Assessing global transpiration estimates: Insights from tree-scale sap flow analysis.

• A comprehensive evaluation of global gridded transpiration products based on SAPFLUXNET for the first time and collocation analysis validation as a promising proxy for ungauged regions. • GLEAM and the weighted average based on collocation analysis perform well across diverse vegetation types but show precision limitations at lower and higher value ranges. • Accurately capturing how vegetation responds to changes in VPD, root-zone soil moisture, and radiation is crucial for precise transpiration estimates in these products. Ecosystem transpiration estimation presents significant uncertainties, prompting the need for direct assessment through sap flow measurements to validate T products and their associated modeling mechanisms. Additionally, the scarcity of global site data urges us to seek reliable error assessment and integration methods that do not rely on observations. This pioneering study conducts a comprehensive global-scale analysis, evaluating uncertainties in four prominent products using SAPFLUXNET data and delving into multi-source data assessment and fusion using collocation analysis. Results highlight GLEAM's superior performance across diverse vegetation types, closely trailed by weighted average and PMLv2, with ERA5L displaying the poorest performance. While discrepancies exist among product performances, their estimations across low, median, and high percentiles demonstrate moderate differences. Further sensitivity analysis underscores the critical role of accurately representing vegetation responses to VPD, root-zone soil moisture, and radiation for improved transpiration estimates. Additionally, collocation analysis emerges as a reliable tool for error analysis, with collocation-based fusion results effectively reducing input errors and demonstrating the best estimation of multi-year mean and trend. This study strongly advocates for heightened precision in these products and continual assessments to refine ecological models. [ABSTRACT FROM AUTHOR]