Assessment and Comparison of Sapwood Diameter Measurement Techniques in Scots pine: A Multi-Method Analysis

Accurate determination of the sapwood area in trees is crucial for understanding stem hydraulic capacities and sap to heartwood transitions. Hence it is essential to analyse sap flow data.

This study investigates the efficacy of various methods - electric resistivity tomography (ERT), visual inspection, thermography and staining - in measuring sapwood widths in Scots pine (Pinus sylvestris) growing in a mountainous pine forest in Mieming, Austria (AT-Mmg). Twenty trees were probed at breast height utilizing these techniques, and comparative analyses were conducted to assess their accuracy and efficiency.

ERT was performed with a PICUS system (Argus electronic, Germany). Low resistivities indicated high water content, and values along west and east oriented radii were extracted to determine sap wood borders. For the remaining measurements, wood cores (diameter 5mm, west and east oriented) were taken with an increment borer. Visual determination of the sapwood width was carried out immediately after coring, right before thermal images of the cores were taken. Thermography relied on temperature variation along the core, due to evaporative cooling of the sapwood. Finally, cores were sealed in a chamber which enabled axial flow of safranin solution and thus staining of sap wood areas.

All methods allowed identifying the transition zone between sap and heartwood. Remarkably, the comparative analysis among these methods unveiled close alignment and consistency in sapwood width measurements. Compared with staining, serving as the benchmark as based on the xylem hydraulic function, visual inspection, thermography, and ERT yielded results congruent with the stained cores.

The results demonstrate that all techniques under study enabled reliable measurements of sap wood widths in Scots pine. Analyses based on wood cores were easy and less time consuming than ERT, though the latter enabled insights into the entire cross-sectional sapwood. The capability and agreement of these methods for use with other conifers and/or angiosperms remains to be tested.