Table 1 Division of the research stages of aesthetic value protection and tourism development at WNHSs

Exploration phase (before 2004)

The Convention Concerning the Protection of the World Cultural and Natural Heritage (hereinafter referred to as "the Convention"). The Convention was promulgated, which clearly stipulates that WHNSs must meet the value criteria and conservation management requirements. In the 1990s, the global economy recovered and tourism activities developed rapidly, and the tourist destinations were mainly heritage sites satisfying aesthetic value. However, with the threat of tourism activities to the natural beauty of the heritage site landscape, whether to carry out tourism has aroused controversy

The application of aesthetic value standard is controversial, and the exploration of the conservation of heritage as a whole are available, and there is no research literature on the conservation of aesthetic value. The study of tourism examples in WNHSs is relatively small and mainly focuses on theoretical research

Start-up phase (2005–2014)

At the beginning of the twenty-first century, there is a rapid development of domestic and foreign tourism, the world's pursuit of high-quality tourism that is accompanied by a serious environmental crisis. With the development of tourism, some scholars admit that the economic benefits generated by aesthetic value can be beneficial to the protection of WNHSs. However, the aesthetic value OUV of heritage sites is also threatened by the construction of tourism infrastructure and tourists. People gradually begin to discuss the protection of aesthetic value OUV from ecology and integrity of heritage sites

The depth of research has increased, with more conservation studies highlighting the universal values of WNHSs with actual cases. Conservation studies on aesthetic value are still lacking. The application of aesthetic value standards is greatly affected by subjective factors. Therefore, in 2005, IUCN rearranged the order of standards in the Operation Guide. Scholars began to make an in-depth interpretation of aesthetic value criteria, but the protection of aesthetic value is lack of pertinence and microscopical research. Researchers began to focus on the importance of sustainability theory in the synergy between conservation and tourism development of WNHSs

Development Phase (2015-present)

WNHSs have become a hot topic both at home and abroad, and the International Union for Conservation of Nature (IUCN) makes further revisions to the OUV criteria, reflecting a change in the concept of WHSs conservation, with heritage development and utilization based on the conservation of OUV. People beginning to value the question of how to improve coordination between conservation and tourism. The importance of the aesthetic value of heritage sites as an increase in human well-being and the importance of preserving aesthetic value were recognized

WNHS research is richer, the recognition of OUV is strengthened, the protection of other OUV including aesthetic value is beginning to be emphasized, and the concept of sustainable development is more emphasized in tourism at heritage sites. The synergistic development of aesthetic value conservation and tourism is in the exploration stage