The characterization of wetwood in western hemlock

Summary One of the problem areas in the kiln drying of western hemlock lumber is the wide variation in final moisture content of the wood. This variation in moisture content is due to the presence of sinker or wetwood in the heartwood. The features of wetwood which differentiate it from the normal heartwood include higher specific gravity, higher extractives content, and lower permeability. The apparent higher specific gravity can be fully accounted for by the higher extractives content. The principial extractive is α-conidendrin. The wetwood in western hemlock often occurs together with ring shake and under these circumstances the white deposit on the shake surfaces is also α-conidendrin and not matairesinol, the substance usually associated with ring shake in western hemlock. A viewpoint is presented on the origin of wetwood as the endproduct of a reaction by the tree to injury, i.e., ring shake, in which additional extractives are deposited. The extractives result in a greatly lowered permeability, which prevents loss of moisture during heartwood formation and thereby resulting in wetwood. Bacteria usually found in wetwood and responsible for many of the symptoms associated with wetwood are a result of the high moisture content which favors bacterial growth in wood. Presumably, the two primary sources of loss in kiln drying of western hemlock, shake and wetwood, are often intimately associated. The authors appreciate the assistance of Allen H. Doerksen and Louis W. Hamlin of the Forest Research Laboratory. All samples collected for this study were donated by Willamette Industries, Inc., Dallas, Oregon, and a portion of the research was supported by the Forest Research Laboratory, Oregon State University.