Long-term forest understory recovery and tradeoffs in response to prescribed fire return interval

Abstract Background Prescribed fire is a critical tool for managers of dry fire-prone forests used to create more fire resilient forests, improve wildlife habitat and increase understory diversity. However, there are still knowledge gaps around how to apply fire to achieve and balance desired conditions, such as how the forest understory responds to frequency of burning. We leveraged a unique long-term prescribed burn experiment and evaluated the responses of different understory vegetation components to differing fire return intervals (5-, 10-, 20-year) including two common shrub species of interest in western forests, antelope bitterbrush (bitterbrush) and snowbrush ceanothus (snowbrush). Results Five- and 10-year burn treatments suppressed bitterbrush cover throughout the study but after cessation of 5-year burning, cover rebounded substantially within a decade. Bitterbrush density demonstrated rapid recovery after burning later in the study and with 5-year burning. We found that all burn treatments increased snowbrush cover and density to some degree, although cover remained low with 5-year burning. In some areas, 10- and 20-year burning and associated substantial overstory loss catalyzed dominance of snowbrush. Burning increased total herbaceous cover and biomass, particularly for the latter part of the study and for longer fire return intervals. Richness was not impacted by the treatments. The positive herbaceous response was driven by the rhizomatous western brackenfern. Ordination results revealed differing plant communities in response to treatments, largely driven by the increase of snowbrush and western brackenfern. Conclusions Wildland fire managers often face substantial tradeoffs as they plan to use prescribed fire to restore dry forests. Burning at 5-year intervals was the most successful at reducing surface litter and shrub cover but came with tradeoffs by reducing wildlife habitat values. Longer intervals (10- and 20-year) can sustain some bitterbrush cover but can also lead to dominance of snowbrush and western brackenfern in some areas. Designs that consider a matrix of unburned areas and a range of fire return intervals may be more likely to succeed in striking a balance. Although long-term studies of this nature are challenging to conduct, they remain critical to our understanding of how management activities affect ecosystem recovery and meet outcomes.