Snag and fuel dynamics after stand-replacing wildfires in the interior Pacific Northwest

These data were collected for a retrospective study of coarse woody debris and woody fuel dynamics following high severity (stand-replacing) wildfires in dry coniferous forests of the interior Pacific Northwest, USA. Between 2007 and 2009, we sampled 255 study sites within a chronosequence of 68 wildfires that burned between 1970 and 2007 in dry coniferous forests of Eastern Washington and Oregon. We selected study sites that represented large patches of dry coniferous forest that burned in stand-replacing wildfire (> 95% overstory tree mortality) within these wildfires and contained a significant component of merchantable ponderosa pine or Douglas-fir trees. This sample included 96 study sites that had been salvage logged following wildfire and 159 sites that had not been logged following wildfire. We avoided sites where Forest Service records or field visits indicated significant post-fire management activities (other than salvage logging) or subsequent fuels management activities (e.g., prescribed burning) that would significantly alter snag or downed wood dynamics. At each site, we established a randomly placed study plot on which we surveyed pre-fire stand structure, snag and log conditions, and surface woody fuel loadings. By surveying all residual live trees, standing and fallen snags, and cut stumps within a fixed-radius plot, we were able to reconstruct pre-fire stand basal area, density, and species composition. For each live tree and snag on the fixed-radius plot – and additional snags in an expanded variable-radius plot – we recorded the species, diameter, and status (alive, standing dead, broken dead, fallen dead, or cut). In 2007 and 2008, we also recorded snag decay class and the presence or absence of wildlife cavities (on standing snags only). We surveyed surface woody fuels, by size class, on 3-7 planar intercept fuel transects at each study site. Data were collected during 2007-2009 on areas that burned during 1970-2007.
As fire-killed trees (hereafter snags) decompose and vegetation recovers over time after fire, forest fuel characteristics change considerably, producing changes in potential fire behavior. We refer to this process as fuel succession. Surface woody fuels accumulate as snags decay, break, and finally fall. Wood decomposition also changes fuel properties and fire behavior. Sound (hard) logs typically contribute little to fire spread rates or local fire severity. As logs decompose, however, they may become more fractured and flammable, increasing the duration of flaming and smoldering combustion, and increasing tree mortality and soil heating. After stand-replacing wildfires, there is often considerable conflict when post-fire (or salvage) logging is proposed. Harvesting fire-killed trees immediately after wildfire can provide economic benefits to local communities and may reduce risks of insect and disease outbreaks that can kill additional trees. However, there is concern that the ecological costs of post-fire logging may outweigh the economic benefits. Specifically, opponents argue that post-fire logging may compound fire effects on soil physical and chemical properties; increase precipitation runoff and soil erosion; harm water quality and aquatic habitats; reduce site productivity and slow future forest development; degrade wildlife habitat for species dependent on snags and coarse woody debris; facilitate the spread of noxious weeds; and alter plant community structure and diversity. Often overlooked in the debate over post-fire logging is the risk of ecological damage to soils, vegetation, and aquatic ecosystems from subsequent high severity fires. High severity fires typically kill most trees and other vegetation; alter wildlife habitat, soil physical, and soil chemical properties; increase erosion rates; and impact aquatic ecosystems. Fuels managers have argued that post-fire logging can reduce future fire severity by removing the boles of dead trees that later contribute to surface woody fuels. However, there has been very little research examining the effects of post-fire logging on future fuels and fire behavior. To better inform this debate, we examined patterns of snag decay and surface fuel accumulations, with and without post-fire logging, across a chronosequence of 68 stand-replacing wildfires that burned between 1970 and 2007 in dry coniferous forests of Eastern Washington and Oregon. Specific research objectives included: 1) describe coarse woody debris dynamics and fuel succession following stand-replacing wildfires in dry coniferous forests of the Pacific Northwest; 2) assess the effects of post-fire logging on post-fire fuel succession and potential fire severity; and 3) describe the snag properties associated with use by primary cavity-nesters.
For additional information about this study, see Peterson et al. (2015) and Peterson et al. (in press).