Defining the pyro-thermal niche: do seed traits, ecosystem type and phylogeny influence thermal thresholds in seeds with physical dormancy?

Seeds are a key pathway for plant population recovery following disturbance. To prevent germination during unsuitable conditions, most species produce dormant seeds. In fire-prone regions, physical dormancy (PY) enables seeds to germinate after fire. The pyro-thermal niche, incorporating temperature effects into seed dormancy and mortality, has not been characterised for PY seeds from fire-prone environments. We aimed to assess variation in thermal thresholds between species with PY seeds and whether the pyro-thermal niche is correlated with seed mass, ecosystem type or phylogenetic relatedness. We collected post heat-shock germination data for 58 Australian species that produce PY seeds. We applied species-specific thermal performance curves to define three critical thresholds (DRT 50, dormancy release temperature; T opt, optimum temperature; and LT 50, lethal temperature), defining the pyro-thermal niche. Each species was assigned a mean seed weight and ecosystem type. We constructed a phylogeny to account for species relatedness and calculated phylogenetic signal (h 2) for DRT 50, T opt and LT 50. We found a consistent inverted u-shaped thermal response curve across all species examined. Seeds from species within Rhamnaceae exhibited higher temperature thresholds than those from Fabaceae. Seed mass was influential in explaining LT 50 variation. The pyro-thermal niche analysis presented here provides a framework for direct comparisons between other fire-prone and nonfire-prone species, in which heat may play a role in postfire germination dynamics.