A stage-based model of carbon dynamics in downed woody debris

Background/Question/Methods

Stage-based matrix models are widely used to predict the rate at which downed woody debris (DWD) transitions between discrete decay classes, because wildlife studies have long used decay classes as a measure of the changing quality of deadwood habitat. In contrast, decomposition studies typically use an exponential decay function to predict the continuous decline in carbon density of DWD. While modeling decay as a continuous exponential process may be more realistic, this approach does not allow one to model habitat supply and carbon storage within a single framework. To examine the effects of removing harvest slash for bioenergy production, we developed a stage-based matrix model for predicting changes in the carbon density of DWD as it transitions between discrete decay classes, then calibrated the model using destructive sampling of logs.

Results/Conclusions

Results indicate that as DWD progresses from decay class 1 to 5, carbon content (C) in logs declines by 63% on a mass basis, outpacing the decline DWD volume. Eight years following harvest, the carbon content of slash declines by 50%, whereas an equivalent decline in DWD volume takes 16 years. C was lost from DWD primarily via respiration, which peaked in decay class 2 DWD and a smaller proportion of C was leached out as water-borne dissolved organic carbon in decay class 3 and later stages of decomposition. The results challenge the widely used assumption that all C in DWD is respired, and demonstrate the feasibility of modeling habitat supply and carbon storage within a single framework.