Decomposition of coarse woody debris originating by clearcutting of an old-growth conifer forest

Decomposition constants (k) for above-ground logs and stumps and sub-surface coarse roots originating from harvested old-growth forest (estimated age 400–600 y) were assessed by volume–density change methods along a 70-y chronosequence of clearcuts on the Wind River Ranger District, Washington, USA. Principal species sampled were Tsuga heterophylla and Pseudotsuga menziesii.Wood and bark tissue densities were weighted by sample fraction, adjusted for fragmentation, then regressed to determine k by tissue type for each species. After accounting for stand age, no significant differences were found between log and stump density within species, but P. menziesii decomposed more slowly (k = 0.015·y-¹) than T. heterophylla (k = 0.036·y-¹), a species pattern repeated both above- and below-ground. Small-diameter (1-3 cm) P. menziesii roots decomposed faster (k = 0.014·y-¹) than large-diameter (3-8 cm) roots (k = 0.008·y-¹), a pattern echoed by T. heterophylla roots (1-3 cm, k = 0.023·y-¹; 3-8 cm, k = 0.017·y-¹), suggesting a relationship between diameter and k.Given our mean k and mean mass of coarse woody debris stores in each stand (determined earlier), we estimate decomposing logs, stumps, and snags are releasing back to the atmosphere between 0.3 and 0.9 Mg C·ha-¹·y-¹ (assuming all coarse woody debris is P. menziesii) or 0.8-2.3 Mg C·ha-¹·y-¹ (assuming all coarse woody debris is T. heterophylla). Including coarse roots increases these loss calculations (averages of all decomposition classes for the study year) to 0.5-1.9 Mg C·ha-¹·y-¹ or 1.0-3.5 Mg C·ha-¹·y-¹, respectively. Our results support substitution of log k in C flux models when stump k is unknown. Substitution of log k for coarse root k could, however, substantially overestimate C flux back to the atmosphere from these forests.