Coarse Woody Debris and Nutrient Dynamics in a Southern Appalachian Spruce-Fir Forest

Spruce-fir forests in the southern Appalachian mountains have been hypothesized to be in the latter stages of nitrogen saturation. These forests receive high atmospheric nitrogen inputs and show high nitrate levels in soil solution and streamwater. The invasion of the balsam woolly adelgid (Adelges piceae Ratz.) in the late 1970s killed the majority of endemic Fraser fir (Abies fraseri (Pursh) Poir.) trees, resulting in large amounts of coarse woody debris. In an effort to quantify nitrogen cycling in spruce-fir ecosystems, the 17.4 ha Noland Divide Watershed in the Great Smoky Mountains National Park has been intensively studied since the mid 1980s. Permanent plots (20 x 20 m) were used to determine the volume and biomass of down and standing dead wood in this watershed. Red spruce (Picea rubens Sarg.), Fraser fir, and yellow birch (Betula alleghaniensis Britt) boles were sampled to determine decay rates, change in density, change in concentration and content of carbon and nitrogen over the decomposition process, and the extent to which dead wood was acting as a sink for nitrogen. Dead wood volume was highly variable across the watershed, ranging from 5 m3/ha to 307 m3/ha for standing boles, and from 25 m3/ha to 400 m3/ha for down boles. The average total volume of coarse woody debris in this watershed exceeded averages typical of other forest types in the Great Smoky Mountains. Annual decay rate constants (k) for slightly decayed down boles were 0.029, 0.036, and 0.217 for Fraser fir, red spruce, and yellow birch, respectively, indicating that up to 100 years may be necessary for the complete disappearance of most of the dead wood in this ecosystem. Decay rate constants changed markedly between decay classes. For all decay classes considered together, k-values were 0.051, 0.062, and 0.097 for Fraser fir, red spruce, and yellow birch respectively. The projected time for the disappearance of wood of all decay classes combined was 59 years for Fraser fir, 48 years for red spruce, and 31 years for yellow birch. Density decreased significantly for all three species by approximately 70% from live tissue to highly decayed wood. Carbon concentrations remained fairly stable at approximately 47% for all decay classes and all species. Nitrogen concentrations increased sharply between moderately decayed wood and highly decayed wood. Average nitrogen content on a unit volume basis increased by approximately 35% (270 g/m3 to 368 g/m3), 75% (189 g/m3 to 330 g/m3), and 95% (547 g/m3 to 1055 g/m3) in live tissue to decay class III wood for Fraser fir, red spruce, and yellow birch respectively