Lodgepole pine resilience in Troublesome times: the influence of stand-level characteristics on regeneration following bark beetle mortality and wildfire

Abstract

Climate-driven increases in disturbance activity from bark beetle outbreaks and wildfire have prompted concerns for potential declines in tree regeneration. In forests of the Southern Rocky Mountains, widespread tree mortality from bark beetles over the last few decades has resulted in forest conditions that may drive alternate patterns of recovery following subsequent wildfire. As such, the extreme fire season of 2020 presents a unique opportunity to examine the patterns of forest recovery following two disturbances that are unprecedented in their severity and extent. In this study, I collected field data on lodgepole pine seedling recruitment following the East Troublesome fire, which burned through beetle-killed lodgepole pine stands (11-19 years post outbreak), to 1) assess the potential for natural forest recovery and 2) determine the primary stand-level factors that influence the abundance of post-fire lodgepole pine seedlings. In the summer of 2022, I sampled 116 plots across gradients of beetle severity, fire severity, and stand age in Rocky Mountain National Park, Colorado. In each plot, I measured variables capturing fire effects, forest structure, topography, and post-fire regeneration. General linear mixed models were used to determine the relative influence of fire effects, seed source, stand characteristics, and topography on post-fire lodgepole pine seedling density. I found that 97% of the study area had lodgepole pine seedlings established at 2 years post-fire, and 70% of the study area had seedling densities exceeding 370 t ha -1, however, seedling densities were highly variable across the landscape. The abundance of lodgepole pine seedlings at the plot-level was strongly associated with the density of cone-bearing trees, elevation, and tree consumption. These results highlight the mechanisms by which pre-fire forest structure and canopy consumption alter post-fire seed availability to influence post-fire lodgepole pine recruitment. My findings help inform predictions of future forest trajectories following sequential disturbance by bark beetle outbreaks and wildfire and underscore the need for tailored forest management approaches that consider both the legacy of bark beetle outbreaks as well as the variable nature of fire effects.