Alpine treelines as ecological indicators of global climate change: Who has studied? What has been studied?

As the "front line " of coping with global climate change, the alpine treeline has been widely investigated by ecologists for a long time. In this study, 3180 articles in the Web of Science database from 2000 to 2021 are visualized by using CiteSpace bibliometric software from the perspectives of basic literature features (volume, discipline, journal, author, institution, and country), academic layout, research hotspot, and research frontier to have a deeper understanding of the development laws and hot spots in the alpine treeline. Results show that: (1) The number of papers published in alpine treeline research field is increasing annually, and the professional degree of papers (Sp) is 0.13-0.14, it is lower than that in other fields, implying a tendency of multi-disciplinary integration . (2) The core journals are headed by Arctic and Alpine Research, and J. Julio Camarero is the most prolific author in the field. (3) From the analysis of countries and institutions, the United States and the Chinese Academy of Sciences have leading positions in this research field. (4) Over the last 20 years, alpine treeline researchers have primarily focused on three key words (climate change, environmental characteristics, inter specific interactions, etc.), research sites (National Glacier Parks, the Alps, the Sygera Mountains, etc.), and research species (Norway spruce, European spruce, Abies georgei, etc.). (5) The first phase (2000-2009) focuses on the formation and changes of the alpine treeline in the context of climate change, and the second phase (2010-2021) focuses on the driving mechanisms of climatic factors, physiological change of tree species and grasslands on the treeline ecotones, and the influence of human activities (logging, grazing, etc.) on the alpine treeline. (6) Academic publications in this discipline have a short half-life (3.85 +/- 1.70 years), and the aging rate is high and changing quickly. In the coming years, more research on alpine treeline will be devoted to explain the mechanism of tree species limitation in alpine treeline under climate change, influence of treeline changes the carbon and water cycles. There is a press need to explore the driving mechanism for treeline shift from the perspectives of plant physiology and soil carbon and nitrogen cycles.