Dynamics of coarse woody debris following wildfire in a mountain hemlock (Tsuga mertensiana) forest

Coarse woody debris (CWD, comprised of snags and downed logs) is an important component of the structure and function of forest ecosystems, one which both influences the availability of fuel for wildfires and can be a result of wildfires. We studied snag persistence, and changes in mass of CWD in 10 years following a 1996 wildfire in mountain hemlock (Tsuga mertensiana (Bong.) Carrière) forest in the Cascade Range of Oregon, comparing unburned areas to areas with varying fire severity. Though mountain hemlock forest is a dominant type at high elevations in the Pacific Northwest, previous studies have not addressed the species. From studies of other conifers in western North America, we hypothesized that probability of snag persistence would be >75% 5 years and <50% 10 years after fire, and would increase with tree size. We modeled snag persistence on characteristics of the snags and the plots on which they occurred, using generalized mixed effects models. Probability of snag persistence was >75% 5 years after fire and increased with increasing tree diameter, as predicted. However, probability of persistence was >50% 10 years after fire. The cold climate, with a brief interval of optimal temperatures for decay organisms, probably contributed to prolonged snag persistence. Although probability of snag persistence remained high, estimated mass of snags declined and estimated mass of logs increased. Both breakage of snags and decreasing wood density due to advancing decay contributed to the decrease in estimated snag mass. Due to the nearly complete transformation of live trees to snags and the paucity of tree regeneration, we expect that patches of high fire severity will be effective barriers to the spread of crown fires for decades.
Keywords: Snag persistence; Fire severity; CWD mass