Fire suppression and past selective logging of large trees have fundamentally changed frequent-fire adapted forests in California. The culmination of these changes produced forests that are vulnerable to catastrophic change by wildfire, drought, and bark beetles, with climate change exacerbating this vulnerability. Management options available to address this problem include mechanical treatments (Mech), prescribed fire (Fire), or combinations of these treatments (Mech + Fire). We quantify changes in forest structure and composition, fuel accumulation, modeled fire behavior, inter-tree competition, and economics from a 20-year forest restoration study in the northern Sierra Nevada. All three active treatments (Fire, Mech, Mech + Fire) produced forest conditions that were much more resistant to wildfire than the untreated control. The treatments that included prescribed fire (Fire, Mech + Fire) produced the lowest surface and duff fuel loads and the lowest modeled fire hazards. Mech produced low fire hazards beginning 7-years after the initial treatment and Mech + Fire had lower tree growth than controls. The only treatment that produced inter-tree competition similar to historical California mixed-conifer forests was Mech + Fire, indicating that stands under this treatment would likely be more resilient to enhanced forest stressors. While Fire reduced modeled fire hazard and reintroduced a fundamental ecosystem process, it was done at a net cost to the landowner. Using Mech that included mastication and commercial thinning resulted in positive revenues and was also relatively strong as an investment in reducing modeled fire hazard. The Mech + Fire treatment represents a compromise between the desire to sustain financial feasibility and the desire to reintroduce fire. One key component to long-term forest conservation will be continued treatments to maintain or improve the conditions from forest restoration. Many Indigenous people speak of 'active stewardship' as one of the key principles in land management and this aligns well with the need for increased restoration in western US forests. If we do not use the knowledge from 20+ years of forest research and the much longer tradition of Indigenous cultural practices and knowledge, frequent-fire forests will continue to be degraded and lost.

Funding provided by: Joint Fire Science ProgramCrossref Funder Registry ID: http://dx.doi.org/10.13039/100015231Award Number: Funding provided by: California Fourth Climate Change Assessment*Crossref Funder Registry ID: Award Number: Funding provided by: McIntire-Stinnis Program*Crossref Funder Registry ID: Award Number: Funding provided by: California Greenhouse Gas Reduction Fund*Crossref Funder Registry ID: Award Number: Funding provided by: UC Laboratory Fees Program*Crossref Funder Registry ID: Award Number: Funding provided by: Smart Practices and Architecture for Prescribed Fires in California*Crossref Funder Registry ID: Award Number: