Limited seed viability in long-dead serotinous lodgepole pine trees in the Southern Rockies, USA

Serotinous cones, those that remain closed until heated, confer post-disturbance resilience on many lodgepole pine forests throughout the Southern Rockies. The record-breaking extent of wildfires in northern Colorado and southern Wyoming in 2020 raised concerns about tree regeneration in areas where crown fires burned lodgepole pine stands that had experienced high levels of mortality during bark beetle outbreaks occurring more than a decade previously. We measured seed germination and assessed the relationship between cone age and seed viability for serotinous cones on beetle-killed lodgepole pine in eight stands adjacent to four of those wildfires (n = 1160 cones). On average, germination was 34 % with three viable germinants produced per cone. Seed germination declined and the proportion of cones that contained no seed increased across the age range of cones sampled (17–51 yrs at the time of the 2020 wildfires). Germination of seed retained within the canopy seedbank of these long-dead stands was roughly half of that measured on live or recently killed lodgepole pine. Though pre-fire tree mortality from bark beetles can exceed 90 % in this area, serotinous cones on the remaining live trees and cones buried in the soil seedbank will contribute viable lodgepole seeds for tree regeneration on some burned landscapes. The decreased germinant levels from these cones may regenerate into pine stands that are less dense than after harvesting, bark beetles or wildfire alone, but they will likely create forest structure within a range of conditions expected for this region.