Fire-killed trees contribute to wildlife habitat and forest fuels during four decades following severe wildfire in dry coniferous forests

Background/Question/Methods: Large wildland fires often produce pulses of fire-killed trees that provide habitat for a wide range of vertebrate and invertebrate species; downed woody debris that contributes to soil stability and nutrient retention; and forest fuels that influence subsequent fire behavior and effects. The relative contributions of fire-killed trees (snags) to each of these functions depends, in part, on temporal patterns of decay and transitions from standing dead trees to downed woody debris. We present results from a regional study designed 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 within a chronosequence of 159 forest stands that burned in stand-replacing wildfires between 1970 and 2007.

Results/Conclusions: Tree species and diameter strongly influenced snag longevity, with small-diameter snags falling faster than large-diameter snags and ponderosa pine snags falling faster than Douglas-fir and true fir snags. Most standing snags developed broken tops within 10-15 years following fire, after which snag fall rates declined notably. Wildlife cavities were found in about 2.5% of standing snags. Cavities were most commonly found in medium-diameter (30-60 cm) snags with broken tops in stands that had burned 10-20 years prior to our survey. Surface woody fuels increased with time since fire, reaching maximum levels within 5-20 years after fire for small-diameter fuels (≤ 7.5 cm) and within 10-30 years after fire for large-diameter fuels (> 7.5 cm). Small diameter snags generally fell without being used as nesting habitat by cavity-nesting species, serving as short-term foraging habitat and as a source of downed woody debris and fuels.  Larger diameter snags stand longer, on average, but still deposit surface woody debris during the first decade after fire as branches and tops break off. Although large-diameter snags are often highly valued as potential nesting habitat, our study suggests that cavity-nesting wildlife select and use snags of varying sizes and species.