Within-stem variation in tropolone content and decay resistance of second-growth Western redcedar

Abstract

Western redcedar (Thuja plicata Donn) is a commercial species valued for its high decay resistance, which is due to the presence of toxic extractives (tropolones) in the heartwood. We measured tropolone content and weight loss in soil block tests using samples taken from 11 second-growth western redcedar trees. Tropolone content increased with rings from the pith, and in the outer heartwood, increased from the top to the base of the tree. We did not find any consistent increase or decrease in tropolone content with height in the tree for samples the same number of rings from the pith. Our data suggest that the lower tropolone content near the pith is largely a juvenile effect, associated with wood formed near the active crown. This has implications for choosing a rotation length if wood of uniformly high decay resistance is desired, because younger trees contain a greater proportion of wood close to the pith. However, we also found large tree-to-tree differences in the rate of tropolone increase with age from the pith. This may have promise for maintaining uniformly high decay resistance if the factors underlying the tree-to-tree differences can be discovered and managed. Our results from soil block tests for decay resistance were more variable, but in general, wood near the pith, which was low in tropolones throughout the tree, was extremely variable in decay resistance; outer heartwood with high tropolone content showed higher, less variable decay resistance, while outer heartwood with low tropolone content remained variable in decay resistance.