Natural conifer regeneration after megafire and heat dome in western Oregon

The Holiday Farm Fire burned over 173,900 acres over Labor Day weekend in 2020 due to a combination of prolonged dry and hot conditions and an east-wind event. The June 2021 heat dome event followed the fire, and the two events provided an opportunity to study forest regeneration after large-scale fire and through a heat wave event. Data were collected in 33 plots across the footprint of the fire, four growing seasons after the fire. The objectives were (1) to model conifer seedling abundance four years after the fire based on the distance to nearest live mature tree (seed source), topographic index, fire severity and other factors, and (2) to investigate the species composition of the conifer regeneration. The models suggested that plots where there was a close live mature tree, that were topographically cool, and that burned at moderate- or high-severity had the most prolific conifer regeneration. Duff depth, litter cover, slope, and soil water storage may also play a role in seedling abundance. Most seedlings (91.4 %) measured were Pseudotsuga menziesii , and the rest were primarily Tsuga heterophylla , however moderate-severity plots had a mixture of P. menziesii and T. heterophylla seedlings, while high-severity plots had P. menziesii seedlings almost exclusively. The results suggest that so far the moist conifer forests of the western Cascades are resilient to severe wildfire and heat wave, however moderate-severity burned areas may lead to a more diverse overstory in the region. • There was strong conifer regeneration four years after the 2020 Holiday Farm Fire and three years after the 2021 Heat Dome. • The highest seedling counts were at moderate- or high-severity plots with close seed sources that were topographically cool. • Duff depth, litter, slope, and soil water storage may also play a role in postfire regeneration. • Although Pseudotsuga menziesii seedlings can regenerate at high-severity sites, moderate-severity plots also had Tsuga heterophylla . • Moderate-severity fire may lead to greater future overstory diversity than high-severity fire in the western Cascades.