Hotter temperatures, prolonged drought and the abundance of dead and downed wood across the Sierra Nevada have led to uncharacteristically large, intense wildfires in recent decades (McKelvey et al. 1996, Hagmann et al. 2021). These fires displace and threaten communities and can exceed the resistance and resilience mechanisms of wildlife and plant populations (e.g. Jones et al. 2021, Shive et al. 2022). During the past five years, approximately 20 percent of the entire population of endemic mature giant sequoias (Sequoiadendron giganteum) in the Sierra Nevada have been lost (Shive et al. 2022, Stephenson et al. 2021). A century of fire suppression in the Sierra Nevada has led to forest conditions with greater tree densities, fuel loads, and proportion of shade-tolerant tree species than pre-settlement conditions (Collins et al. 2011, Knapp et al. 2013, Stephens et al. 2015). These forest conditions can increase rates and size of high-severity fire (Steel et al. 2015, Stephens et al. 2015, Boisramé et al. 2017). Higher tree densities have also been associated with reduced forest health (Hankin and Anderson 2022) and widespread bark beetle-induced tree mortality (Fettig et al. 2007). Historical forest structure through frequent lightning ignitions and Indigenous burning promoted ecosystem resilience by minimizing competition and resisting high-severity wildfire (North et al. 2022). Active fuels management, primarily thinning and prescribed fire, are effective tools to restore forest conditions that limit the spread of large, high-severity wildfires and maintain natural ecosystem processes (Fulé et al. 2012, Restaino et al. 2019, North et al. 2021).
For forests with historically frequent fire (e.g. mixed-conifer, giant sequoia, ponderosa pine (Pinus ponderosa)), reducing forest density through mechanical fuel treatments and prescribed fire can prevent high-severity fire and restore natural ecosystem processes (Stephens et al. 2013). Fuel treatments and previous burn footprints are effective barriers to the spread and probability of high-severity fire (Collins et al. 2010, Lydersen et al. 2017, Cansler et al. 2022). Mechanical thinning treatments followed by prescribed fire effectively reduce fireline intensities, the likelihood of passive and active crown fire, surface fuel loads, and retain larger trees through increased stand resistance (Pollet and Omi 2002, Stephens and Moghaddas 2005, Stephens et al. 2009, Fule et al. 2012). For example, the largest fire in Yosemite National Park history, the 2013 Rim Fire, had significantly lower proportions of high-severity fire in areas with previous fuels treatments or were burned by low- to moderate-severity fire (Lydersen et al. 2017), limiting negative impacts of plume-dominated wildfire. These reductions in fire intensity increase human safety and allow for more effective firefighting in the wildland-urban interface. Beyond moderating fire behavior, restoration thinning and prescribed fire treatments provide numerous co-benefits (Stephens et al. 2020). Associated increases in soil moisture have been shown to significantly increase dead woody debris fuel moisture content and shorten fire seasons (Rakhmatulina et al. 2021). Furthermore, treatments have been shown to enhance biodiversity, increase water availability, increase long-term carbon storage stability (Krofcheck et al. 2017), and improve forest resilience to beetles, and drought (Churchill et al. 2013, Young et al. 2017).
Like much of the Sierra Nevada, Yosemite’s mixed-conifer forests are dense with ladder fuels and dead biomass after a century of fire suppression and the recent severe drought and bark beetle outbreaks. These fuel conditions pose significant risks to the Park’s communities and numerous natural resources. Strategically-placed prescribed fire and other fuels reduction treatments have been implemented in Yosemite to achieve protection and resource objectives, particularly in the wildland-urban interface and surrounding the Park’s giant sequoia groves. By reducing heavy fuel loads and removing abundant dead biomass, the Park can create more resilient communities and landscapes to ongoing climatic change and increasing fire activity. Prescribed fire is the primary tool for reducing fire risk, however, fuel conditions in some areas of the Park require significant preparation prior to first-entry burning to meet objectives of firefighter safety and ecological benefit. This need has increased with expanding drought- and bark beetle-caused tree mortality. Thinning and brush removal are therefore important components to the fire management program to allow for safe prescribed burning implementation. Recent roadside thinning projects using mechanical biomass removal have greatly increased the Park’s capacity for prescribed burn projects. Furthermore, reducing tree densities and surface fuel loads along roadways maintain safe evacuation routes, increase firefighter safety by creating significant fuel breaks, and allow for suppression tactics such as backfire operations. This proactive management strategy facilitates more effective resource management and wildfire suppression in the face of longer fire seasons, hotter temperatures and associated drought, and rapidly changing fuel conditions.
While research has demonstrated the effectiveness of fuels treatments such as thinning and prescribed fire in moderating fire behavior and reducing the risk of large, catastrophic fire, there are few opportunities to directly link historical and contemporary fuels treatments to unplanned wildfire operations and outcomes. In July 2022, the Washburn Fire started in Yosemite National Park in a location immediately threatening the Park’s largest giant sequoia grove, critical habitat for the threatened Pacific fisher (Pekania pennanti), and the community of Wawona. It was also an area with a rich history of fuels treatments and prescribed fire surrounding these highly valued natural resources, therefore the Washburn Fire presented an opportunity to evaluate the direct impact of a long-term fire management program. Here, we join together historical fuels treatments, recent biomass removal operations, and an unplanned wildfire in Yosemite National Park to highlight how fuels treatments influenced decisions and tactics for the Washburn fire - a wildfire with a full suppression response, and significantly impacted successful protection of the Mariposa Grove of Giant Sequoias and the community of Wawona. First, we provide background and data on the use of prescribed fire and thinning treatments in the areas surrounding the Washburn Fire, second, we use airborne LiDAR data to highlight forest structure differences between these treatment areas and the footprint of the Washburn fire, and third, we discuss how this treatment history influenced the outcome of the Washburn Fire.