Cork of Douglas-fir bark: Impact of structural and anatomical features on usage

Abstract

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is one of the best conifer timber species. Its bark contains a substantial proportion of cork that may have a valorization potential, given adequate structural and cellular features. This study was made here onbark samples from mature trees from the north and central mountains of Portugal. The area proportion of cork was determined by image analysis, the cork tissue was observed with electron scanning microscopy (SEM) and cell dimensions measured.The cork is not continuous within the rhytidome, and the layers are interspersed with phloem regions. The cork layers are not continuous along the tangential or axial directions. Older trees contain on average a thicker rhytidome and a higher proportion of cork. The cork tissue was characterized by the presence of extensive areas of cells that are crushed or completely collapsed, making up a compressed and very compact structure with patches of uncompressed cork. The compression occurs in the radial direction and is clearly observed in transverse and radial sections. In the uncompressed regions the majority of the cork cells are hexagonal and pentagonal prisms stacked base-to-base and aligned in the radial direction in parallel rows. On average, the prism height and base area are 55μm and 1388μm2, respectively, with a 1.3μm cell wall thickness. To obtain pure cork fractions from Douglas-fir bark, trituration and fractionation processes are needed. Also the use of Douglas-fir cork as a cellular material will be restricted by the extensive cell compression.