Less than 20-min time lags between transpiration and stem sap flow in emergent trees in a Bornean tropical rainforest

Sap flow velocities in stems (Sfs) were measured in four 50-m tall emergent trees in a Bornean tropical rainforest. Cross-correlation analysis was used to estimate time lags between diurnal patterns of Sfs and canopy-scale transpiration (Tr) calculated from a big-leaf model validated by water-vapor flux measurements at this site. The time lags between Sfs and Tr were found to be less than 20 min in these trees in both wet and medium dry conditions. In addition, the time lags between Sfs and sap flow velocities measured in a branch at the top of an emergent tree were also less than 20 min. These results are in contrast to those of previous studies conducted in a neo-tropical forest ecosystem, which reported significant time lags of 40–120 min between transpiration and stem sap flow in emergent trees. The results of this study suggest that hydraulic resistance along the conductive pathway and internal water storage between tree crown and stem are insignificant for this forest. Our results also suggest that sap flow measurements conducted at accessible points can be used to estimate canopy transpiration on a diurnal time scale without accounting for the time lags between canopy transpiration and stem sap flows, despite the 50-m tall forest canopy at our site.