Background

Silvicultural treatments that modify forest structure and reduce fuel loads can help mitigate future wildfire effects, but treatment efficacy declines over time. In 2021, we remeasured a southern Sierra Nevada mixed conifer forest 13 years after prescribed fire, mastication, and surface-fuel pull-back treatments to assess changes in forest structure and fuels. One month after our remeasurement, the 2021 French Wildfire burned across the study site, providing a serendipitous opportunity for additional remeasurements to investigate wildfire interactions with fuel treatments, where data from original and 13-year posttreatment forest characteristics exist. Here, we report on changes in forest and fuel structure and understory plant composition 3 years after wildfire and relate wildfire outcomes to the legacy of silvicultural treatments.

Results

Wildfire induced large declines in the live overstory biomass for control (47%) and prescribed fire plots (32%) though remotely sensed burn severity was lower in treated plots relative to the control. Downed woody fuels and duff were consumed equivalently in both control and treated plots, ranging from 24 to 99% consumption. Grass loading increased 78-fold in control plots and 22-fold in prescribed fire plots after wildfire, largely driven by invasive cheatgrass, which comprised 79% to 99% of grass cover. However, overstory canopy cover was negatively correlated with cheatgrass loadings (R² = 0.81) and cover (R² = 0.84).

Conclusions

Prescribed fire treatments 13 years prior to the wildfire reduced wildfire effects and mitigated cheatgrass invasion where intervening drought and bark beetle outbreak had not caused high overstory mortality. In contrast, where tree mortality reduced canopy cover before wildfire, cheatgrass biomass increased to dominate the herbaceous layer. The dramatic shift in understory composition and loadings may lead to increased fire frequency and further loss of native vegetation. Cheatgrass invasions after wildfire in low-elevation, mixed conifer forests have the potential to shift fire frequencies and increase the risk of forest conversion toward alternative stable states. Interventions to increase resilience to compounding disturbances may be necessary to mitigate cheatgrass invasion, including prescribed fire and restoration of forest cover where it has been degraded by disturbances.