Forest resilience and post-fire conifer regeneration in the southern Cascades, Lassen Volcanic National Park California, USA

Abstract

Contemporary fire regimes in Californian forests are shifting, with fires becoming larger, more frequent, and increasingly severe. As landscapes transition back to active fire regimes, understanding how the physical environment and biological legacies of past disturbance interact with and determine forest development becomes an increasingly important management concern. We surveyed post-fire regeneration in a wilderness area of Lassen Volcanic National Park (LAVO) with a three-decade history of fires. We tallied conifer seedlings , and measured shrub and ground cover and mature trees in the field, and used geospatial data and water balance models to account for the possible effects of , terrain, live seed sources, vegetation characteristics, and immediate post-fire climate as measured by water balance on conifer regeneration. We used a Random Forest machine learning technique to model how landscape and local factors affect post-fire regeneration stocking and species regeneration occurrence. Post-fire conifer regeneration in LAVO was common: Median post-fire seedling density was close to desired stocking density, 73% of our plots had at least one conifer seedling, and only 16% of plots lacked tree regeneration or mature trees. Likelihood of stocking and individual species’ occurrence was most strongly related to time since fire, distance to forest, elevation, and species of nearest seed source. Tree regeneration increased with time since fire, but this effect was more pronounced at high elevation suggesting an interaction between terrain and time since fire on regeneration. Stocking was most likely on cooler (northeastern) slopes and at lower (<1950 m) elevations, nearer (within 200 m) to live forest patches, and where shrub cover was present but low (<20%). Climatic conditions as represented by water balance for the first five years after a fire did not influence likelihood of stocking but it did influence species occurrence. Distance to conspecific trees was important for species’ regeneration occurrence, and the landscape pattern of species occurrence was consistent with the elevational zonation of forest dominants in LAVO. Adequate tree regeneration of forests with an active fire regime in LAVO indicate that California montane forests are resilient to prescribed fire and managed wildfire and that expanded use of fire to reduce potential for severe fire promotes resilience in California forest landscapes.