Natural Resources in Health Tourism: A Systematic Literature Review

Natural resources are recognized among the key determinants for the improvement of wellness, and thus the development and sustainability of health tourism destinations. This study applied a systematic review to investigate the contributions mapping and analyzing under different perspectives the value of the natural resources of a destination and related activities for health tourism. The main research topics identified from a review of 52 papers include the analysis and exploitation of natural resources in health tourism, the nature-based factors considered in clustering of tourists and their motivations, the development of value offer and marketing, as well as the cultural issues. Research gaps and future directions are summarized in a research agenda laying the foundations for the development of a multidisciplinary research stream focused on nature-based health tourism. Results also represent a key reference for managers and policy makers to identify key issues, areas of intervention and practices for industry development in the health tourism destinations through an effective and sustainable exploitation of natural resources.


Introduction
Health tourism (HT) is a branch of general tourism foreseeing tourists travel with the aim of receiving specific healing treatments or enhancing their mental, physical, or spiritual well-being [1,2]. Health travels can include rewarding elements of beauty, indulgence, and regeneration, and more demanding elements such as stimulating activities and sports [3]. According to several scholars, HT comprises a continuum of medical, wellness, and spa tourism [4]. Medical tourism is associated with curing a form of illness and foresees tourists traveling to other countries to access medical treatments [5]; wellness tourism (WT) is devoted to maintain or enhance the tourists' health and well-being through services provided by "wellness centers" [6]. Spa tourism-positioned in between the previous ones-is focused on healing and relaxation by exploiting curative medical techniques [7].
Different sources also highlight that HT services can be strengthened by some key determinants as natural resources (NR) [8,9]: certain destinations, in fact, can leverage resources such as mineral waters, clean air, or a peculiar microclimate to promote HT and enhance the local economy. By definition, NR are intended as encompassing both the destination physical features (i.e., mountains, lakes, rivers, sea, forests) and their related by-products (such as local food and remedies), together with their traditional cultures and heritage [10]. Some works underline that NR constitute a necessary condition for the improvement of wellness and also for the formation of the health and wellness destination's image [11] (e.g., in the case of the alpine arc [12] and for the development of spa tourism [13]), considering their properties and effects on health [14,15]. HT literature

Methodology
Aiming to appraise the evidence on the value of NR in HT research, a systematic literature review was performed. The process of source selection and analysis was adapted from the approaches by [20][21][22], which allowed to comprehensively identify, evaluate, and synthesize the relevant studies on the topic with a transparent and replicable process. The review process was conducted following a shared review protocol, as suggested by [23], and included the three phases described below.
Identification and search strategy. This phase involved the selection of the keywords, databases, and filters to be used. The keywords were identified considering the different concepts of literature reviews [4,10,24] on tourism activities concerned with health, wellness, and well-being. Therefore, the search was performed with the keywords "health touris*", "medical touris*", "wellness touris*", "spa touris*", in combination with natur* or environment*. The electronic databases Scopus, ISI Web of Science, and PubMed were selected for retrieving the articles including at least one of the research strings in Title, Keywords, or Abstract (Scopus), Topic (ISI Web of science), and Title or Abstract (PubMed). The integration of the three databases enabled to cover a wide journal range and a large time span as well as an optimal update frequency [25]. The search in the electronic databases was conducted in March 2020. Limitations in timespan were not considered as the research base in the field of tourism has gradually expanded since the 1970s [23].
Selection and inclusion. The sample was then limited to English peer-reviewed articles, resulting in a first selection of 498 contributions. After eliminating duplicates, titles and abstracts of the selected works were thoroughly scanned separately by three researchers in order to avoid personal bias while ensuring consistence with the review aims [26]. The selection included papers considering at least one natural resource in their analysis of HT and WT. 80 relevant articles were identified after discussion and resolution of eventual disagreements between the researchers by consensus.
The subsequent step of full-text reading excluded the works not considering the destination NR or the activities linked to the natural environment among key sources of well-being, or having a focus only on facilities (e.g., medical centers) or economic perspectives of travelling for medical treatments. Indeed, medical tourism does not require, by definition, the presence of a natural setting or natural healing resources [27]. The selection was further discussed by the three authors, obtaining a total of 47 contributions after a joint agreement.
Finally, the results were cross-checked by reviewing the references listed in the publications, leading to a total of 52 papers. Table 1 summarizes the outcomes of these review phases.
Articles classification and analysis. In the final step, a quantitative and content analysis was conducted to synthesize the data retrieved in the 52 selected papers, and to classify them accordingly. The categories identified in the review protocol were considered and thus revised after the analysis of the first 10% of papers (six papers), following the suggestion by [21]. First, the quantitative analysis allowed us to draw the descriptive statistics. Selected articles were classified according to year of publication, journal and journal's subject area(s), authors' affiliation, and methodology. Second, the content analysis identified the focus (i.e., the industry studied), the list of NR, the list of activities considered for wellness purposes and exploiting destination NR (if present), the list of effects or benefits for health and wellness (if present), and finally, the main theme of the paper around NR, revealing the research gaps to be addressed with future research.

Distribution of Articles by Year of Publication
The temporal distribution ( Figure 1) shows that the first contributions advancing research on the identification and use of NR considered for HT belong to the late nineties. Articles classification and analysis. In the final step, a quantitative and content analysis was conducted to synthesize the data retrieved in the 52 selected papers, and to classify them accordingly. The categories identified in the review protocol were considered and thus revised after the analysis of the first 10% of papers (six papers), following the suggestion by [21]. First, the quantitative analysis allowed us to draw the descriptive statistics. Selected articles were classified according to year of publication, journal and journal's subject area(s), authors' affiliation, and methodology. Second, the content analysis identified the focus (i.e. the industry studied), the list of NR, the list of activities considered for wellness purposes and exploiting destination NR (if present), the list of effects or benefits for health and wellness (if present), and finally, the main theme of the paper around NR, revealing the research gaps to be addressed with future research.

Distribution of Articles by Year of Publication
The temporal distribution ( Figure 1) shows that the first contributions advancing research on the identification and use of NR considered for HT belong to the late nineties. The number of papers on this topic started growing significantly after 2007, reflecting the qualitative and quantitative change in how HT and related destinations were analyzed and understood [4]. Publications in 2010 focused on factors and effective strategies to promote and develop the WT industry including the destination's local resources (e.g., [28]), The number of papers on this topic started growing significantly after 2007, reflecting the qualitative and quantitative change in how HT and related destinations were analyzed and understood [4]. Publications in 2010 focused on factors and effective strategies to promote and develop the WT industry including the destination's local resources (e.g., [28]), while papers in 2015 showed a major interest toward sustainable development and the use of specific NR (e.g., [29]). Indeed, the last decade testifies to an increasing awareness on the importance of nature in contributing to tourists' well-being [24].

Distribution of Articles by Journals and Subject Areas
The 52 articles were published in a total of 36 journals. The journals with the higher number of papers were: Sustainability (five papers), International Journal of Tourism Research (four papers), and Journal of Hospitality and Tourism Management (three papers). The number and variety of journals and the distribution of articles by subject areas (Figure 2) reveal that the exploitation of NR in HT destinations is a cross-disciplinary topic with fragmented contributions. while papers in 2015 showed a major interest toward sustainable development and the use of specific NR (e.g., [29]). Indeed, the last decade testifies to an increasing awareness on the importance of nature in contributing to tourists' well-being [24].

Distribution of Articles by Journals and Subject Areas
The 52 articles were published in a total of 36 journals. The journals with the higher number of papers were: Sustainability (five papers), International Journal of Tourism Research (four papers), and Journal of Hospitality and Tourism Management (three papers). The number and variety of journals and the distribution of articles by subject areas (Figure 2) reveal that the exploitation of NR in HT destinations is a cross-disciplinary topic with fragmented contributions. The majority of articles belong to the areas of: Tourism, Leisure and Hospitality Management (29 papers), Geography, Planning and Development (20 papers), Management, Monitoring, Policy and Law (13 papers), Renewable Energy, Sustainability, and the Environment (eight papers). The topic attracted attention from journals focused on cultural issues (e.g., belonging to subject areas of Cultural Studies and Arts and Humanities (miscellaneous), and only partially from journals in medicine and health-related fields (e.g., Medicine (miscellaneous)).

Distribution of Articles by Authors' Affiliation
The geographical distribution of authors (Table 2) shows that the topic of NR in HT has attracted the interest of scholars worldwide. The majority of articles belong to the areas of: Tourism, Leisure and Hospitality Management (29 papers), Geography, Planning and Development (20 papers), Management, Monitoring, Policy and Law (13 papers), Renewable Energy, Sustainability, and the Environment (eight papers). The topic attracted attention from journals focused on cultural issues (e.g., belonging to subject areas of Cultural Studies and Arts and Humanities (miscellaneous), and only partially from journals in medicine and health-related fields (e.g., Medicine (miscellaneous)).

Distribution of Articles by Authors' Affiliation
The geographical distribution of authors (Table 2) shows that the topic of NR in HT has attracted the interest of scholars worldwide.  Institutions in Asian areas (led by Mainland China with 18 authors and Taiwan with 10 authors), and in the Eastern and Balkan regions of Europe (Romania, Hungary, and Croatia, with 16, 11, and 10 authors respectively) are the most contributors. This spread reflects the geographical distribution of cases selected in empirical studies, which are the most well-known destinations for HT and WT (i.e., Mainland China, Hungary, and Croatia).

Distribution of Articles by Methodology
The analysis on the methodologies adopted to investigate the use, exploitation, or importance of NR in HT literature (Table 3) shows that 81% of selected papers (42 out of 52) are empirical ones. This further highlights the high level of fragmentation and the local scope of the contributions on this topic. Among them, there is a balanced distribution between case study (16 papers) and survey (14 papers) research, and in general, between qualitative and quantitative studies. Only 10 out of 52 papers performed a conceptual analysis, with the main efforts toward refinement or extension of exiting theories or models. Secondary data analyses were performed to identify factors, actors, or resources in specific geographical areas.

Focus of Articles
The analysis of the focus (i.e., the specific industry addressed in selected contributions) revealed a fragmented scenario. Several papers (19 out of 52) focus on spa tourism and mainly address the use of related resources as thermal/natural mineral waters or muds. Rocha and Brandao [30] directly refer to thermalism, distinguishing among "classical" and "wellness hydrotherapies" based on multiple uses of natural mineral waters. Four contributions [31][32][33][34], mainly referring to Mainland China and Japan, focused on hot spring tourism, highlighting the link between cultural factors and the use of hot springs. Eleven articles mention HT in general, grounding their definition on different references, starting from the broad one from Goodrich and Goodrich [35]. Other works examine WT, mainly referring to the definition from Mueller and Kaufmann [36]. The concept of health is often linked to wellness, as HT is "oriented toward the promotion of a healthy lifestyle, recreational opportunities, and general wellness" [37]. Destination features also affect the development of country-specific HT forms, as highlighted by Konu et al. [38], which introduce the concept of "lake wellness" due to the peculiar nature of Finnish NR. Finally, some scholars (e.g., [39]) study well-being tourism in rural areas. It represents a rising sector addressed by tourists looking for wellness and relaxation derived from the stay in such natural contexts [40], and the literature shows an increasing awareness on the importance of rural area resources for the development of wellness and health practices [41,42]. Finally, the exploitation of NR is considered in developing authentic experiences in the medical tourism context [43].

Overview of Natural Resources and Their Effects on Health and Related Activities
The quantitative analysis of the NR investigated (Figure 3) clearly shows a preponderance for hydrological resources (natural mineral waters, muds, lakes, rivers, streams, waterfalls). Hydromineral resources are indeed beneficial for a wide range of treatments [44]. Other assets for HT include forests, mountain areas, and local climate (including air and light). Specifically, the presence of mountain areas and forests represents a vital resource for developing WT, with positive effects on both physical and mental health [45]. Authentic local food and traditional local health remedies involving at least one of the above-mentioned NR [41,46] have been less studied. As a consequence, the majority of the healing effects addressed in the selected papers involve the use of mineral waters (Figure 4). The most evident benefit is related to the psychological aspect of health, with a main focus on relaxation, stress relief, and spiritual well-being. The literature has also investigated positive effects on musculoskeletal conditions (e.g., chronic pain in limbs, back and neck, arthritis, recovery from physical traumas), especially with the use of muds in wraps or massages [47], and chronic diseases including cardiovascular, respiratory, endocrine, and digestive, which benefits from the combination of warm climate and mineral springs [44]. Additionally, skin conditions can As a consequence, the majority of the healing effects addressed in the selected papers involve the use of mineral waters (Figure 4). The most evident benefit is related to the psychological aspect of health, with a main focus on relaxation, stress relief, and spiritual well-being. The literature has also investigated positive effects on musculoskeletal condi- tions (e.g., chronic pain in limbs, back and neck, arthritis, recovery from physical traumas), especially with the use of muds in wraps or massages [47], and chronic diseases including cardiovascular, respiratory, endocrine, and digestive, which benefits from the combination of warm climate and mineral springs [44]. Additionally, skin conditions can take some benefits from NR, as for example, psoriasis [48]. As a consequence, the majority of the healing effects addressed in the selected papers involve the use of mineral waters (Figure 4). The most evident benefit is related to the psychological aspect of health, with a main focus on relaxation, stress relief, and spiritual well-being. The literature has also investigated positive effects on musculoskeletal conditions (e.g., chronic pain in limbs, back and neck, arthritis, recovery from physical traumas), especially with the use of muds in wraps or massages [47], and chronic diseases including cardiovascular, respiratory, endocrine, and digestive, which benefits from the combination of warm climate and mineral springs [44]. Additionally, skin conditions can take some benefits from NR, as for example, psoriasis [48]. The therapies addressed in the analyzed works (see Figure 5) reflect the predominance of hydrological NR, with balneotherapy, hydrotherapy, thalassotherapy, and crenotherapy. Indeed, therapeutic waters have several natural properties benefiting health including temperature, hydrostatic pressure, viscosity, and electric conductivity, which designate them as a key raw material for conducting spa therapies [49,50]. Interestingly, the array of therapies is wider and includes treatments requiring specific equipment such as magnet therapy and electrotherapy, and trained personnel like kinesiotherapy and thermotherapy [44]. These are usually present in combination with classical hydrotherapies in the value offer of thermal institutions [30]. Few therapies from alternative The therapies addressed in the analyzed works (see Figure 5) reflect the predominance of hydrological NR, with balneotherapy, hydrotherapy, thalassotherapy, and crenotherapy. Indeed, therapeutic waters have several natural properties benefiting health including temperature, hydrostatic pressure, viscosity, and electric conductivity, which designate them as a key raw material for conducting spa therapies [49,50]. Interestingly, the array of therapies is wider and includes treatments requiring specific equipment such as magnet therapy and electrotherapy, and trained personnel like kinesiotherapy and thermotherapy [44]. These are usually present in combination with classical hydrotherapies in the value offer of thermal institutions [30]. Few therapies from alternative medicinecrystal therapy [51], aromatherapy [37] and acupuncture [51]-and purely nature-based therapies-forest bathing [39] and subterranean therapy [50]-are also mentioned. medicine-crystal therapy [51], aromatherapy [37] and acupuncture [51]-and purely nature-based therapies-forest bathing [39] and subterranean therapy [50]-are also mentioned. The investigation on the activities ( Figure 6) confirmed a strong orientation to waterbased ones, with bathing being the most mentioned. For example, special experiences as baths in hot water barrels are proposed in specific wellness products for relaxation and comfort [38]. Sport and fitness activities range from golf to gymnastic programs and swimming. More nature-related physical activities are cited in relation to mountain areas  The investigation on the activities ( Figure 6) confirmed a strong orientation to waterbased ones, with bathing being the most mentioned. For example, special experiences as baths in hot water barrels are proposed in specific wellness products for relaxation and comfort [38]. Sport and fitness activities range from golf to gymnastic programs and swimming. More nature-related physical activities are cited in relation to mountain areas including hiking, trekking and climbing [31,44]. Some works mention biking [4,12], yoga, and excursions to natural or anthropological sites of interest. Indeed, leisure contents such as trip excursions should be considered in a holistic offer approach, as they influence the decision-making of travelers in the selection of HT services [37]. This set of activities also includes treatments that are not properly therapies, but concur to tourists' wellness such as massages, cosmetic treatments, and sauna [39]. The investigation on the activities ( Figure 6) confirmed a strong orientation to waterbased ones, with bathing being the most mentioned. For example, special experiences as baths in hot water barrels are proposed in specific wellness products for relaxation and comfort [38]. Sport and fitness activities range from golf to gymnastic programs and swimming. More nature-related physical activities are cited in relation to mountain areas including hiking, trekking and climbing [31,44]. Some works mention biking [4,12], yoga, and excursions to natural or anthropological sites of interest. Indeed, leisure contents such as trip excursions should be considered in a holistic offer approach, as they influence the decision-making of travelers in the selection of HT services [37]. This set of activities also includes treatments that are not properly therapies, but concur to tourists' wellness such as massages, cosmetic treatments, and sauna [39].

Main Themes of Research on Natural Resources in Health Tourism
The content analysis allowed us to inductively identify five thematic clusters: • Analysis and exploitation of NR in HT: this thematic cluster consists of research in different scientific fields and investigates the characteristics of NR and how these could be exploited (or are already exploited) in HT. Local anthropological aspects connected to health and HT potentialities of geographical areas are also analyzed; • Nature-based factors for tourists and motivations clustering: works on this theme analyze tourist characteristics, the motivations underlying their choices, and the drivers of specific HT services including NR to identify specific clusters; • Analysis, management, and development of nature-based value offer for HT: papers in this cluster investigate potentialities, issues, and strategies fostering the development of areas or structures in HT. Several studies touch on the economic potentialities of NR as assets for development, or consider a sustainability perspective; • NR for competitiveness and marketing of HT: this set of studies investigates the determinants for competitiveness, the efficacy of communication channels, and the elicitation of key factors such as the local resources in the marketing of HT structures; • Cultural perspectives on nature-based HT: although cultural artifacts are not linked to healing effects, some works address the role and possible reuses of historical HT buildings to strengthen the therapeutic effects of NR.
These clusters reflect the main themes addressed in the selected papers and are described in the following paragraphs.

Analysis and Exploitation of Natural Resources in Health Tourism
Works pertaining to the analysis and exploitation of NR for HT purposes highlight the predominance of water as a research topic, thus mainly dealing with spa tourism, balneal tourism, and marine tourism. However, only a few works [29,52] explicitly address the healthy properties: they generally make reference to the waters' properties and their significance in balneotherapy, but lack in indicating how they are beneficial for health. Similarly, in Bacsi et al. [53] and Chernova et al. [54] the mineral properties of spa waters are only briefly mentioned. In contrast, in Pleşoianu and Draconescu [47] and Yessengabylova et al. [49], the therapeutic uses of waters and muds are explicitly described. It is worth noting that none of the above-mentioned studies cite scientific works or reports to corroborate the described waters' healing effects.
Other works [43,45,51,[55][56][57] approach the exploitation of a set of natural assets from a more general point of view to promote HT. These studies provide valuable insights for the identification of those natural features a territory should leverage to foster local HT development. Together with waters, forests, green areas, countryside, and mountains are indicated as very important, especially for WT [55][56][57]. Therefore, the quality of the NR is considered a necessary condition for HT development opportunities, both under therapeutic and recreational perspectives. Some studies [43,51] address local and authentic (i.e., indigenous and folkloristic) NR as competitive assets to develop HT for those destination characterized by a very unique set of cultural and geographical features.

Nature-Based Factors for Tourists and Motivations Clustering
As observed in the broader HT research [24], the majority of papers deal with the characteristics and motivations of tourists to reach specific destinations.
In order to determine tourist clusters, the literature distinguishes between pull and push factors (i.e., when NR are defined from the supply or demand perspective). On the supply side, NR are among the destination attributes to be considered in the identification of tourist clusters (e.g., [58]). On the demand side, groups of tourists are identified according to their socio-demographic, attitudinal, and behavioral traits [59]. Interestingly, only few papers investigate the actual tourists' satisfaction (e.g., [60]) and experience (e.g., [61]), which are fundamental in determining the attractiveness of the destination for their healing effects. Other scholars [1,34,41] analyze the tourists' perceptions of the destination or regarding the offering of health-promoting facilities, services, and activities. Some papers (e.g., [62]) further highlight that behavioral intentions driving the tourists' choice differ among their country of origin.
Finally, most of the studies in this cluster investigate the classifications of factors and drivers in the fields of hot springs, thermalism, and spas (e.g., [27]). The NR recognized as factors influencing the tourists' decision-making are the restorative/relaxation elements of natural sights [63], the mineral water [64], and the climate [37].

Analysis, Management, and Development of Nature-Based Value Offer for Health Tourism
Products offered by HT destinations represent a complex experience including, besides health and wellness treatments, attractions, entertainment, accommodation, food, accessibility, local transport, and hospitality [53]. Considering that regional NR and local culture are significant drivers, the development process has to be properly configured in order to be sustainable and overcome both the structural and operational challenges involved. Therefore, the majority of works examined in this thematic cluster underline a set of factors and key practices that need to be carefully considered. These can be classified according to different strategic areas of intervention, as highlighted in Table 4. Table 4. Areas of intervention and practices for management and development of health tourism (HT).

Value offer
Diversification of product offering [33] Increase the quality of medical services and the adoption of an overall quality-oriented approach in supply development. [48,52,66] International service standards should be adopted by wellness service providers. [55] Need to expand the concept of the value proposition, in order to include the value for a customer, the value captured by the enterprise, and the value for the community as well as benefits for the natural environment. [65] Need to improve management systems and internalize management processes. [70] Modernization of the treatment base and accommodation facilities. [52] Cooperation Development of national databases for mapping natural resources and structures. [33,48,69] Enlargement of the target group to attract an intermediate category of spa tourists. [68] Close cooperation of all stakeholders (also through private and public partnership): tourism service providers, health care institutions, local community and public sector, the environment, and the targeted tourists. [33,53,55,66,67,70] Development plans should pay attention to the interests of all stakeholders including tourists. [53] Importance of adopting a systemic approach built on a modern and integrated spatial and economic planning base. [68] Foundation of recreation-tourist clusters with healthcare and wellness mono-specialization as a way to design opportunities for development and effectively build and implement a long-term development strategy. [69] Importance of the integration of new facilities with the therapeutic facilities already in place. [68] Policy and standard The establishment of a national scale of quality assurance to provide a sound background for the top quality spa services. [33,48] Government support is crucial: proper policies and regulations are needed to stimulate investment and reduce financial and operational risks of investors. [33,48,49,55,69,70] Preventive measures for safety, security, and sanitation of wellness products should be formulated as a government policy. [55] Special training or certification programs with educational institutions. [70] Technology & Science Availability of the relevant technology (adopt new machineries and high quality equipment). [44,45,48] Research should be carried out to provide scientific evidence for the effectivity of medicinal waters. [48,52] Organization & skills Need for specialized staff with relevant qualifications (i.e., physicians, therapists, . . . ). [44,53] The full range of training should be established to correspond to the job requirements in health services. [48,55] Sustainability Modernization of water management, water treatment, and sewage facilities; exploration and mobilization of underground water for drinking water supply and rational use of available sources. [69] Implementation of sustainable water resource management and environmental protection practices. [33] Developing strategies consistent with the sustainable use of lands, hot springs, and other natural resources. [33] Intensification of the adoption of energy-saving technologies and the development of renewable energy utilization in facilities as well as other use of thermal water. [48,69] Monitoring is needed to preserve sub-terrain water resources against pollution and over exploitation. [48] Estimation of recreational resources in the framework of geo-environmental analysis is a necessary prerequisite for the rationalization of natural resources and health tourism development. [49] The value offer area refers to facilities, with the necessity to expand the concept of value proposition in order to include value for customers, enterprises, community, and benefits for the natural environment [50,65]. Other relevant practices identified in this area include the diversification of product offering, the increase in the quality of medical services [52], and the adoption of an overall quality-oriented approach in supply development [66]. In the cooperation area, analyzed practices involve the perspectives of multiple actors. Research primarily underlines that there is a crucial need for a close cooperation of all the stakeholders of HT, starting from tourism service providers, health care institutions, municipalities, and public authorities [33,53,55,66,67]. A primary task of tourism destination management organizations is in fact the adoption of a collaborative approach through the establishment of partnerships (both private and public) among key players [66]. Development plans should address the interests of all of these stakeholders including tourists [53]. It is thus important to adopt a systemic approach built on an integrated spatial and economic planning base [68] that comprises the implementation of national databases for mapping NR and structures [33,69], and the integration of new facilities with therapeutic facilities already in place [68]. Some interesting insights are provided by Shablii et al. [69] for the foundation of recreation-tourist clusters with health care and wellness mono-specialization. In the policy and standard area, government support is crucial at all stages of WT product development. Interventions include defining proper policies and regulations to stimulate investment and reduce the financial and operational risks of investors [33,49,55,69,70], and to establish a national scale of quality assurance [48]. Regarding the technology and science area, different scholars (e.g., [48,69]) highlight the need for providing evidence on the effects of medicinal waters to leverage recommendations on the use of natural areas of resorts and provide patients with complete documentation on the impact of treatments. Other important issues in this area concern the need for investing in relevant technology [44,48,53]. The practices collected in the organization and skills sphere stress the necessity for qualified human resources and specialized staff to guarantee high professionalism [44,48,53,70], and suggest special training or certification programs with educational institutions [70].
Finally, the sustainable development of HT is argued to be necessary for long-term competitiveness in different contributions [50,53,67]. The importance of sustainability practices and developing strategies consistent with the sustainable use of lands and other NR [71] is highlighted by Shablii et al. [69] and Lee and King [33], who suggest the implementation of sustainable water resource management and environmental protection practices. Indeed, management of the natural environment is considered fundamental by tourists as it affects the health of both the residents and visitors [11]. In general, a complex evaluation of HT potential elements through an estimation of NR in the framework of a geo-environmental analysis is a necessary prerequisite for HT development [49].

Natural Resources for Competitiveness and Marketing of Health Tourism
The successful marketing of HT destinations, especially those with healing NR, is fundamental to valorize and differentiate local tourism products. Research on this theme focuses on areas considered as fundamental for enhancing the competitiveness of HT in general, or in a specific geographical zone. Marketing strategies are discussed to be implemented in single businesses [72], or carried out by HT stakeholders including national health supporting organizations [48]. Examples of means for promotion of a HT destination include the use of effective communication media as websites [28], and activities aimed at bestowing destinations with wellness-related meanings [38]. Schalber and Peters [12] highlight the importance of considering the sustainable management of NR when determining the strengths of territories to be leveraged in a marketing perspective.
NR can have a key role in competitiveness as they represent a source of differentiation of health-related destinations and harness the potential for new business opportunities [32]. Differentiation in terms of environmental experience (the feelings experienced by tourists from the physical environment) is a key determinant of customer loyalty to a HT destination [73]. When different organizations rely on the same NR, they have to innovate both their services and infrastructures as well as their marketing [28]. In this case, the creation of uniqueness represents a dual challenge: there is the need to diversify the destinations' product structure and to exploit the role of geographical features in maintaining competitiveness [66]. The richness entailed in the NR of specific geographic areas can represent the core content of a wellness-related experience product [38].

Cultural Perspectives on Nature-Based Health Tourism
Cultural aspects related to destinations can enhance HT stays and impact on individual well-being. Papers in this cluster share the idea that health and wellness are related to the cultural and historical background of destinations. In some works, nature-based activities like hiking or bathing are linked with aspects other than NR, like spirituality [31], relaxation, and socialization [34]. The concept of the authenticity of a destination and its nature-based activities and remedies is rooted in the cultural tradition of the indigenous populations and plays a relevant role in helping the tourists experience otherness and pristine environments [43,51]. Some works investigate the role of therapeutic landscapes (places with a considerable number of visitors and a long-established reputation for healing) [42,46]: these are characterized by unique characteristics (either natural or anthropological) that can attract tourists such as springs, fresh air, or an uncontaminated environment upon which local populations built a tradition of wellness, both physical and spiritual.

Toward a Research Agenda for Nature-Based Health Tourism
A deeper analysis of the different perspectives on the use and exploitation of NR in the HT and WT literature shows that the role of NR is still receiving little attention. Considered the key importance of HT within the wider tourism management research, and the need to focus on sustainable ways of exploiting NR, this paper calls for the development of a research stream entirely focused on nature-based HT.
The review results enabled us to elaborate a research agenda around the key research areas surrounding this stream of studies and the main topics requiring further investigation in a comprehensive research framework (Figure 7). (places with a considerable number of visitors and a long-established reputation for healing) [42,46]: these are characterized by unique characteristics (either natural or anthropological) that can attract tourists such as springs, fresh air, or an uncontaminated environment upon which local populations built a tradition of wellness, both physical and spiritual.

Toward a Research Agenda for Nature-Based Health Tourism
A deeper analysis of the different perspectives on the use and exploitation of NR in the HT and WT literature shows that the role of NR is still receiving little attention. Considered the key importance of HT within the wider tourism management research, and the need to focus on sustainable ways of exploiting NR, this paper calls for the development of a research stream entirely focused on nature-based HT.
The review results enabled us to elaborate a research agenda around the key research areas surrounding this stream of studies and the main topics requiring further investigation in a comprehensive research framework (Figure 7). The research framework highlights that literature on analysis and exploitation of NR in HT should inform and provide the knowledge base for the "building blocks" considered as essential in future studies on nature-based HT (i.e., NR, their healing effects, related activities and therapies, as highlighted in Section 4.2). Concerning this theme, scholarly attention in the HT literature appears to focus mostly on hydrological NR, mainly thermal The research framework highlights that literature on analysis and exploitation of NR in HT should inform and provide the knowledge base for the "building blocks" considered as essential in future studies on nature-based HT (i.e., NR, their healing effects, related activities and therapies, as highlighted in Section 4.2). Concerning this theme, scholarly attention in the HT literature appears to focus mostly on hydrological NR, mainly thermal waters and their use in different therapies. However, very few papers listed the specific properties and none of the works referred to scientific studies assessing the waters' properties on health. Consequently, the most mentioned healing effect of thermal waters is stress relief, as spas and hot springs are normally associated with a state of physical and psychological relaxation. These are beneficial for many different medical conditions, but there is still need to understand the mechanisms for these effects as well as for the development of related facilities [74]. Other NR are only mentioned, and their effects on health are rarely described or supported with references. Therefore, future studies should focus on underlining both short-and long-term effects of specific NR existing in HT scenarios (e.g., mountains and their altitudes, seashores and their specific microclimates, caves and their ionized air). Such efforts could strengthen the link between NR and their effects on health, clarifying whether some folkloristic and generally accepted ideas (e.g., specific locations in China characterized by a particular microclimate help people live longer and be healthy) have a positive feedback on tourist choices.
Hydrological NR predominance also impacts the analyzed activities: excluding those related to thermal waters (the majority), the proposal of HT activities includes very common touristic activities, also not strictly related to health issues (like sports, cosmetic treatments, and excursions). Although the benefits of sporting activities on health are known, it is arguable that strengthening the relationship between NR and their effects on health can have an impact on both research and industry. Further research on new kinds of nature-based product configurations and offers may inject new life into the HT industry.
Future studies on nature-based factors for tourists and motivations clustering should distinguish the change in perception of the importance of specific NR in different destinations, and how this drives the choice of tourists searching for well-being. Moreover, differences between the service providers' perceptions and the experience of customers in the use of specific NR should be considered in the analysis of drivers, both from the supply and demand side. The proposed agenda also calls for future studies dealing with the analysis, management, and development of nature-based value offer for HT. Although most works report practices for developing HT areas and structures, a structured analysis of these practices is missing, and their systematization according to specific HT scenarios is required to support practitioners in strategic planning and operations. Future studies could also investigate whether the presence of multiple NR in the HT destination could lead to the definition of value and product offers that exploit their synergic combination.
Moreover, sustainability issues in the development of HT are faced in some contributions under the perspective of preservation of NR and responsible growth. Nevertheless, the concepts of sustainability are only partially explored and further research would be necessary for the definition of specific guidelines for sustainable development in the HT framework, according to the specific resources and contexts analyzed under both the economic and environmental perspectives.
Focusing on the research area of NR for competitiveness and marketing of HT, it emerged that marketing concepts seem to focus on the therapies based on NR as thermal water. Future studies should advance research on the different means exploitable to promote the true value and potentialities of NR in HT destinations. More comprehensive research should also consider the marketing actions involving NR along the multiple phases of tourists travelling for well-being, i.e., the pre-and post-visit to the destination, the en-route, and the on-site experience as well as their influence in generating positive and negative experience outcomes [17].
Finally, concerning cultural perspectives on nature-based HT, few papers addressed the role of the cultural characteristics of destinations in HT. Moreover, it has to be taken into account that geographical and cultural factors could have influenced the results of the existing research models [2]. Future studies should consider the role of culture and heritage in HT, the effects of tourists immersing themselves into a new cultural perspective, and especially both the related short-and long-term effects on health.

Conclusions
This study reviewed methodologies and frameworks for mapping and analyzing under different perspectives the value of the NR of a region and related activities for HT. Considering the key importance of a better use of NR in the HT context [10], this systematic review focused on identifying which are being currently investigated, their proposed exploitation, and what role they play in HT business. This has allowed us to identify the most investigated resources and highlight those areas and NR presenting a not fully researched potential.
This study contributes to the HT and WT literature by laying the foundations for the development of a research stream entirely focused on nature-based HT. It also proposes a framework based on the key research areas involved, and identifying the gaps calling for future research in each considered area. Moreover, the results highlight the need for multidisciplinary research efforts, confirming the relevance of the HT topic in different fields.
From a managerial perspective, the performed analyses stress the relevance of the correct exploitation and parallel preservation of NR in specific areas to enhance tourism activities. The synergic combination of NR, healing effects, activities, and treatments represents a key reference for managers and policy makers to identify areas of intervention, marketing actions, and practices for industry development. This is corroborated by the importance of health issues due the rise in chronical diseases and an aging society, also compounded by the long-term effects of the current COVID-19 pandemic on business and people.
This work presents some limitations. First, only papers published in peer-reviewed academic journals were considered. Future studies may thus expand the sources to be included in the review. Second, even if the integration of three databases allowed us to cover a wide journal range, keywords selected in the identification and search strategy may have potentially left out interesting works considering the debates and the dynamics in the wider HT literature.