*1.4. Natural Capital*

Several researchers have emphasized the importance of natural areas as natural capital contributing to SWB by estimating their economic value. For example, Kopman and Rehdanz identified correlations between the ratios of natural environments and SWB in European countries and analyzed their monetary values as marginal willingness-to-pay (WTP) estimates [20]. Ambrey and Fleming reported relationships between ecosystem services (i.e., visual amenity and biodiversity) and SWB in Australia, and estimated WTP values for the improvement of these services [21,22]. An inverted U-shaped (concave) relationship between the distance to urban green spaces and life satisfaction was found for Berlin and this relationship was replicated in other German cities in a separate study [23,24]. In another study, Tsurumi and Managi identified relationships between SWB and the distance from the green spaces from residential areas, and calculated the marginal WTP for green spaces in metropolitan areas in Japan [25]. Tsurumi et al. further investigated various well-being measures such as the Cantril ladder, life satisfaction, subjective happiness, affect balance, and mental health, and their relationships with diverse green spaces in a metropolitan area in Japan and suggested possible positive effect of greenery investments on SWB [26]. A national-level survey in Japan found that respondents living in areas with more plantation forests or open water had relatively higher levels of well-being [27]. Based on these results, the authors recommended additional investments in plantation forest management and open water areas. Apart from land use types, Holms et al. captured the negative impacts of bark beetle epidemics on SWB in the western US [28].

Several studies have further explored the impact of interactions, not only physical closeness, with nature on SWB. Jang et al. reported that the frequency of forest visits is more relevant to SWB than the distance to urban forests in South Korea [29]. It was found in Italy and the UK that longer and more frequent visits to urban green spaces improved the perceived benefits and well-being of respondents [30]. Another study confirmed that the types of activities during visits to forest areas ((a) reading, talking, socializing; (b) walking/exercising, and (c) contemplating the setting) influenced well-being differently [31]. Bieling et al. noted that practices (e.g., hiking, walking) and perceived relationships (e.g., naturalness, tranquility, accessibility) in natural settings were also important factors for SWB compared to physical factors (e.g., mountains, forests, water bodies), based on open-ended interviews in Germany and Austria [32].

Notably, several studies have examined urban–rural differences based on SWB. In secondary analysis of existing data, lower levels of urban sprawl were associated with higher SWB as measured by personal financial issues for individuals living in urban areas in the USA [33]. Carrus et al. investigated how urban residents perceived peri-urban natural areas in large- to medium-scale cities in Italy and found that biodiversity had a positive relationship with perceived restorative properties and self-reported benefits from urban and peri-urban green spaces [31]. The aforementioned studies mainly focused on urban resident perspectives. Thus, we highlighted the well-being of residents in rural areas in the second (upper watershed) study reviewed below.
