Tropical forest transpiration estimates are geographically, ecologically and methodologically biased: a systematic review of sap flow research

Tropical forest transpiration is essential for the maintenance of the water and carbon cycles and regulation of the global climate. However, recent collective efforts to compile sap flow data, which is the most widely used method for quantifying transpiration in woody stems, have revealed a large data gap in the tropics. Furthermore, accurately upscaling point measurements of sap flow to whole-plant water use (PWU) requires data inputs and corrections that may be difficult to determine in highly diverse tropical forests. This systematic review has two main goals: to assess the methodological limitations of PWU research in tropical forests and to highlight geographical and ecological gaps among current tropical forest PWU research. Our Web of Science search used terms related to ‘plant water use’ and ‘tropical forest’ and gathered all peer-reviewed studies that used sap flow sensors and upscaled point measurements to PWU in tropical forests. Based on 72 studies conducted in 85 locations, we found that drier forest types (based on Holdridge life zones) are largely underrepresented in the literature, with more than 75% of the research output concentrated in wetter forest types. We also identified a substantial knowledge gap on liana sap flow, which appeared in less than 6% of the studies. Regarding the methodological limitations, most studies omitted information regarding the number of sensors deployed per plant and the method of zero-flow determination. Other shortcomings specific to each sap flow sensor type are highlighted. Our study underscores the need for standardising key methodological aspects and identifies significant geographical and ecological gaps in tropical forest PWU research. It provides a foundation for further studies to refine estimates of the present and future contributions of tropical forest transpiration to the global water cycle.