Does Bark-Decortication for Hiwada Production Change Mechanical Properties of Xylem in Chamaecyparis obtusa ?

Abstract

Bark isreproducible by the tree when its cambium is carefully left without damaging it by decorticate technicians.Throughout the world, reproduced bark has been traditionally used for daily commodities such as cork from cork oak (Quercussuber)in Portugal, or house buildings,such as roof tilesof 굴피rom sawtooth oak (Quercusacutissima) in Korea and Hiwadaroof plate from japanese cypress (Chamaecyparisobtusa) in Japan, etc.For traditional roof constructionin Japan, especially for national heritages, Hiwada bark is indispensable, and a number of cultural properties are awaiting for its supply for repairesthat arenecessary approximatedlyevery 40 years. However, Hiwada production is suffering from a shortageof old trees, which must bejapanese cypresstrees of more than 80 years old. Also,there is alack of traditional master technicians, motokawashi, who can decorticate the bark without damaging the cambium, and due tothe misunderstanding oftimber sellersthat decorticationimpaires the mechanical properties of the tree. We have been engaged in a series of research projects to solve this shortage of Hiwada. This study aimed to clarify whether bark decortication for Hiwada production changes the mechanical properties of the xylem,focusing on xylem production in the cambium after decortication, by measuring Young’s modulus and the cellulose-microfibrilangle. It is difficult to discuss the impact of decortication accuratelywithout considering other effectssuch asindividual variabilityand climate change. Bark was carefully removedincontrol experiments and by quantative treatments.A pair of trees, grown under very similar conditions, one of which was decorticated and the other was not treated, were compared. In order to remove the individual variability at each cell level, a series of daughter cells with the same mother cell in the cambium were sampled in annual rings and compared. Then, five to ten annual rings each before and after decortication and the changes in ratios were compared to the control, andthe impact of decortication itself was discussedandinvestigated.