Abstract
AbstractAs the size and extent of wildfires has increased in recent decades, so has the cost and extent of post-fire management, including seeding and salvage logging. However, we know little about how burn severity, salvage logging, and post-fire seeding interact to influence vegetation recovery long-term. We sampled understory plant species richness, diversity, and canopy cover one to six years post fire (2006 to 2009, and 2011) on 72 permanent plots selected in a stratified random sample to define post-fire vegetation response to burn severity, post-fire seeding with native grasses, and salvage logging on the 2005 School Fire in eastern Washington. Understory vegetation responded rapidly post fire due, in part, to ample low intensity rainfall events in the first post-fire growing season. Vegetation was more diverse with greater plant species richness and diversity (Shannon-Wiener index) in low and moderate burn severity plots in 2006 (species richness 18; diversity 2.3) compared to high burn severity plots (species richness 10; diversity 1.8), with species richness on the high severity plots reaching 19 in the sixth post-fire year, similar to the initial values on the low and moderate burn severity plots. Plants that commonly resprout from rhizomes, bulbs, and other surviving belowground sources were abundant post fire, while those establishing from off-site seed sources, including non-native species, were present but not abundant. Plots seeded with native grass post fire and not salvage logged had the highest canopy cover of graminoid species: more than 30 % six years after the fire (in 2011), with low forb (15 %) and shrub (1 %) canopy cover and species richness. For comparison, high severity plots that were not seeded and not salvage logged had 3 % graminoid cover, 14 % forb cover, and 26 % shrub cover. Plots that had been salvage logged from one to three years after the fire produced less canopy cover of shrubs and forbs, but three times more canopy cover of graminoids on the high burn severity plots by 2011. High severity plots that were salvage logged and not seeded with native grasses had the lowest species richness, diversity, and cover. Very few non-native species were found, regardless of salvage logging and seeding. Rapid post-fire growth dominated by native plants of high diversity suggests that this forest’s vegetation and soils are highly resilient to disturbance. Overall, burn severity and post-fire seeding with native grasses were more influential than salvage logging on understory plant abundance one to six years after fire.
Highlights
Characterizing post-fire vegetation response is important for predicting how landscapes will respond to large fires, subsequent management activities, and their interactions
Many experts have predicted that the large fires of recent decades, portions of which burn with high severity (Dillon et al 2011), will become increasingly common in the future (Littell et al 2009, Spracklen et al 2009)
We quantified the effects of burn severity, salvage logging, and post-fire seeding to help define their individual and combined effects on four different aspects of post-fire vegetation, including understory plant species richness and diversity, and percent canopy cover by plant growth form and regeneration strategy as a functional trait
Summary
Characterizing post-fire vegetation response is important for predicting how landscapes will respond to large fires, subsequent management activities, and their interactions. Post-fire management after large, severe wildfires can often include seeding or mulching to reduce erosion potential and the spread of invasive species, and salvage logging to remove standing dead trees and recover economic value of some of the trees killed by the fire. The number and size of large fires and total area burned has increased in recent decades (Westerling et al 2006, Littell et al 2009), as have the costs of post-fire rehabilitation (Robichaud et al 2000, 2010, 2014), with long-term implications for ecosystem resilience (Abella and Fornwalt 2015). Within large forest fires, high burn severity alters vegetation (Lentile et al 2007) and prompts post-fire rehabilitation treatments to reduce erosion and invasion by non-native plant species (Robichaud et al 2010), which could alter post-fire vegetation community development. Many experts have predicted that the large fires of recent decades, portions of which burn with high severity (Dillon et al 2011), will become increasingly common in the future (Littell et al 2009, Spracklen et al 2009)
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