An integrated model for snag and downed woody debris decay class transitions

Abstract

In order to project the long-term implications of forest management for coarse woody debris (CWD) and its habitat-related ecological functions, the time frame of snag and downed woody debris (DWD) decomposition needs to be quantified. In the present study, we parameterized models of CWD decay class transitions that explicitly represent decomposition-related changes in white and red pine ( Pinus strobus, P. resinosa) snags and DWD over time. Remeasurements of permanent sample plots in Ontario, Canada, were used to derive models of a tree's condition at death (standing or fallen), snag fall rates, transitions among snag decay classes, and the entry of fallen snags into DWD decay classes. DWD decay class transitions were modelled from the condition of harvest residues across a 27-year chronosequence of shelterwood-harvested stands in central Ontario, and on the quasi-equilibrium decay class distribution of DWD in a series of relatively undisturbed sites. By combining these various model components, an integrated model of CWD decay class transitions was developed to project the condition of snags and DWD pieces over time. Projections showed that white and red pine snags had median fall times of 15–20 and 30 years, respectively. DWD from trees that fell at death was projected to advance through four decay classes over a 55–60-year period, whereas DWD originating from snags was modelled to persist for up to 90 years after tree death because of slower decomposition while standing. For both snags and DWD, decay classes I and II were short–lived relative to later classes. Diameter was found to have a significant effect on DWD transitions into decay classes II and IV but not III, with projections accordingly showing smaller-sized DWD to be less abundant in class III than larger material. Incorporating CWD dynamics such as those modelled here into stand growth and mortality models would enable managers to simulate long-term changes in the quality and quantity of snags and DWD under various forest management regimes, and to evaluate the degree to which CWD-associated habitat features may be maintained over time.