The increased frequency of large, destructive wildfires in the Sierra Nevada has been in part driven by historical management decisions that have altered the structure of forests, creating dense stands dominated by shade tolerant species with thick, flammable understory fuels. Prior to European settlement c. 1850 and the onset of fire suppression policies, abundant archaeological evidence suggests that Indigenous groups occupying the Sierra National Forest (SNF) set frequent, low-intensity fires to maintain “park-like” woodlands with clear travel corridors, productive food and game forage patches, and culturally-important plant materials such as basket-weaving grasses and oak acorns. The history of frequent fire in the SNF has been documented in the paleorecord over at least the past 1700 years as fire scars on tree rings. To this point, this fire history and coincident and subsequent shifts in forest structure have been largely attributed to climate and lightning trends rather than anthropogenic causes, even though Native Americans occupied the area since at least 6000 B.C.E. This discrepancy may be partly due to the low-intensity, slow-moving nature of Indigenous burning, potentially reducing the likelihood of scarring trees and producing sedimentary charcoal. Partitioning the effects of human vs. climatically-induced fire on Sierra Nevada forests over time is essential for understanding baseline forest structures pre-fire suppression and guiding modern restoration.

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In the mixed conifer forests of the Sierra Nevada, long-term fire exclusion has increased fuel loads and stem densities while fundamentally altering species compositions and other stand metrics relative to historical estimates. These stand conditions, along with hotter, drier, and longer fire seasons, have created increasingly large and severe wildfires across the western United States, with many areas experiencing successive reburns. Given the growing number of challenges faced by managers to increase forest resilience to future wildfires, it is crucial to understand the implications of reintroducing repeated fire into fire-excluded landscapes, and to assess the capacity of successive low-to-moderate severity fires to restore forests to resilient structural parameters.

Researchers at UC Berkeley and the US Forest Service sought to evaluate the influence of forest structure and composition, topography, and weather on fire severity in a third successive fire. They investigated the structural conditions emerging after successive burns, whether these conditions contributed to fire severity, and how these conditions compared to historical estimates. Their study utilized a network of Forest Service field plots in the Plumas and Lassen National Forests that had been initially burned in the Storrie and Rich Fires in 2000 and 2008, reburned in the Chips Fire in 2012, and were then subject to a second reburn in the 2021 Dixie Fire. Plots were sampled in 2017 and 2018 following the Chips Fire and in 2023 following the Dixie Fire.

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Interacting disturbances have increasingly come to shape stand structure and fuel characteristics in seasonally dry western forests. Between 2012 and 2016, concurrent multi-year drought and outbreaks of western pine beetle, fir engraver beetle, and pinyon ips resulted in the mortality of an estimated 147 million trees across California’s forests, with central and southern California being most impacted. Peak tree mortality occurred between 2015 and 2016, leading researchers to investigate changes in stand structure, fuel loading, and fire behavior in the four following years (2015-2019). Data collection occurred in forested plot networks in the Los Padres (LPF) and Sierra National Forests (SNF). Southern plots (all on the LPF) were pinyon pine and canyon live oak-dominated, while northern plots (SNF) were mixed-conifer, including ponderosa, lodgepole, and sugar pine, incense cedar, black oak, and white and red fir at higher elevation sites. Both forest types have experienced densification as a result of fire suppression over the past two centuries, leading to standing fuel continuity that, when combined with heavy, dry surface fuel loads and appropriate weather conditions, could produce severe fire behavior.

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Despite the vast area and large numbers of trees affected by drought- and bark beetle-induced tree mortality worldwide, relatively little is known about how post-mortality management practices affect forest recovery, particularly in forests historically adapted to frequent fire. Cutting and removing dead trees after a mass-mortality event provides an opportunity to salvage timber and lessen wildfire risk by reducing fuel loads, but the ecological impacts of this strategy extend beyond fuel reduction. A tree mortality event of locally unprecedented extent and severity occurred in the Sierra Nevada of California, USA, during and after the exceptional 2012-2016 hot drought, providing an opportunity to evaluate the ecological effects of the management practice of dead-tree removal.

The authors compared adjacent mixed conifer forest areas with and without dead-tree removal, with the aim of assessing how dead-tree removal affects: 1) the amount and composition of forest fuels; 2) tree regeneration in the form of seedling and sapling density and species composition; 3) carbon pools; and 4) fire behavior and severity and fuel loads over the following century.

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