Seed maturation and mortality patterns support non-serotinous conifer regeneration mechanism following high-severity fire
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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 we refer to as conditional pyriscence, is possible if the fire occurs in the 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 this temporal window, 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. murrayana ), 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 overlayed the period of viability with the timing of fires for northern California to identify the proportion of fires that may be conducive to conditional pyriscence.
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 °C, respectively. Higher cone heat exposure was negatively associated with seed survival and some withstood temperatures as high as 400 °C for 150 s. Seeds of the smaller cone species tended to have lower survivability to heat treatments than the larger cone species. The period of availability of mature seeds overlapped with 60% of the area burned during wildfires for northern California.
Conclusions
We identify circumstances suitable for conditional pyriscence following high-severity fires for four non-serotinous conifers in northern California. The temporal window that permits conditional pyriscence for these species may provide one explanation as to why some large high severity patches can result in substantive seedling regeneration. Inclusion of this information has potential to improve post-fire regeneration modeling and can aid forest management decision-making in fire-prone ecosystems containing non-serotinous conifers.
