Heat tracer-based sap flow methods for tree transpiration measurements: A mini review and bibliometric analysis.

Plant transpiration is one of the main water flux components on the earth's land surface. Accurate measurement of plant transpiration is critical to better understand plant water use, to develop water management strategies, and to explore the influence of plants on regional and even global climate. At present, heat tracer based sap flow (HTSF) techniques are the dominant method to measure sap flow and then to estimate plant transpiration at the individual plant level. However, the majority of current research focuses on specific applications or the evaluation of the method itself, while the overall picture of the use of HTSF methods is lacking. The objectives of this study were two fold: (1) briefly review the theories and categories of various HTSP methods, and (2) investigate the use of HTSF methods in measuring plant transpiration using bibliometrics. The HTSF methods were divided into six transient and three steady-state techniques based on their governing theory. Each method was described mathematically and their applications and pros and cons were briefly discussed. The bibliometric analysis was conducted using 3,964 papers published between 1992 and 2020 archived in the Web of Science core collection. The number of articles published in this field has increased exponentially over the past three decades. The analysis identified the most productive authors, organizations, and countries, as well as the most utilized HTSF methods and journals in which the papers were published. USA, China, and Germany are the top three countries applying HTSF methods to measure transpiration. The thermal dissipation method is most widely used mainly because they are the most easily self-manufactured sensors as well as the availability of commercially-available sensors. Agricultural and Forest Meteorology, Tree Physiology, and Agricultural Water Management are the most widely utilized journals for publication of results. In addition, world distribution maps of the use of HTSF methods and tree species measured were drawn based on the 741 selected publications with in situ measurements.