Within‐tree spatial heterogeneity of sap flow velocities and wood properties and its effect on the uncertainty of tree level sap flux estimates

Societal Impact Statement As forests are important for biodiversity and recreation and provide vital resources globally, it is essential that their health can be accurately monitored, for example, by using sap flow probes to measure tree water uptake. We investigated whether sap flow measurements might differ at various locations within the same tree due to variation in wood properties. We found high variability in sap flow measurements across one tree, meaning that accurate measurements might require a large number of probes to be used per tree. This information is critical for ensuring accurate measurements and uncertainty estimation in future research. Summary To derive the transpiration flux of a tree from the measurements of a sap flow probe several parameters, including the thermal properties of the wood, the conducting sapwood area, and radial sap flow velocity profiles, have to be determined. Uncertainties in these parameters can be reduced by repeated sampling, which conflicts with the intention of minimally invasive investigations and efficient use of limited resources. In this study, we instrumented a single tree with 23 sap flow sensors, sampled 30 wood cores from it, and finally cut the whole tree to acquire stems disks for additional measurements. We assessed the heterogeneity within the tree stem and quantified the uncertainties and their propagation into the final transpiration flux estimate. We found that estimated transpiration fluxes based on one single sap flow probe deviated from the pooled average value by more than 15% in one of three cases. The main source of this uncertainty was the spatial heterogeneity of sap flux densities, followed by the uncertainty of the conducting sapwood area, which could effectively be reduced by including additional nondestructive observations of the stem cross section. This study's unusually high number of sap flow sensor installations and wood core samples for one single tree exemplarily shows, that even four sensors installed around a tree stem may still leave a considerable amount of uncertainty for tree level transpiration flux estimates.