Seed maturation and mortality patterns support non-serotinous conifer regeneration mechanism following high-severity fire

Background Climate warming increases fire activity for many regions around the world, prompting concern over the long-term persistence of conifer species that regenerate poorly from seed after high-severity fire. However, substantive regeneration of non-serotinous conifer species within a large high-severity patch, a process termed facultative serotiny, is possible if the fire occurs in the brief window following seed maturation but before cone opening, the enclosed seeds can withstand the heat range of the fire, and the non-serotinous cone crop is sufficient. To define the temporal window of facultative serotiny, we collected closed cones from June to September over two seasons, examining seed maturation as a function of the heat sum for four non-serotinous California conifer species: ponderosa pine ( Pinus ponderosa ), Sierra lodgepole pine ( Pinus contorta var. murryana ), incense cedar ( Calocedrus decurrens ), and Douglas-fir ( Pseudotsuga menziesii ). Additionally, we examined seed survival based on viability testing in closed cones following heat treatments ranging from ~ 20 to 600°C. Finally, we compared this temporal window of viability to the proportion of area burned during that same interval using recent fire data for northern California to identify the proportion of fires that may be conducive to facultative serotiny. Results The accumulated heat sum was positively associated with seed maturity; the proportion of seeds that were viable varied by species but generally ranged from late-July (10%) to mid-September (90%) with heat sums ranging from 1285℃ to 2081℃, respectively. Higher heat exposure was negatively associated with seed survival and some withstood temperatures as high as 400°C for 150 seconds. Seeds of the smaller cone species, incense cedar and Douglas-fir, tended to have lower survivability to heat treatments than the larger cone species, lodgepole pine and ponderosa pine. The period of availability of mature seeds overlapped with 60% of the area burned during wildfires for northern California. Conclusions We identify conditions suitable for the occurrence of facultative serotiny following high-severity fires for four non-serotinous conifers in northern California. The temporal window that permits facultative serotiny for these species can be incorporated into post-fire regeneration modeling and can aid forest management decision-making in fire-prone ecosystems containing non-serotinous conifers.