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In historically frequent-fire forests of the western US there has been an increase in stand-replacing wildfire that is well outside of the historical range of variability (HRV), leading to forest loss at unprecedented scales. As a result, forest managers are increasingly applying forest management treatments designed to reduce the probability of stand-replacing wildfire, by reducing the tree densities and woody debris that have accumulated after more than a century of fire exclusion. Although these treatments have generally been shown to be effective, increasingly warm and dry conditions may challenge the efficacy of these treatments. We compared fire severity (% basal area mortality) in areas that had mechanical thinning with a follow up fire treatment (broadcast burning or pile burning), mechanical thinning without a follow up fire treatment, and untreated areas in California's largest wildfire to date, the 2021 Dixie Fire. We found that the probability of stand-replacing wildfire (defined here as 100 % basal area mortality) was highest on large fire growth days, and lowest in areas that were treated with mechanical thinning and fire; mechanical thinning treatments with no follow up fire treatment did not differ from untreated areas. Where stand-replacing wildfire did not occur, percent basal area mortality was reduced in both the mechanical plus fire treatment and the mechanical-only treatment, both of which were characterized by larger trees and lower densities. This suggests that the addition of the fire treatment is critical for reducing fire severity under more extreme burning conditions, but that the mechanical-only treatments can still be effective under milder burning conditions. We also found that the majority (93 %) of our treated plots were within the HRV for tree density prefire. Postfire, 79 % of the plots with a mechanical thinning plus fire treatment remained within HRV target conditions. In contrast, 48 % of the mechanical thinning only plots, and 58 % of untreated plots, had no live trees. This work contributes to a growing body of evidence that forest treatments to reduce both tree densities and surface fuels are critical for reducing fire severity and forest loss. • Mechanical thinning with prescribed fire or pile burning reduced stand replacing wildfire. • Stand-replacing wildfire was also predicted by large growth days and steep slopes. • Outside of stand replacement, mechanical only treatments also reduced fire severity. [ABSTRACT FROM AUTHOR]