Study of Douglas-fir bark development and characterization of its cork component

Doutoramento em Engenharia Florestal e dos Recursos Naturais - Instituto Superior de Agronomia / UL

Douglas-fir (Pseudotsuga menziesii) is a valued timber conifer providing long sawnwood components, recognized for its fast growth, natural regeneration and adaptability. Original from North America, it was planted in Europe on approximately 550 thousand ha including Portugal, where its potential area is estimated at 250,000 ha. The Douglas-fir bark has a high cork proportion and its potential use was already discussed in early studies. This research was made on mature trees from the north and central mountains of Portugal. Stem development regarding heartwood, sapwood and bark was analyzed as well as bark structural development. Heartwood represented 49 % of the cross section in the lower stem and sapwood width was on average 75 mm at stem base, both decreasing upwards. The bark was approximately 3 cm thick at stem base, where the rhytidome corresponded to 84 % of the bark, including nearly 50 % of cork, and decreased to 3–5 mm at the top. The cork is not continuous within the rhytidome, and the layers are interspersed with phloem. The cork was studied regarding structural and anatomical features. It was characterized by extensive areas of crushed cells, making up a very compact structure, with patches of uncompressed cork. In the uncompressed regions the cellular dimensions were: prism height 55 mm, base area 1388 mm2 and 1.3 mm cell wall thickness. Chemical composition of Douglas-fir bark varies with age: at 45, 30 and 17 years of age, the bark contained respectively 25.4 %, 2.6 % and, 0.9 % suberin. Cork and phloem differ in extractives (49.8 % vs. 17.0 %), suberin (30.1 % in cork vs. not present in phloem) and hemicelluloses composition regarding arabinose content (25.3 % vs. 4.8 % of monosaccharides). Douglas-fir bark proved to be an interesting feedstock for biorefineries while a valorization targeting cork should consider using the lower stem parts of mature trees

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