Defining the pyro-thermal niche: do seed traits, ecosystem type and phylogeny influence thermal thresholds in seeds...

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 (DRT₅₀, dormancy release temperature; T_opt, optimum temperature; and LT₅₀, 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 (h²) for DRT₅₀, T_opt and LT₅₀. 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 LT₅₀ 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.