Wildfire severity and postfire salvage harvest effects on long-term forest regeneration

TitleWildfire severity and postfire salvage harvest effects on long-term forest regeneration
Publication TypeJournal Article
Year of Publication2020
AuthorsPovak, NA, Churchill, DJ, Cansler, CA, Hessburg, PF, Kane, VR, Kane, JT, Lutz, JA, Larson, AJ
Date Published08/2020
Keywordsclimate tolerance, Douglas fir, dry forest, high severity, salvage harvest, technical reports and journal articles

Following a wildfire, regeneration to forest can take decades to centuries and is no longer
assured in many western U.S. environments given escalating wildfire severity and warming trends. After
large fire years, managers prioritize where to allocate scarce planting resources, often with limited information
on the factors that drive successful forest establishment. Where occurring, long-term effects of postfire
salvage operations can increase uncertainty of establishment. Here, we collected field data on postfire
regeneration patterns within 13- to 28-yr-old burned patches in eastern Washington State. Across 248 plots,
we sampled tree stems <4 m height using a factorial design that considered (1) fire severity, moderate vs.
high severity; (2) salvage harvesting, salvaged vs. no management; and (3) potential vegetation type (PVT),
sample resides in a dry, moist, or cold mixed-conifer forest environment. We found that regeneration was
abundant throughout the study region, with a median of 4414 (IQR 19,618) stems/ha across all plots. Only
15% of plots fell below minimum timber production stocking standards (350 trees/ha), and <2% of plots
were unstocked. Densities were generally highest in high-severity patches and following salvage harvesting,
although high variability among plots and across sites led to variable significance for these factors.
Post hoc analyses suggested that mild postfire weather conditions may have reduced water stress on tree
establishment and early growth, contributing to overall high stem densities. Douglas fir was the most
abundant species, particularly in moderate-severity patches, followed by ponderosa pine, lodgepole pine,
western larch, and Engelmann spruce. Generalized additive models (GAMs) revealed species-level climatic
tolerances and seed dispersal limits that portend future challenges to regeneration with expected future climate
warming and increased fire activity. Postfire regeneration will occur on sites with adequate seed
sources within their climatic tolerances.