Field Tests of Naturally Durable Species

Six wood species generally accepted to represent a range of natural durability were exposed in American Wood Protection Association (AWPA) E7-09 (ground-contact) and AWPA E25-08 (aboveground) decay tests at field sites located near Maple Ridge, British Columbia, and Petawawa, Ontario, in Canada and Gainesville, Florida, and Hilo, Hawaii, in the United States. Variables examined included comparisons of sapwood to heartwood, old growth to second growth, and effect of a protective coating. The tests began between October 2004 and February 2005. Results are reported after 5 years of exposure. Ground-contact decay rates were fastest at sites in Florida and Hawaii. Yellow cedar (Callitropsis nootkatensis (D. Don) Orsted), western red cedar (Thuja plicata Donn ex D. Don), and eastern white cedar (Thuja occidentalis L.) had the highest condition ratings (least decay) for this measure after 5 years of exposure, followed by western larch (Larix occidentalis Nutt.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and then tamarack (Larix laricina (Du Roi) K. Koch). The aboveground decay rate was highest in Hawaii. For this measure, yellow cedar and western red cedar again had the highest average decay ratings (least decay) after 5 years of field exposure, followed by Douglas-fir, western larch, and tamarack. Eastern white cedar did not fit neatly into this pattern. It was durable at three of the four sites but failed rapidly in Hawaii. Sapwood appeared to have a larger impact on aboveground decay than on ground-contact decay. No substantial difference was found between old-growth and second-growth decay rates.