Author(s):
Susan J. Prichard, Robert W. Gray, Vivian Griffey, Paul F. Hessburg, Becky K. Kerns, Rebecca Lemons, Roger D. Ottmar, Nicholas A. Povak, R. Brion Salter
Year Published:

Cataloging Information

Topic(s):
Fuels
Fuel Treatments & Effects
Management Approaches

NRFSN number: 23784
Record updated: November 2, 2021

As land managers strive to implement the National Cohesive Wildland Fire Management Strategy, guidance is critically needed on where and how landscape fuel reduction treatments can mitigate future fire impacts and assist in active fire management. In this project, we evaluated the effects of past fuel reduction treatments, including prior wildfires, on fire severity and firefighting operations within recent large fires of north-central Washington State. Past treatments spanned multiple agencies and land ownerships, including private holdings, Colville Indian Reservation (CIR), Colville (CNF) and Okanogan-Wenatchee National Forests (OWNFs), Washington Department of Fish and Wildlife (WDFW) and Washington Department of Natural Resources (WA DNR) lands. We compiled geospatial layers of past wildfires and fuel treatments and evaluated how treatments contributed to two central goals of the Cohesive Strategy -- restoring fire resilient landscapes and promoting safe and effective firefighting response. Through landscape modeling of fire weather, biophysical variables and past fuel treatments, we assessed how fuel treatments in the context of other drivers of fire severity performed in recent large wildfires events.

Fuel reduction treatments occupied a small portion of the total study area but were still significant factors in mitigating fire severity. Compared with first-entry fires, reburned areas exhibited a greater influence from bottom-up factors (e.g., topography, fuels, and past wildfire burn patterns) compared to top-down drivers (i.e., climate, fire weather). Maximum wind speed and direction were strongly correlated with higher burn severity, particularly for first-entry fires. Study fires burned across large environmental gradients, and models revealed a general trend of higher severities in cooler and less arid climatic settings with higher fuel moistures, typical of higher elevation mixed conifer forests. Given that weather and vegetation conditions can vary widely across a given fire -- particularly those that burn for multiple weeks and months -- we also assessed drivers of severity within the 10 largest progression intervals separately to better understand what drove severity under the most extreme conditions. The relative importance and direction of drivers varied more across burn days than across fires. Furthermore, bottom-up drivers were still influential within these burn periods suggesting that large spread days are not driven by top-down factors alone. These findings underscore how each fire spread event has a unique set of factors that drive fire behavior and severity. As such, building predictive models of fire severity for future fires will be difficult given the complexity and non-stationarity of the relationships within and among top-down and bottom-up factors.

To evaluate how treatments assisted firefighting operations, we interviewed local area fire and fuels managers to compile lessons learned about specific ways treatment type, configuration, and landscape position assisted in safe and effective wildfire response. One of the key findings was that forest thinning and especially past burning provided opportunities for low-intensity burnouts that effectively corralled summer wildfires but also, where crews practiced patience and used burning techniques to mitigate fire behavior and severity, served as maintenance burns within treated areas. We also evaluated a new method for measuring fireline effectiveness. Overall, a total of 2205 km of fireline was evaluated in this analysis; not counting roads as completed line, a total of 1742 km of dozer and hand lines were constructed. Because wildfires had such large perimeters, the amount of fireline did not exceed the total fire perimeter for most wildfires. Fireline engagement varied considerably with very low engagement where fires were mainly in roadless areas, and firelines were constructed as contingency lines. Not surprisingly, firelines were more effective in low severity burned areas vs. high severity areas. Finally, using operational fire models within the Interagency Fuel Treatment Decision Support System, we evaluated how a range of fuel treatment intensities influenced burn probability and predicted flame length surfaces. Treatment scenarios that had over 40% of the area treated resulted in substantial reductions to both burn probability and conditional flame length, suggesting that if scaled to 30 to 40% of the landscape, fuel reduction treatments can effectively reduce fire potential and if fires still occur, reduce the intensity and potential severity of summer wildfires. Because fire heeds no administrative boundaries, our emphasis on multiple land ownerships is particularly relevant in evaluating how future fuel reduction treatments can be coordinated across ownerships and land allocations.

Citation

Prichard S, Gray R, Griffey V, Hessburg P, Kerns B, Lemons R, Ottmar R, Povak N, and Salter RB, 2021. Landscape fuel treatments and wildland fire management strategies within recent large fire events: JFSP Final Report for Project 17-1-01: 45 p.

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