Research Article: Redefining Old-Growth in Forested Wetlands of Western Washington

Abstract

In the Pacific Northwest, state agencies have adopted definitions of mature and old-growth forest, with minimum size and age criteria for the largest trees, as part of a program to protect rare forest habitats. State wetland rating and priority habitat protection guidelines use these criteria to identify mature and old-growth forested wetlands; however, these definitions are based on the characteristics of upland habitats and may not be applicable to wetlands. In this study, data from forested wetlands in western Washington were analyzed to estimate growth rates for five tree species: western red cedar, Sitka spruce, western hemlock, red alder, and shore pine. Estimated average diameter for mature forest is 46 cm (18 inches) and for old-growth 69 cm (27 inches), significantly smaller than the standard size criteria of 53 cm (21 inches) and 81 cm (32 inches), respectively. These results may be more appropriate as size criteria for these forest stages; however, growth rates varied widely between different wetlands and between trees in the same wetland. The great variation in growth rates calls into question the validity of estimating the age of a stand based on the size of trees. Consideration should be given to applying a higher degree of protection to all forested wetlands dominated by coniferous trees, regardless of the size of the trees.Environmental Practice 11:68–83 (2009)