Comment on Wang et al. ‘Quantifying sapwood width for three Australian native species using electrical resistivity tomography’

Scaling water use of individual trees to estimate the water use of entire forest stands is a widespread practice. Errors must be kept to a minimum, as any initial error will propagate during scaling, rendering final estimates inaccurate. One parameter that is crucial for the determination of volumetric use of water of individual trees is the area of conducting sapwood (AS). This parameter can easily be calculated once the diameter of the stem (mostly measured at breast height, i.e. 1.3m above ground), as well as its bark and sapwood depth (DS) at measurement height is known. While measurements of stem diameter and bark width are straightforward, the measurement of DS is not. More often than not, trees have to remain intact, and collection of wood discs that can be used to assess DS is not possible. As a result, a range of invasive and non-invasive methods can be employed to determine DS. As currently no non-invasive method is available to determine AS on standing trees accurately, any methodological progress or development of new techniques is highly valuable.