Douglas-Fir Stump Decomposition:Modeling Carbon Residence Times

Abstract

Stumps are a significant portion of the woody debris in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation forests in western Washington State. Very few studies have attempted to quantify the amount of carbon that is held in stumps or the rate at which that carbon is lost. This study assessed carbon and nitrogen concentration and stump density in a chronsequence of thinned or harvested Douglas-fir plantation forests. Using a negative log transformation of density versus age, a model for stump decay was determined. Stumps were found to have a decay rate (k) of 0.019 year-1, starting at an average density of 0.35 g cm-3 at one year of age and declining to a density of 0.26 g cm-3 over 15 years. The carbon concentration of stumps decreased steadily over time, while the nitrogen content remained constant. Stumps contain up to 33% of total tree-based carbon and 22% of total ecosystem carbon in Douglas-fir plantation forests.