Identification of Mixed Tropical Hardwood (MTH) by characteristic morphological features – a contribution to species protection

Abstract

This study deals with the investigation of 38 Asian timbers for identification purposes of their genera, thus making it possible to detect these timber genera in fibrous materials such as pulp and paper products as well as fiberboards. Currently, the standard method for determining wood genera in fibrous materials is to compare the anatomy of the cells with references. The identification is made possible by the examination of certain cells of hardwood: the vessel elements. These have the most numerous and conspicuous characteristic morphological features which differ from genus to genus, thus enabling these genera to be differentiated. There are a number of atlases that list references to temperate woods, which are frequently used for pulp and paper. The most comprehensive sample is the "Fiber Atlas" by Ilvessalo-Pfäffli (1995). The chief objective of the present investigation is to complement the hitherto existent fiber atlases with a whole new set of genera references, namely those of the Asian tree genera. Thus, it is now for the first time possible to determine these Asian tree genera in fibrous materials as well. Many of these belong to the group “Mixed Tropical Hardwood” (MTH). Particularly with regard to the increase in pulp production in Asia, and especially in Southeast Asia, the need has arisen to provide references for the indigenous Asian tree genera. The investigation procedure consisted of macerating each genus of a total of 35 genera (38 species). Each macerate was then stained and the vessel elements were separated out. Permanent microscope slides of the vessel elements were made and quantitative data on the vessel elements were collected. High-quality microphotographic images of each genus were taken and a selection of representative elements was compiled on one page. In addition, descriptions of the diagnostic features, including quantitative data, were noted. Of the respective genera, selected dimensions of the vessel elements and their intervessel pits as well as of the fibers were recorded. A significance analysis was carried out for the 23 species of the first publication (Helmling et al. 2016). In addition, comparisons of the investigated genera within two families were made. For this study, four wood genera of the Anacardiaceae and six genera or subgenera of the Dipterocarpaceae were compared with each other. The compared genera of the Anacardiaceae are distinguishable, as a blind test showed. Campnosperma shows the biggest differences, while the other three genera Gluta, Mangifera and Swintonia are more similar. The investigated genera of the Dipterocarpaceae (Dipterocarpus, Parashorea and the four subgenera of Shorea: Anthoshorea, Richetia, Rubroshorea and Shorea) show great similarities and are hardly distinguishable in mixed samples. For the topochemical characterization, UV spectroscopic investigations were done. The lignin content of mechanical pulp, as compared with the lignin content of macerated tissue, was analyzed by investigating three genera (Eucalyptus, Hevea and Shorea subg. Rubroshorea). It could be demonstrated that, in contrast to mechanical pulp, macerated tissue contains nearly no residual lignin. Furthermore, the variability of characteristic feature dimensions within a genus (Gonystylus) and a subgenus (Shorea subg. Rubroshorea) from different (geographical) origins was investigated. This investigation not only confirms the variability of the dimensions of characteristic morphological features within a genus, but also affirms the expediency of size classes as determined by the standards of the “International Association of Wood Anatomists” (IAWA). The entire study contributes substantially to the implementation of the European Timber Regulation (EUTR), which was introduced in 2013 in order to thwart trade in illegally logged timber. With the “Atlas of Vessel Elements“ (Helmling et al. 2018) and the documented references therein, the inspectors authorized to implement the EUTR are provided with a means to effectively control pulp and paper products that are imported by market participants into the European Union (EU). This work equips the competent authorities – as well as the traders who carry out voluntary self-checks – with an array of data tools that extends to the vast domain of Asian timbers, thus now enabling their determination in pulp and paper.