Regeneration of fire-adapted species in the Sierra Nevada contingent upon repeated fuel treatments and available soil moisture

Abstract

Management practices over the last 150 years have driven a significant fire deficit in the forests of western North America, leading to widespread changes in the composition, structure, and fuels of these historically fire-adapted ecosystems. Over the last few decades, fuel treatments have been widely applied to restore forest structure and mitigate fire hazard, but prescriptions often do not explicitly consider regeneration dynamics. The increasing ubiquity of fuel treatments, and the necessity to implement them repeatedly, calls for a better understanding of how treatments may influence tree seedling communities and shape stand trajectories over time, yet this remains a critical knowledge gap in forest management. Here, we leveraged a long-term study of repeated fuel treatments in the Sierra Nevada mixed-conifer forest type to examine the regeneration response of six native tree species to repeated applications of three different treatments: prescribed fire, mechanical treatments, and mechanical treatments followed by prescribed fire. With stand and climate data over a twenty-year period, we examined 1) changes in seedling community composition among treatments, 2) important stand and climatic drivers of seedling dynamics, 3) treatment and temporal effects within species, and 4) conditions that promote regeneration of fire-adapted species. We found that post-treatment moisture availability (June-September and October-March precipitation) increased densities and occupancy probabilities, particularly after burning for all conifers. Treatments that excluded fire were most favorable for black oak seedling densities and site occupancy, while the combination of mechanical and fire treatments increased these responses for ponderosa pine. While repeated treatments have begun to restore conditions, further management is necessary to ensure continual recruitment of these fire-adapted species. Treatment systems should be designed to encourage regeneration of fire-adapted species to ensure that future forest structure and composition maintains resilient characteristics.