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

Abstract

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.

Large-diameter trees dominate snag and surface biomass following reintroduced fire

“…Because fire rarely consumes the boles of live trees, this pulse of delayed mortality can increase post-fire fuel levels by inputting additional biomass to snag and deadwood (down woody debris) pools (Fulé et al 2004;Stephens 2004;Hyde et al 2012, Stenzel et al 2019. Efforts to reduce these fuels through silvicultural treatment (i.e., salvage logging) have had mixed results, sometimes increasing fuel loads over the short term (i.e., Donato et al 2006, Johnson et al 2020a and sometimes reducing subsequent fire severity (i.e., Povak et al 2020).…”

Spatial and temporal drivers of post-fire tree establishment and height growth in a managed forest landscape

“…To assess the likelihood for post-fire recovery to forest (i.e., resilience), we compared post-fire stem densities to local silvicultural stocking recommendations and pre-fire stem densities. On lands managed by the USDA Forest Service in the Blue Mountains, minimum stocking recommendations range from ~60 trees ha −1 in lower elevation and drier forest types to >350 trees ha −1 in higher elevation and cooler-moister forest types (Powell 1999), and these stocking rates were used in similar post-fire regeneration studies of dry to moist mixed conifer forest types in the PNW (Povak et al 2020;Boag et al 2020). Specifically, we compared postfire stem density (sum of juveniles and surviving trees) for each plot to four post-fire stem density thresholds:…”

“…The spatial configuration of fire-killed trees creates variability in distance to and quantity of available seeds (Turner 2010), and annual seed delivery varies with the infrequent production of large seed crops (Kelly 1994). When seeds are available, survival and growth of seedlings are modified by the topographic effects on soil moisture (Donato et al 2016;Hoecker et al 2020;Stewart et al 2021), competition from understory vegetation (Povak et al 2020;Owen et al 2020), species traits (Harvey et al 2016;Rodman et al 2020;Hoecker and Turner 2022), soil substrates (Burns and Honkala 1990), and subsequent disturbance (Turner et al 2019;Busby et al 2020). Most researchers have focused on the factors affecting the spatial variability in post-fire seedling density (as reviewed by Stevens-Rumann and Morgan 2019), whereas few researchers have examined factors affecting height growth, a critical factor for forecasting the future forest canopy.…”

“…Salvage logging, often conducted within 1-2 years post-fire, removes some or all merchantable firekilled and fire-damaged trees (Peterson et al 2009). The machinery used in salvage operations, such as feller bunchers and skidders, may have positive (e.g., exposes mineral soils and reduces competition from understory vegetation), negative (e.g., damage to seedlings or surviving trees and soil disturbance), or no effect on post-fire regeneration (Donato et al 2006;Povak et al 2020;Leverkus et al 2021). Another common post-fire management practice is seeding of native and non-native grasses, with the intended goal of reducing soil erosion and limiting the expansion of invasive species into burned areas (Robichaud et al 2006;Peppin et al 2010).…”

Large-diameter trees, snags, and deadwood in southern Utah, USA

“…Removal of snags may reduce fire risk but also reduce or eliminate ecosystem function (Janisch and Harmon 2002;Lutz and Halpern 2006;Thorn et al 2020). Previous work in other forest types has shown that the results of salvage logging can sometimes increase fuel loads (i.e., Donato et al 2006) and sometimes reduce subsequent fire risk (i.e., Povak et al 2020).…”