Farm Forestry New ZealandSapwood depth tool – proof of concept field prototype
By Nurzhan Nursultanov, Bill Heffernan, June 2019.
Download SWP-T077 (pdf)
Executive summary
The sapwood depth (or size of heartwood) in standing trees can be measured by (1) the destructive method, (2) the semi-destructive method, or (3) the non-destructive method, with minimal damage to a tree. The destructive method involves cutting a tree and measuring the sapwood depth on the crosscut. This method relies on the colour difference between heartwood and sapwood or may require a special dye if the colour difference is not obvious. The semi-destructive method such as coring (Li, Apiolaza, & Altaner, 2018) or the sap flow velocity method (Pearsall, Williams, Castorani, Bleby, & McElrone, 2014) results in minor localized damage to a tree. Coring is fast but relies on the colour difference. Relatively non-invasive methods such as Picus TreeTronic (Argus Electronic GmbH, Rostock, Germany) are complex, expensive, and have a long set-up time.
This report describes the concept of a new sapwood depth tool that determines the location of sapwood/heartwood interface by measuring the spatial electrical current change between two energized electrodes versus displacement below the bark surface and uses this to determine and record the heartwood depth with a specially developed android app. The report describes the main operational principles of the field prototype tool, including the updated algorithm of sapwood depth identification. The sapwood tool was tested on a rig that mimics the change of electrical resistance in a tree and on logs of Eucalyptus globoidea and Eucalyptus nitens.
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