Quantifying sapwood width for three Australian native species using electrical resistivity tomography

Sap flow measurement techniques have been successfully applied in ecohydrological studies as they can be used to estimate watershed transpiration. Sapwood area (A_s) is one of the most important tree parameters for estimating transpiration from point sap flow measurements. Accurate and efficient determination of A_s and the relationship between A_s and other tree parameters (e.g. diameter at 130 cm, DBH) is essential for the practical upscaling of sap flow data. The conventional methods for determining sapwood area are accurate (although coring can damage the trees) but prohibitive when a large number of trees need to be sampled. Electrical resistivity tomography (ERT) is a non-destructive geophysical method, which detects the moisture or electrolyte concentration difference in wood tissue to identify sapwood-heartwood boundaries. In this study, a rigorous method for quantifying sapwood width and area using ERT is detailed using measurements on 30 trees of three Australian native species (drooping sheoak and two eucalypts). Results show that sapwood widths estimated by ERT were in good agreement with those obtained from wood core analyses for the three species. A strong linear relationship was observed between A_s and DBH and between heartwood radius and DBH. The A_s-DBH relationship extends the synthesis for eucalypts species in previous studies. Sapwood width was overestimated for trees under wet conditions, which indicates that the ERT technique for sapwood width quantification is limited under such conditions.