Influence of cambial age on the bark structure of Douglas-fir

Douglas-fir (Pseudotsuga menziesii) is a valuable conifer timber species. It has a thick bark with a high proportion of cork in the rhytidome that allows considering its recovery. This study focuses on the characterization of the bark features and their variation with cambial age along the stem using samples of 20 trees from two sites in Portugal at harvest for the sawmilling industry. The morphology and anatomical features of bark were examined including a detailed analysis of the arrangement of tissues, cell biometry, tissue proportion of the phloem, and the development of the rhytidome. Bark structure varied within the tree with cambial age at various height levels, and differences concerned mostly the rhytidome and periderm development, tissue morphology and disarray in the non-conducting phloem. A relationship between cell dimension, proportion of tissues in the phloem and age was observed; the effect of stem height position was statistically significant for sieve cell length, fiber–sclereid length and wall thickness with a decrease from the base to the top. The rhytidome thickness increased with cambial age: At the stem base (45–50 years of cambial age), the bark includes a rhytidome of about 3 cm thickness corresponding to 84% of the bark, with 5–8 periderms, containing nearly 50% of cork. The cork cells were thin-walled and oriented in radial rows, and the occurrence of thick-walled lignified cells was associated with the increments of the phellem layer. In the youngest periderm, the occurrence of phellem cells with empty lumens and thin suberized walls started at 25–30 years of cambial age. The results show that trees with 45–50 years of age and their logs up to 5 m of height may be suitable for bark and cork exploitation.