1. Introduction
Nearly 33 years have elapsed since the restorative effects of nature proposed by Ulrich [
1] became a subject of scientific analysis, and since then, the issue has been studied from psychological perspectives compared with urban settings [
2,
3,
4,
5,
6]. According to Haluza et al. [
7], since the beginning of this century, the physiological and psychological restorative effects of forests have been investigated, primarily in developed countries where aging populations and declining birth rates have become serious issues. In Japan, since the concept of Shinrin-yoku (taking in a forest atmosphere or forest bathing) was proposed in 1982 [
5], primarily in the second half of the 1990s, studies on the physiological and psychological restorative effects of forests progressed rapidly. Findings suggest that if citizens’ physical and mental health can be maintained by utilizing nature, including forests, medical expenses, and other costs could decrease, which would greatly benefit societies. For example, in the areas of medicine and physiology, Ohira et al. [
8] clarified that Shinrin-yoku was effective in immune restoration, and Li et al. [
9,
10] reported that immune cells became more active after a three-day stay in the forest. Li et al. [
11] also reported that the benefits of Shinrin-yoku were attributable not only to the differences in air quality but also to the overall environment compared with living in urban areas for the same duration. Ochiai et al. [
12,
13] reported that two hours of Shinrin-yoku reduced physiological parameters such as stress hormones, adrenaline, and blood pressure in middle-aged and elderly people. The Profile of Mood States (POMS; McNair et al. [
14]) is generally applied to investigate improvements in feelings [
15,
16,
17,
18], and the State-Trait Anxiety Inventory (Spielberger et al. [
19,
20]) is used to study feelings of insecurity [
21]. Furthermore, using other developed indices such as the Subjective Vitality Scale [
22], the Restorative Outcome Scale (ROS; Korpela et al. [
23,
24]), and the Perceived Restorativeness Scale (PRS; Korpela and Hartig [
25]; Hartig et al. [
26,
27]), it was clarified that Shinrin-yoku improved vigor [
28,
29,
30] and was provided psychological restorativeness. Kobayashi et al. [
31] conducted a large-scale field experiment known as forest therapy in 62 areas maintained for Shinrin-yoku. Moreover, a few researchers have examined the restorative effects of not only remote and suburban forests but also urban forests and confirmed considerable physiological and psychological effects [
30,
32]. In brief, researchers have confirmed the physiological and mental restorative effects of forest environments; previous research on Shinrin-yoku has made great contributions to the public health field by scientifically clarifying the restorative effects of nature.
However, in forest management, planning is necessary to enhance the health and recreational functions of forests; to use them continuously and comfortably, more information on how to manage the physical environments of forests. Regarding the relationship between the physical forest environment and Shinrin-yoku, Horiuchi et al. [
33] observed that when individuals viewed real forest scenes, the cerebral oxygenated hemoglobin in the prefrontal area declined and their feelings improved more than they did when the participants were cut off from forest environments. Park et al. [
34] reported a relationship between impressions and mood-restoring effects and physical features such as temperature or illumination. Takayama et al. [
35] and Fujisawa and Takayama [
36] investigated the effects of light in a forest and demonstrated that in the bright environment with sunlight passing through the trees, the blood volume in the participants’ brains decreased and their moods improved more than they did in the dark area in the same forest. Summarizing the results of the previous studies, an appropriate forecast of the forest environment is better for Shinrin-yoku. However, for effective forest management, it is important to understand each forest setting based on its intended use [
37,
38,
39]. For instance, compared with remote artificial forests, suburban forests or forests near tourist areas require management to maintain biological diversity and at the same time satisfy users’ needs [
37,
38,
39]. Accordingly, adequately managing urban forests with highly diverse uses such as Shinrin-yoku and other recreation, managers must be well informed about what aspects of management are required based on how the forests will be used.
The following studies have assessed the relationship between people’s evaluations of forests and forest management methods could be helpful. Buhyoff and Leuschner [
40] showed that forest favorability ratings decreased suddenly and sharply when damage from pinewood nematodes exceeded 10%; Takahashi et al. [
41] suggested that people’s evaluations of forests vary depending on the mixes of hardwood and softwood; and Oishi et al. [
42], Takayama et al. [
43], and Takayama et al. [
44] reported on the relationship between people’s assessments of the comfort of forest environments and tree density. Other researchers have also investigated the effects of types and numbers of trees, tree density, planting patterns, presence or absence of fallen trees, and other factors on people’s evaluations of landscape beauty.
Meanwhile, one method of controlling forest density is known as thinning. Thinning improves light environment and soil, which encourages the growth of the remaining trees even when growth is still dense. In addition, thinning is not only useful for trees but also highly related to users’ forest experiences. However, few studies have investigated the relationship between thinning and perceptions of forest beauty [
45,
46,
47]. Daniel [
48] reported that forest visitors in general prefer managed forests in which trees grow well to entirely unmanaged natural forests, and Edwards et al. [
49] highlighted that thinning not only maintains forests in good condition, but also improves people’s evaluations of forests’ scenic beauty. However, it is necessary to further organize the knowledge about the impact on users’ evaluations of forests’ esthetic value of controlling stand age, forest type, thinning, and other management factors. Studies have been performed that considered the connections between forest management and recreation. Brunson and Shelby [
50] reported that the landscape quality of forests and recreational activities are closely linked. Kunisaki and Imada [
51] suggested that thinning is essential for effectively managing forest density along roads and sidewalks to increase user satisfaction; researchers at the Gifu Prefectural Research Institute for Forests [
52] found that visitor ratings increase when trees are maintained approximately 20 m from either side of forest roads; and Oku [
53] studied the ease of performing forest activities including analyzing the management of the physical environment based on intended use.
These studies lead to a number of questions. How does our evaluation of forests change depending on the degree of thinning in the forest, and how do these factors relate to psychological and physiological restoration? Unfortunately, there are only a few previous studies such as the one by Oishi et al. [
15], and the data on the subject are still insufficient.
Meanwhile, in urban forests or those in resort areas that are already in use for forest recreation, some facilities are reasonably managed, unlike in remote forests where care has been abandoned. Health consciousness has been recently promoted, and forest managers should aim to sustainably provide Shinrin-yoku experiences that increase the restorative effects of forests on visitors, which should improve users’ general evaluations of forests. Thus, managers need to be aware of the best criteria for frequency of forest thinning, weeding, and pruning based on their intended goals.
Currently, as we mentioned above, the related research is insufficient, and hence forest administrators must make arbitrary choices about thinning based on their own judgment and experience without scientific evidence. In this context, if users cannot efficiently realize the benefits of thinning because of the lack of information on forest management and on the psychological and physiological restorative effects of forest environments, forest managers and users both miss excellent opportunities to take advantage of forests.
However, if there is scientific evidence on how to improve and maintain the physical and psychological restorative features of forests, managers will gain objective guidelines for managing forest environments and for increasing the effects of Shinrin-yoku. In turn, users will enjoy their Shinrin-yoku experiences in more comfortable forest environments, and as noted earlier, if the mind and body can be restored by Shinrin-yoku, the increasing medical expenses that accompany aging populations could begin to decrease, which will significantly contribute to public health (
Figure 1). Therefore, we believed that it would be most effective to understand whether forest maintenance affects the restorative features of individual forests. Beginning with such an investigation would give us fundamental information on the relationships between forest improvement, their restorative effects, and users’ appraisals of similar forest environments.
For this research, we planned an experiment in an urban forest located in a suburban summer resort. The goal of the research was to determine how thinning influenced users’ impressions and evaluations of the forest and the restorative effects of the forest environment.
5. Conclusions
In general, forest environments that provide Shinrin-yoku and recreational use are often considered problems in terms of trees in forest density management, promenade design, resting facilities, signboards, and pavilions from the beginning of their design, because forests appear to enhance users’ physical and psychological restorative effects and their experience of quality. However, forests consist of a variety of animals and plants, and in addition, forests with characteristic facilities degrade more quickly than urban areas because of the higher humidity. Accordingly, compared with other environments, it is very important to have not only an initial arrangement but also a method and planning for later management for proper maintenance. Here, considering the results of this research, the user impressions and restorative traits of forest environments show the differences caused by forest management; however, we can think of management plans based on our finding that psychological restorative effects will not differ greatly. For instance, when performing maintenance for Shinrin-yoku and recreational use, we could attempt to thoroughly manage all forests by conducting thinning and/or undergrowing. For all forests, the costs will increase in terms of expenses and manpower in terms of management and maintenance. Therefore, we suggest that it would be more useful to not attempt to perfectly manage all forests but to partially manage forests that users will not approach from forest roads or promenades, thereby reducing the maintenance costs and preserving the diversity of flora and fauna. If the environment changes in a certain forest, the diversity of the environment will increase in the whole forest area. Regarding the places where it is assumed that users intend to enter the forest from forest roads or to conduct active activities, it is necessary to manage forest environments with sharp focus, such as concentrating on maintenance. This increases the possibility of users’ experiencing a variety of forest environments and experiencing psychological restorative effects. Therefore, we will formulate an effective and cost-effective forest management plan.
Summary
Finally, the findings obtained in this research can be summarized in the following three points:
- (1)
The impressions and evaluations of the restorative traits for the crowding versus thinned forest environments differed greatly, and the thinned forest were evaluated more positively than the crowding one.
- (2)
The differences in the physical environments between the crowding and thinned forests did not appear to be reflected in respondents’ impressions except for illuminance (on the SD scale).
- (3)
In terms of appreciating landscapes while sitting, it is possible that forest environments can bring about psychological restorative effects whether or not forest management is being conducted but that these effects can be partially enhanced by managing the forests.