Coarse woody debris dynamics following stand-replacing wildfires in dry coniferous forests of the interior Pacific Northwest

Background/Question/Methods Fire-killed trees provide critical habitat for wildlife, but also add to available surface fuels as they decay and fall. Debates over the effects of post-fire logging in dry coniferous forests have highlighted the need for a greater understanding of post-fire coarse woody debris dynamics and possible trade-offs between the ecological benefits of retaining fire-killed trees as potential wildlife habitat and the potential fire hazards that high levels of surface fuels produce. The purpose of this study was to describe temporal patterns of snag decay and fall, surface fuel accumulations, and snag usage by cavity-nesting birds following stand-replacing wildfires in dry coniferous forests of the interior Pacific Northwest. We sampled fire-killed trees and surface fuels on 126 plots within a chronosequence of 49 wildfires that burned dry coniferous forests of eastern Washington and Oregon during 1970-2007.

Results/Conclusions Snag fall rates varied by species and size, with ponderosa pine snags falling faster the Douglas-fir and true fir snags and small-diameter snags falling faster than large-diameter snags. More than half of small ponderosa pines (< 30 cm dbh) fell within 10 years after wildfire. Large Douglas-fir snags (> 45 cm dbh) were retained the longest, with less than 50% of snags falling in the first 30 years after wildfire. The probability of a standing snag hosting a wildlife cavity increased over time.  Snags with broken tops were more likely to have cavities than intact snags, and ponderosa pine snags were more likely to host a cavity than Douglas-fir snags.  Surface accumulations of small and large diameter woody debris increased with time since fire, reaching a maximum at 10-20 years after fire.  The proportion of rotten (soft) logs increased monotonically with time since fire up to 37 years. Pre-fire stand basal area was positively correlated with amounts of large diameter surface woody debris.  Small diameter snags generally fell without being used as nesting habitat by cavity-nesting species, but served as a major source of downed woody debris and fuels.  Larger snags stand longer, but still experience considerable decay furing the first decade after fire as branches and tops break off and are deposited as surface fuels. Although large-diameter snags are often highly valued as potential nesting habitat, our study suggests that wildlife select nesting snags of varying sizes and species and sizes of snags through time following wildfire.