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Influence of past wildfires on wildfire effects in northern Rockies mixed-conifer forest - Final Report to the Joint Fire Science Program

Author(s): Andrew J. Larson, R. Travis Belote
Year Published: 2018

Natural resource managers need to know how past wildfires influence the severity and ecological effects of subsequent wildfires fires in order to make informed decisions during and after wildfire events, and to effectively plan for the future. The overarching goals for this study were to quantify and compare the effects of single and repeat wildfires on forest ecosystem structure and fuels; to determine if past wildfires influence the severity of subsequent wildfires; and to determine if short-interval reburns are causing transitions to non-forest communities. Since the early 1980s managers have allowed many lightning-ignited fires to burn with minimal interference in forests of the Bob Marshall Wilderness in northwestern Montana, USA. Substantial portions of this landscape burned twice between 1985 and 2013. We used this active fire regime to investigate fire-effects and post-fire fuel loads, tree regeneration, and forest structure in mixed-conifer forest communities. We used an extensive network of field plots (n = 264 total plots) distributed among three sampling protocols to meet our objectives. Fire history strongly affected forest structure and fuels. Surface and canopy fuels exhibited contrasting responses, with the surface fuel complex buffered by post-fire inputs from the overstory. A second fire is required to cause meaningful reductions to the surface fuel loads, and three or more fires may be required to reduce the largest (≥1000 h) fuels, except in situations where high severity patches are reburned. Fire effects on canopy fuels are much more predictable, with steady reductions of canopy fuels, tree biomass, and total aboveground biomass along the fire history gradient from unburned, to once-burned, to reburned sites. Live tree density was best explained by an interaction between initial fire severity and topography: live tree density was lowest on steep southerly aspects that burned in at high severity. Environmental variables related to topographic position and the severity of the initial fire, but not necessarily the occurrence of a reburn, were important in explaining transitions to non-forest following fire. Our most important finding is that surface fuel loads are maintained or increased in the years following an initial wildfire after a long fire-free period as fire killed trees and branches fall to the ground. This unsurprising result nevertheless deserves highlighting because the current conventional wisdom is that an initial fire can be thought of as a 'fuel treatment.' Our most surprising finding was the unimportance of reburns as a cause of transitions to non-forest. We maintained a very active and successful science delivery and outreach program during this project. Science delivery activities included eight scientific, workshop, and public presentations; two completed and two in preparation publications; and multiple media contacts and interviews resulting in this research being featured in two different news articles, a book, and a short documentary film. This diversified science delivery program reached managers, scientists, students, and the general public through multiple platforms. Managers need to plan for multiple fire entries (i.e., two or more fires) if their goal is to use wildfires as surface fuel reduction treatments. Our results demonstrate that some transitions to a putative non-forest condition are to be expected following both initial fires and short-interval reburns. Thus, managers may wish to incorporate this outcome into their expectations, and into their outreach and education efforts, in order to prepare policy makers and the public for forest conversion. Numerous historical reconstructions have shown that many formerly fire-maintained open areas have been encroached by forest during the period of fire exclusion-returning some areas to a putative non-forest condition may actually be restorative from a landscape perspective.

Citation: Larson, Andrew J.; Belote, R. Travis. 2018. Influence of past wildfires on wildfire effects in northern Rockies mixed-conifer forest - Final Report to the Joint Fire Science Program. JFSP Project No. 14-1-02-9. Missoula, MT: University of Montana. 50 p.
Topic(s): Fire Regime, Fire Intensity / Burn Severity, Fire and Landscape Mosaics, Recovery after fire
Ecosystem(s): None
Document Type: Technical Report or White Paper
NRFSN number: 18251
FRAMES RCS number: 56786
Record updated: Nov 5, 2018