Improving Our Understanding of Water Flow and Transpiration in Plants
Understanding water flow and transpiration in plants is an important component of understanding land-atmosphere interactions, but methods to make these measurements are poorly developed. Thermal dissipation probes are widely used to estimate the movement of water through woody plant stems, branches, and roots. The mathematical treatment of the heat-transfer characteristics that underlie this technique are complex. Models that allow ecologists to evaluate the performance of those techniques are lacking, thus limiting advancements in process-level understanding and technology development. Scientists at Oak Ridge National Laboratory (ORNL) have now developed a model of conductive and convective heat transfer in sapwood that takes into account the thermal properties of wood and the physical dimensions and thermal characteristics of the probes that can be used to identify shortcomings in the thermal dissipation approach to measuring water use in trees. After validating the model’s performance using data from field studies, the team observed that the fundamental calibration equation upon which the technique is based was highly sensitive to variation in water content, sapwood density, radial gradients, wound diameter, and other operational characteristics of this technique. Uncertainty analysis suggested that significant over- and under-estimation of sap flow was possible using the traditional calibration equation. Improved estimates of water use and latent energy exchange should further understanding of land-atmosphere interactions when applied to a variety of ecosystems such as the AmeriFlux study sites.
Wullschleger, S. D., K. W. Childs, A. W. King and P. J. Hanson. 2011. “A Model of Heat Transfer in Sapwood and Implications for Sap Flux Density Measurements Using Thermal Dissipation Probes,” Tree Physiology 31, 669-79. DOI: 10.1093/treephys/tpr051.