Incidence of wetwood and decay in precommercially thinned western hemlock stands

Abstract

The occurrence and pattern of stains in the base and lower stem of young-growth western hemlock (Tsugaheterophylla (Raf.) Sarg.) and its relationship to root disease were studied at 16 sites in coastal Washington. Stands ranged in age from 17 to 35 years and were precommercially thinned 5–17 years previously. The most common stain was discolored wood or wetwood, which is a type of heartwood in standing trees that has been internally infused with water. Wetwood occurred in more than 50% of the 1215 trees examined, and within some stands it occurred in 80% of the trees examined. The presence of wetwood did not indicate root disease or other decay. However, all wood decay infections were associated with wetwood. Wetwood can be economically important because it interferes with drying and causes shake and warping in lumber. Incipient decay stain was less common (<6% of the trees) and was caused predominantly by the basidiomycete fungi Heterobasidionannosum (Fr.) Bref., Armillaria spp., and Perenniporiasubacida (Pk.) Donk. Wetwood appearance varied from bright pinkish, purplish, or pinkish brown to light brown, depending on moisture and extractives content. Incipient decay stain of H. annosum was distinguished from wetwood by its mostly brown, dark brown, or brownish black color. Heterobasidionannosum also has incipient decay stain that may be light brown to pinkish and closely resembles wetwood. Experimental wounding of trees resulted in both wetwood and incipient decay formation. Wetwood and incipient decay stain can occur in the same bole section, so there is a potential for confusing them. Cultural isolation of fungi is the only sure way to distinguish wetwood from incipient decay.