Transpiration is a key process driving energy, water and thus carbon dynamics. Global T products are fundamental for understanding and predicting vegetation processes. However, validation of these transpiration products is limited, mainly due to lack of suitable datasets. We propose a method to use SAPFLUXNET, the first quality-controlled global tree sap flow database, for evaluating transpiration products at global scale. Our method is based on evaluating temporal mismatches, rather than absolute values, by standardizing both transpiration and sap flow products. We evaluate how transpiration responses to hydro-meteorological variation from the Global Land Evaporation Amsterdam Model (GLEAM), a widely used global transpiration product, compare to in-situ responses from SAPFLUXNET field data. Our results show GLEAM and SAPFLUXNET temporal trends are in good agreement, but diverge under extreme conditions. Their temporal mismatches differ depending on the magnitude of transpiration and are not random, but linked to energy and water availability. Despite limitations, we show that the new global SAPFLUXNET dataset is a valuable tool to evaluate T products and identify problematic assumptions and processes embedded in models. The approach we propose can, therefore, be the foundation for a wider use of SAPFLUXNET, a new, independent, source of information, to understand the mechanisms controlling global transpiration fluxes.

The future of tropical forests is dependent on the capacity of young trees to adjust to drought. We evaluated multiple hydraulic traits indicative of the drought tolerance of small trees across nine common genera at the world’s longest-running tropical throughfall exclusion experiment and compared their responses with surviving large canopy trees. Small understorey trees increased specific hydraulic conductivity by 56.3% and leaf:sapwood area ratio by 45.6% in response to the drought treatment. However, understorey trees in both a control and the throughfall exclusion treatment had significantly lower minimum stomatal conductance and maximum hydraulic leaf-specific conductivity relative to the large trees, as well as significantly greater hydraulic safety margin (HSM) and PLC and embolism resitance, occupying a distinctly different hydrualic niche. The greater HSM of small understorey trees relative to large canopy trees likely enables them to adjust other aspects of their hydraulic systems to take advantage of increases in light availability in the understorey, driven by drought-induced mortality of canopy trees. Our results suggest that small understorey trees can adjust their hydraulic systems in response to changes in water and light availability and this has major implications for the regeneration potential of tropical forests following droughts.