Low seed viability and other obstacles to post-fire tree regeneration of serotinous lodgepole pine in the wake of pine beetle outbreaks in the Southern Rockies
Wildfires typically stimulate dense tree regeneration in serotinous lodgepole pine forests of the southern Rockies. However, a recent survey of stands with extensive bark beetle-related tree mortality found that seed viability had declined significantly since the mid-2000s outbreak. This may have consequences for regeneration following wildfires that burned along the Colorado-Wyoming border in lodgepole stands with trees that have been dead since those outbreaks. We inventoried seedling recruitment and stand and site conditions in 88 lodgepole pine-dominated stands burned at high severity by the 2020 Mullen and East Troublesome fires. The density of pine recruits was generally low in old growth forest, regenerating stands and recent harvests. None of the areas sampled met regional stocking thresholds, and new seedlings were entirely absent from 40 and 75% of recent harvests sampled in the two fires. Post-fire recruits increased with the pre-fire density of live, serotinous overstory trees and with understory plant cover and site moisture. Conversely, harvest units lacking overstory trees and stands with high cone consumption had low recruitment. Beetle outbreaks have lasting impacts on seed availability and implications for post-fire regeneration due to decreased seed viability in serotinous cones, and high consumption of residual canopy and ground layer cones.