**5. Conclusions**

This paper evaluated the global research and publication trends in the remote sensing of PAs monitoring from 1991 to 2018. This analysis comprised eight main aspects: document types, publication output, subject categories, source journals, countries, organizations, and keywords. The results showed that since 2004, the number of publications increased rapidly and steadily. Environmental Sciences was the largest subject category. The highest number of papers was published in the two journals on Remote Sensing of Environment and Remote Sensing. The USA published the most in application of remote sensing technology in PA monitoring. Institutions a ffiliated to the USA have a massive number of publications and strong international collaboration in such type of explorations. Landsat, MODIS, and LiDAR are the most commonly used satellites and sensors. Most of the research was focused on classification, time-series analysis, and model methods. Keywords selections indicate that "Landsat", "deforestation", "LiDAR", "conservation", and "biodiversity" are among the most common subjects in the remote sensing of PAs. Studies on PA monitoring using remote sensing are mainly focused on change detection, biodiversity conservation, and the impact of climate change. In the future, we should continue to pay attention to the development trends and hot spots for the remote sensing monitoring of PAs. Researchers from all countries should strengthen the international exchanges of ideas and actively promote international research cooperation.

**Author Contributions:** P.D. designed the research, collected the data, analyzed the results, and wrote the first draft. Y.W. conceived the research and reviewed the manuscript. P.Y. processed and visualized the data. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Humanity and Social Science Foundation of the Ministry of Education (19YJCZH229) and the China Postdoctoral Science Foundation (2018M632719).

**Acknowledgments:** We appreciate the insightful and constructive comments and suggestions from the anonymous reviewers that helped improve the quality and presentation of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
