Quantitative metrics of deadwood decay classes as a function of moisture regime and tree species

Abstract

The patterns and rates of decomposition of dead woody debris (DWD) in the boreal ecosystem throughout the wood decay process have been hypothesized to vary across moisture regimes and between species. A fundamental aspect of examining these patterns of temporal wood decay rates and patterns is distinguishing between deadwood decay classes. The study sites for this project were set up across northern Ontario, with 8 in the Northeast (Abitibi Lake Ecoregion) and 8 in the Northwest (Lake Nipigon Ecoregion) to allow for an examination of the impacts of ecoregion on temporal wood decay in future studies. Within each ecoregion, three study sites of Dry, Fresh, and Wet, moisture regimes were included to examine the impacts of moisture regime. Within each site species differences were examined by sampling of conifer and poplar (Populus tremuloides (Michx.)). A qualitative five decay class system was used to classify wood in situ via non-destructive methods, in order to examine how DWD would reclassify upon further quantitative analysis of physical and chemical properties. The patterns of decay observed in poplar DWD were significantly faster compared to conifer decay, as represented by shifts in density and the proportion of solid wood to decayed wood across the decay class continuum. DWD appeared to decay more quickly on fresh sites compared to dry and wet sites, which seemed to have an early delay in decomposition. C:N ratios decreased across the decay class continuum in our results, and appeared to reflect a three phase decay process, consisting of an initial slow phase, a second rapid stage of decay, and a third slow phase and coinciding release of N. Further reclassification of decay classes based on quantitative metrics was required and the middle decay classes were found to have significant amounts of overlap in quantitative patterns and classification. Despite this, some reclassification of the middle decay classes into decay classes 1 and 5 occurred indicating that a three decay class system may not be ideal. However, if all analysis was repeated using the quantitatively assigned decay classes, patterns across the decay class continuum may better reflect the three-phase decay process, and thus imply that DWD classification is well suited to a three decay class system.