Prescribed fires effects on actual and modeled fuel loads and forest structure in southern coast redwood (Sequoia sempervirens) forests

Abstract Background: Fire suppression, timber harvesting, and the forced removal of Indigenous burning have fundamentally changed conditions in coast redwood forests. The contemporary approach of forest preservation and fire exclusion has produced high densities of small trees, elevated fuel loads, sparse understories with limited herbaceous plant diversity and cover, and increased vulnerability to wildfire and climate change. Prescribed broadcast burning presents a viable treatment option to meet forest restoration goals, especially where mechanical treatments are not feasible. Forest and fire managers utilizing fire modeling software such as the Fire and Fuels Extension of Forest Vegetation Simulator (FFE) to predict prescribed fire effects in coast redwoods are limited by model accuracy due to a lack of empirical research and model verification across a breadth of site and weather conditions. Results: We compared the difference between pre- and post-treatment conditions for two fall-season prescribed burns in Sonoma and Santa Cruz counties in California to quantify changes to forest structure, fuel loads, and treatment effectiveness in mitigating future wildfire behavior. This observed data was used to analyze the accuracy of FFE modeled prescribed fire treatment outputs for post-treatment forest and fuels conditions. Observed burn treatments were low intensity and resulted in no significant change to forest structure and composition, but there was a reduction in seedling and sapling densities and an increase in resprout density. There was a reduction in duff and litter fuels, and we found litter and fine woody debris reduction was driven by pre-treatment total fuel loads. The modeled probability of torching was very low pre- and post-treatment. FFE underpredicted scorch height, duff fuel reduction, and redwood regeneration, but slightly overpredicted tree mortality and significantly overpredicted reduction of litter and fine woody debris. Conclusion: Our results highlight a need for model refinement in regards to species-specific mortality, tree regeneration dynamics, and moisture-dependent fuel consumption. In order to achieve desired forest restoration goals, fire practitioners may need to burn at moderate to higher intensities than those typically implemented in these forests, and potentially pair burning with mechanical thinning. Long-term health of coast redwood forests also relies on the restoration of cultural fire and stewardship partnerships that equally share decision making power between western science and Indigenous knowledge bearers.