose a flat area in the forest to build trails for people to walk as almost all participants in forest therapy activities take walks at a steady and slow speed. The slope of the trail should be controlled within 8%, and the minimum width should be sufficient for one person to pass, preferably 1.5 m or more. The pavement material for the walkway can be made of soft ecological pavement materials such as soil, grass and humus that make people feel comfortable. At the same time, seats should be set up midway along the trail for rests. The results of this study showed that 30 min sitting and walking activities all decreased subjects’ negative emotions and increased positive emotions. Therefore, 30 min should be considered as a suitable time period in the design of forest therapy activities. If the time is too long, it will make people feel tired. In addition, the priority should be given to multi-tree mixed forest stands or forests with abundant plant communities in the development of forest therapy activities as a mixed forest has better health improvement effects. A coniferous forest is suitable for passive activities, such as forest sitting, forest meditation and forest viewing, so exercise platforms can be added to flat areas in the forest.